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Learn about chemical gold plating for jewelry. It uses special solutions to put a thin layer of gold on items. Some methods need cyanide, others don't. The process can be slow and tricky, but it makes jewelry look great. Perfect for jewelry makers and sellers who want to add a touch of gold.

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Chemical Gold Plating: Techniques, Solutions, and Applications for Jewelry

Εισαγωγή:

What is chemical gold plating? It’s a process used to deposit a thin layer of gold onto various materials, enhancing their appearance and durability. How does it work? Chemical gold plating involves using special solutions with gold ions and reducing agents. Traditional methods often use cyanide, but there are also cyanide-free options. Why choose chemical gold plating? It offers a uniform, high-quality finish that’s ideal for jewelry and other decorative items. Whether you’re a jewelry maker, designer, or retailer, understanding these techniques can elevate your products.

Section I Cyanide Chemical Gold Plating

1. Overview

With the high density of electronic components, the complexity of line design, the microfabrication of circuits, and the independence of electrical properties, chemical gold plating, which solves the complexity of the plating process, has become an inevitable way of gold plating. However, chemical gold plating has the following disadvantages:

① Slow speed;

② Narrow usage conditions and operating range, increasing management difficulty;

③ The material surface must be thoroughly clean;

④ The plating solution life is short (prone to self-redox reactions);

⑤ The plating thickness is very sensitive to stirring conditions.

Therefore, chemical gold plating must be carried out using special equipment. The commonly used chemical gold plating solution composition is shown in Table 1-24, prepared by combining various gold coordination salts and reducing agents.

Table 1-24 Types of Gold Coordination Salt and Reducing Agents in Chemical Gold Plating Solution

Αναγωγικό μέσο	KAu(CN)₂	KAu(CN)₄	Na₃Au(SO₂)₂	HAuCl₄ • 3Η₂O	HAuCl₄ ・ 3H₂O	AuCN	KAu(CN)₄	KAu ・ O₂	KAu(OH)₄	AuI	Not specific
Sodium Hypophosphite	57,58		71			77
Formaldehyde			71				78
Biazide	59		71		76		78
Borohydride	60,61		72				78		79
Methylborane	60	69		74	75			79			80
Hydroxylamine	62
Unpublished	63		73								81
Θειουρία	64
Ammonium hydroxide	65
Potassium sodium tartrate tetrahydrate (combined)	66	70
N,N-Dimethylamide										82
N,N-Dimethylglycine	67
Κανένα	68

The various components of the chemical gold plating solution and the plating process are shown in Table 1-25. Below, the reducing agents hypophosphite and borohydride in two practical plating solutions are introduced.

Table 1-25 Functions of Components in Chemical Gold Plating Solution

Component	Purpose	Compound Name
Gold Ion	Supply of Metallic Gold	KAu(CN)₂、KAu(CN)₄、AUCN、 Na₃Au(SO₂)₂. HAuCl₄ • 3Η₂O、KAu ・ O₂
Reducing Agent	Reducing agents for gold deposition	Potassium sodium tartrate tetrahydrate, borohydride, amine borane, formaldehyde, hypophosphite, N,N-dimethylglycine, ascorbic acid
Organic chelating agents	Buffer action to prevent rapid gold deposition	EDTA, potassium citrate, tartaric acid
Stabilizer	Mask active nuclei to prevent plating solution decomposition	Thiourea, metal cyanide, acetylacetone
Activator	The slowing effect of the moderating chelating agent	Succinic acid
Buffer agent	Maintain a certain pH	Phosphates, citrates, tartrates
Surfactant	Make the plated parts easy to wet	Aliphatic sulfonates, alcohol sulfates

2. Hypophosphite Gold Plating Solution

When the base metal and the gold in the plating solution undergo a displacement reaction, the metal with a lower electrochemical potential ionizes more easily than gold. During the reaction, once the surface of the base metal is completely covered by gold, the reaction immediately stops, so only 0.1~0.3μm thin gold plating layer can be obtained. This plating solution, or Brookshire’s solution, is an old plating solution and is the basic composition of currently marketed chemical gold plating solutions. Characteristics of the plating solution: the base layer for gold plating must be nickel for the displacement reaction; gold cannot be displacement-plated on copper material.

(1) Composition and conditions of hypophosphite chemical gold plating solution

Table 1-26 show the typical composition of hypophosphite chemical gold plating solution.

Table 1-26 Composition and Conditions of Hypophosphite Chemical Gold Plating Solution

Composition and Operating Conditions	Σειριακός αριθμός
Composition and Operating Conditions	1	2	3	4	5
Potassium gold cyanide/(g/L)	2	2	20	2	2
Sodium Hypophosphite/(g/L)	10	10	10	10	10
Potassium cyanide /(g/L)	0.2	0.2	80	0.4 ^①	0.2
Θερμοκρασία/°C	96	96	96	96	96
pH	13.5	7.5	13.5	13.5	7.1
Load rate/(cm²/cm³)	0.25	0.25	0.25	0.25	0.25
Plating rate/[mg/(cm²· h)]	9.8	9.85	12.3	8.2	3.86

① Potassium cyanide is replaced by sodium cyanide.

(2) The effect of nickel on the deposition rate of hypophosphite plating solution

In the chemical gold plating solution with hypophosphite as the reducing agent, active metal ions (Ni²⁺) are added. Metal (nickel) and gold are co-deposited in trace amounts on the plated parts’ surface, creating a large chemical driving force and increasing the gold plating speed. The principle is shown in the model diagram of Figure 1-25.

Experiments were conducted using the plating solution from Table 1-27. The test sample was 20mm×25mm×0.1mm. The gold concentration of two types of plating solutions are 2g/L, 5g/L, 7g/L, temperature of 70℃and time are 10min, 30min, 50min, 60min. Pre-plating treatment included immersion degreasing for 2min, electrochemical degreasing for 1min, activation, and hydrochloric (1+1) acid treatment for 1min before gold plating. The results are shown in Figure 1-26.

Table 1-27 Composition and Operating Conditions of Hypophosphite Gold Plating Solution

Composition and operating conditions	I	II	Composition and operating conditions	I	II
Potassium gold cyanide/(g/L)	2, 5, 7	2, 5, 7	Potassium cyanide /(g/L)	-	2
Sodium citrate/(g/L)	75	75	H₂NNH₂ · H₂O /(g/L)	-	2
Υποφωσφορώδες νάτριο/(g/L)	15	15	pH	8.5	4.3
EDTA-2Na/(g/L)	-	-	Θερμοκρασία/℃	70, 90	70, 90
Sodium chloride/(g/L)	-	5

3. Borohydride Salt Plating Solution

(1) Composition and Properties of Potassium Borohydride Chemical Gold Plating Solution

In 1969, Okinaka et al. proposed using a plating solution with borohydride compounds as the reducing agent. The composition and conditions of the borohydride chemical gold plating solution are shown in Tables 1-28, and the properties of the gold layers plated by these solutions are shown in Tables 1-29. The thickness of the plating layers plated at different times with various plating solutions is indicated in Figure 1-27. The summary results of the reducing agents potassium borohydride and dimethylamine borane are shown in Table 1-30.

Table 1-28 Composition and Operating Conditions of Potassium Borohydride Chemical Gold Plating Solution

Composition and Operating	Σειριακός αριθμός
Composition and Operating	1	2	3
Potassium gold cyanide/(g/L)	0. 02mol/L(5.8g/L)	0. 03mol/L(8.6g/L)	0. 005mol/L(1. 45g/L)
Potassium cyanide	0. 2mol/L(13.0g/L)	0. 2mol/L(13.0g/L)	0. lmol/L(6.5g/L)
Υδροξείδιο του καλίου	0. 2mol/L(11.2g/L)	0. 2mol/L(22.4g/L)	0. 2mol/L(11.2g/L)
Potassium borohydride	0. 4mol/L(21.6g/L)	0. 8mol/L(43.1g/L)	0. 2mol/L(10.8g/L)
Θερμοκρασία/°C	75	75	75

Table 1-29 Properties of Chemical Gold Plating Solution with Potassium Borohydride Reducing Agent

Nature	Data	Nature	Data
Bonding strength	Excellent bonding strength of the metal layer	Porosity	No pores larger than 1μm on the uniform surface
Εμφάνιση	Dull yellow	Καθαρότητα	99. 9%
Density	Gold content 19. 3g/cm³	Resistance value	0. 03Ω/cm² ( Au: 1 μm)
Σκληρότητα	Nup hardness 60〜80	Συγκολλησιμότητα	Excellent

Table 1-30 Functions of Components in Potassium Borohydride Chemical Gold Plating Solution

Influencing Factors	Potassium Borohydride Plating Solution	DMAB (Dimethylaminoborane) plating solution
Effect of Gold Concentration	When the gold concentration reaches 0.002mol/L, the precipitation rate rises sharply and decreases above that.	The rate of precipitation increases linearly at 0.01 mol/l gold concentration, but has no effect above that.
Effects of Potassium Cyanide	Unstable plating solution when below 0.005 mol/l. Gold does not precipitate above 0.2 mol/l.	Same as left
Effect of potassium borohydride	BH is not stable at room temperature. Addition of potassium hydroxide improves the stability of the bath, and the rate of precipitation decreases with increasing concentration.	DMAB is stable at room temperature, the precipitation rate increases with the increase of potassium hydroxide.
Concentration of reducing agent	Increase in precipitation rate with increase in concentration
Effect of plating bath temperature	Increase the precipitation rate by 10℃, and decompose at 85℃.

The maximum plating speed of Okinaka chemical gold plating solution is 2μm/h: After about half a day, the plating solution undergoes a self-decomposition reaction, and it is greatly affected by the strength of air stirring and the amount of air, so this plating solution is not practical.

(2) Influence of plating speed and heavy metal ions

Adding trace amounts of heavy metal ions (lead, thallium, etc.) to the chemical gold plating solution significantly increases the deposition rate. Mcintyre explained this effect as follows. Due to low potential deposition, the catalytic effect of adsorbed lead atoms, as shown in mechanisms (1-4) and (1-5), promotes nucleation.

Pb²⁺ + 2e^– → Pb_ad （1-4）

2Au(CN)₂^– + Pb_ad → 2Au + Pb²⁺ + 4CN^– （1-5）

Due to the large difference in work functions between copper, gold materials, and lead ions, lead is reduced and deposited at a potential more positive than the thermodynamic redox potential, forming a monolayer. Due to the catalytic reaction of adsorbed lead atoms as in (1-4) and (1-5), ) the reduction reaction of Au(CN) occurs rapidly, effectively promoting the deposition of gold.

The relationship between lead concentration and deposition rate in potassium borohydride-reducing gold plating solution is shown in Figure 1-28, and the plating solution composition is shown in Table 1-31. Lead is added in the form of lead nitrate. Figure 1-28 shows that the lead addition concentration in the plating solution can only be about 20ml/L, and adding more has no effect.

Table 1-31 Composition and Operating Conditions of Potassium Borohydride Gold Plating Solution

Composition and operating conditions	I	II	Composition and operating conditions	I	II
Potassium gold cyanide/(g/L)	2	2	Potassium cyanide /(g/L)	10	10
Potassium borohydride/(g/L)	2	10	EDTA-2Na/(g/L)	5	5
Potassium cyanide/(mg/L)	5	5	Plating solution temperature/°C	75	75

On the other hand, when DMAB is used as a reducing agent, the plating solution in Table 1-32 achieves a maximum plating rate of about 7μm/h.

Table 1-32 Composition of BMAB Gold Plating Solution

Composition and operating conditions	Συγκέντρωση	Composition and operating conditions	Συγκέντρωση
Potassium gold cyanide/(g/L)	2	Potassium cyanide /(g/L)	30
BMAB/(g/L)	10	EDTA-2Na/(g/L)	5
Potassium cyanide/(mg/L)	5	pH	13.6

The effects of trace metals other than gold, such as lead, thallium, arsenic, and copper ions, are shown in Figure 1-29.

Generally, the main gold salt commonly used is potassium gold(I) cyanide. Considering the stability of the plating solution, potassium gold(III) cyanide plating solution can also be used. Matsuoka introduced the effect of adding lead and thallium on the gold plating rate in the plating solution, as shown in Table 1-33, and proposed the following conclusions.

Table 1-33 Composition of Potassium Gold(III) Cyanide Chemical Gold Plating Solution

Σύνθεση	Parameters	Σύνθεση	Parameters
KAu(CN)₄/(g/L)	4	KOH/(mg/L)	11.2
KBH₄/(g/L)	3	Θερμοκρασία/℃	70
PbCl₂/(mg/L)	0.5

The effects of temperature and lead concentration on the gold deposition rate are shown in Figure 1-30, and the relationship between gold plating thickness and time is shown in Figure 1-31. The conditions of no stirring (monovalent gold plating solution, curve 1) and no stirring (trivalent gold plating solution, curve 2) are the same (Figures 1-32 and 1-33). After about 1 hour, the plating rate slows down. The change in plating rate at this time is not due to the reduction of plating solution components but because changes occurring on the surface of the gold-deposited layer affect the plating rate. The plating thickness increases proportionally in the stirred plating solution for about 2 hours, and the plating rate slows down. The plating rate of curve 5 recovers due to the renewal of the plating solution. In this stirred plating solution, the plating rate slowdown is consumed by plating solution components. Figure 1-32 shows the relationship between the added concentrations of lead and thallium and the deposition rate.

Adding trace amounts of lead or thallium during the electroplating process has an extremely adverse effect on the bonding of high-purity gold plating layers, so the concentration of PbCl₂ is appropriate 0.5mg/L. For porosity detection of the plating layer, after gold plating on copper, the sample is placed in 20% nitric acid and subjected to ultrasonic treatment; then, the dissolved copper is measured by atomic absorption spectroscopy, followed by evaluation. As shown in Figure 1-33 results, the porosity significantly decreases with increasing thickness up to a plating thickness of 2μm.

The chemical gold plating process of crystallized glass (SiO₂ 46% + Al₂O₃ 16% + MgO 17% + K₂O 10% + F 4% + B₂O₃ 7% ) is shown in Figure 1-34.

4. Reaction Mechanism of Borohydride Salt Chemical Gold Plating Solution

Lin Zhongfu et al. proposed that the potassium borohydride and DMAB (dimethylamine borane) gold plating solutions in Table 1-34 can become practical gold plating solutions after adding stabilizers.

(1) Composition of two plating solutions: Table 1-34 shows the two compositions.

Table 1-34 Potassium Borohydride and DMAB Chemical Plating Solutions

Σύνθεση διαλύματος επιμετάλλωσης	Potassium borohydride plating solution	DMAB plating solution
Potassium gold cyanide/(mol/L)	0.02	0.02
Potassium cyanide/(mol/L)	0.2	0.02
Potassium hydroxide/(mol/L)	0.2	0.8
Potassium borohydride/(mol/L)	0.4	-
DMAB/(mol/L)	-	0.4
Θερμοκρασία/°C	75	85
Plating speed/(μm/h)	0.7	0.4

(2) Reaction mechanism: In the potassium borohydride gold plating solution, BH₄^– itself does not produce a reducing effect, but the intermediate BH₃Ω^– of its hydrolysis product BO₂^– acts as the reducing agent.

ΒΗ₄^– + Η₂O → BH₃Ω^–+ Η₂ (1-6)
ΒΗ₃Ω^– + Η₂O → BO₂^– + 3H₂ (1-7)

At the gold anode, the reducing agent BH₃Ω^– undergoes the following oxidation reaction:

ΒΗ₃Ω^– + 3OH^– → BO₂^– + 3/2 H₂+ 2 ώρες₂Ο + 3ε^– (1-8)

At the cathode, gold ions undergo the following reduction reaction:

Au(CN)₂^– + e^– → Au + 2CN^– (1-9)

The overall reaction of electroless gold plating in potassium borohydride plating solution is:

ΒΗ₃Ω^– + 3Au(CN)₂^– + 3OH^– → BO₂^–+ 3/2 H₂+ 2 ώρες₂O + 3Au + 6CN ^– (1-10)

The hydrolysis reactions of equations (1-6) and (1-7) were analyzed using polarography. The result shows that reaction (1-7) is 500 times faster than reaction (1-6). Therefore, the utilization rate of potassium borohydride in the gold plating reaction is very low. Under typical plating conditions, the effective utilization rate does not exceed 2%~3%, with most reducing agent remaining ineffective due to hydrolysis.

The above hydrolysis reaction proceeds rapidly at low pH (acid-catalyzed reaction), and the rate of gold deposition is fast when the potassium hydroxide concentration is low. To prevent the natural decomposition of the gold plating solution, the potassium hydroxide concentration must be maintained at least above 0.1mol/L. The relationship between the gold deposition rate and the potassium hydroxide concentration is shown in Figure 1-35.

The relationship between the gold deposition rate of DMAB plating solution and the concentration of potassium hydroxide is opposite to that of potassium borohydride plating solution, as shown in Figure 1-36. In the DMAB plating solution, BH₃Ω^– still acts as the reducing agent.

(Χ.Ε.)₃)₂Νιου Χάμσαϊρ₂ • BH₃+ ΩΧ^– → (CH₃)₂Νιου Χάμσαϊρ₂+ BH₃Ω^– (1-11)

In the process of generating BH₃Ω^– from DMAB, DMAB must directly react with OH-when the alkalinity. When the alkalinity is strong, the generation rate of BH₃Ω^– is fast, and the gold deposition rate is also fast.

(3) Issues with Potassium Borohydride Plating Solution and DMAB Plating Solution

Figure 1-37 shows that the plating rate changes with the concentration of copper and nickel ions in the plating solution. When the nickel ion concentration is at 10^-5mol/L, the plating rate begins to decrease, and when the nickel ion concentration reaches 10^-3mol/L, the plating solution decomposes. Copper ions have almost no effect on the plating rate. At the initial stage of gold plating, gold ions and nickel undergo displacement reaction, trace amounts of nickel ions dissolve, and the plating rate slows.

The effect of adding trace amounts of nickel ions on the BH₃Ω^– anodic oxidation current is shown in Figure 1-38. The addition of trace nickel ions significantly inhibits the oxidation reaction of the reducing agent BH₃Ω^– on the gold-rotating disk anode. This is because nickel ions exist in the form of cyanide ligand salt Ni(CN)₄^2- , and this complex ion preferentially adsorbs on the gold surface, hindering the deposition of gold.

Unstable base layers (such as aluminum nitride) cannot use strongly alkaline solutions. The pH of the plating solution depends on the nature of the reducing agent, which is the biggest drawback of potassium borohydride and DMAB plating solutions.

Regarding the above issues, except that the pH of the plating solution cannot be changed, other aspects can be improved.

(4) Improve potassium borohydride gold plating solution and DMAB gold plating solution

① Increase plating speed

a. Change the plating solution’s basic composition and operating conditions to increase plating speed.
b. Increase stirring speed.
c. Increase the temperature of the plating solution.
d. Reduce the concentration of free cyanide.
e. Decrease the alkali concentration in the potassium borohydride plating solution and increase the alkali concentration in the DMAB plating solution.
f. Increase the concentration of the reducing agent. However, when the plating rate without special additives increases by about 2μm/h or more, the plating solution becomes unstable and prone to decomposition.

② High-speed plating solution

a. Add polarization eliminators. In soft gold plating solutions, Pb²⁺ , T1⁺ ions are effective polarization eliminators. These ions strongly adsorb on the gold surface, producing the so-called UPD (under potential deposition) phenomenon, where lead deposits at a potential much more positive than the Pb²⁺ / Pb equilibrium potential and thallium deposits at a potential more positive than the T1⁺ /T1 equilibrium potential. The deposited lead, thallium and Au(CN)₂^– undergoes displacement reactions, causing the gold deposition reaction to shift to a more positive potential (polarization weakening). On the other hand, adding these ions to chemical gold plating solutions, similar to electrolytic plating, shifts the Au(CN)₂^– reduction potential in the positive direction, resulting in an increased plating rate, which can reach about 10μm/h. Plating solutions with added Pb²⁺ , T1⁺ ions are shown in Tables 1-35 and 1-36.

Table 1-35 Potassium Borohydride Plating Solution with Added Pb²⁺

Solution 1		Solution 1
Potassium gold cyanide	5g/L	Colloidal	2g/L
Potassium cyanide	8g/L	Solution 2
Sodium Citrate	50g/L	Potassium borohydride	200g/L
EDTA	5g/L	Sodium hydroxide	120g/L
Lead Chloride	0.5g/L

Note: Solutions 1 and 2 are mixed into the gold plating solution at a volume ratio of 10:1 and plated under 75~85℃, stirring at a plating rate of 4μm/30min.

Table 1-36 Potassium Borohydride Plating Solution with Added T1⁺

Composition and Operating Conditions	Parameters	Composition and Operating Conditions	Parameters
Potassium gold cyanide	Adjust as needed	Potassium borohydride	5. 4 ~ 10. 8g/L
Potassium cyanide	6.5g/L	Θερμοκρασία	70 〜 80℃
Υδροξείδιο του καλίου	11.2g/L	Plating rate	＜10μm/h
Thallium sulfate	5 〜 100mg/L

Adding lead chloride or titanium chloride to the potassium gold(III) cyanide plating solution [KAu(CN)₄], the plating rate increases by 8~9 times compared to the one without addition. The Au(CN)₄^– in the plating solution is reduced by BH4^– to [Au(CN)₂^–]^–, so this plating solution is essentially the same as a newly prepared KAu(CN)₂ plating solution. Because Au(CN)4^–undergoes a reduction reaction, producing Au(CN)₂ and free CN^–, this plating solution does not need to add potassium cyanide when preparing a new plating solution. The reaction formula is as follows:

Au(CN)₄^– + 2e^– → Au(CN)₂^– + 2CN^– (1-12)

The polarization curves of the reduction reaction of Au(CN)₂^– before and after adding Pb²⁺ are shown in Figure 1-39. The gold deposition from Au(CN)₂^– reaction polarization curve shows the effect of Pb²⁺ eliminating polarization. The effect of adding Pb²⁺, T1⁺ ions to promote the gold plating reaction is shown in Figure 1-40.

The Pb²⁺ concentration in the plating solution is too high, causing deterioration of the gold plating appearance. Even if the Pb²⁺ concentration exceeds 10mg/L, adding alkanol amines (such as triethanolamine) does not cause deterioration and allows high-speed gold plating (＞3μm/h). Adding other elements like gallium, indium, germanium, tin, antimony, or bismuth can achieve the same promoting effect.

b. Adding stabilizers. Adding organic stabilizers that reduce the catalytic activity of gold, increasing the plating solution temperature, and changing the concentration of the reducing agent can improve the plating speed of the solution. For example, the plating speed of plating solutions containing amino acetic acid, hydroxyethyl ethylenediaminetetraacetic acid, and diazotriacetic acid or succinic acid derivatives can reach a high rate of 12~23μm/h at 90℃. However, at high temperatures, the hydrolysis reaction of the reducing agent accelerates significantly, making the plating solution difficult to control. Adding trace amounts of p-dimethylamino azobenzene stabilizer does not affect the stability of the plating solution and can increase the usual usage concentration of BH4- by 2 times, raising the plating speed to 5μm/h. Other effective stabilizers include ethylene·glycol. Ethyl·ether, diethylene glycol? ethyl ether, and polyethyleneimine.

c. Use trivalent gold cyanide complex salts. Plating solutions with trivalent gold cyanide complex salts, such as those with added lead chloride or titanium chloride, as shown in Table 1-33, is detailed in Table 1-37.

Table 1-37 Potassium Gold(III) Cyanide Potassium Borohydride Plating Solution

Composition and operating conditions	Parameters	Composition and operating conditions	Parameters
Potassium gold(III)	3g/L	Lead chloride	0. 5mg/L
Υδροξείδιο του καλίου	11.2g/L	Θερμοκρασία	70℃
Potassium borohydride	3g/L

The advantage of trivalent gold cyanide complex salt plating solution is that KAuO₂ and KAu(OH)₄ can be used as supplementary gold salt additives, avoiding excessive accumulation of free cyanide ions in the plating solution, reducing the plating rate, and favoring long-term stable plating rate. The plating rate of additive-free the plating solution is 2~8μm/h.

The study of the electrochemical behavior, structure, and properties of the DMAB plating solution shows that similar to the potassium borohydride plating solution, KAu(CN)₂ only is used when preparing the gold plating solution, and dissolved gold hydroxide in potassium hydroxide solution is added as a supplement to avoid the accumulation of free cyanide ions. The composition of the plating solution is shown in Table 1-38.

Table 1-38 DMAB Plating Solution (Gold Hydroxide Dissolved in Potassium Hydroxide Solution)

Composition and operating conditions	Parameters	Composition and operating conditions	Parameters
Potassium gold cyanide	0.013 ~ 0.018mol/L	Πρόσθετος	Small amount
DMAB	0.07 〜 0.1 mol/L	pH	13.1 ~ 13.4
Υδροξείδιο του καλίου	0.09 〜 0.15mol/L	Θερμοκρασία	70 〜 75℃
Potassium cyanide	0.007 ~ 0.01mol/L	Plating rate	2μm/h

(5) Improve the stability of the plating solution.

Using additives can increase the plating rate and the stability of the plating solution, but the impurities produced will cause the plating solution to decompose.

Substances such as EDTA and ethanolamine added to the plating solution form strong, complex ions with metal impurity ions, inhibiting the reaction between metal impurities and BH₄^– and BH₃Ω^–, thereby, improving the stability of the plating solution. The composition of the potassium borohydride plating solution with added EDTA and ethanolamine is shown in Table 1-39.

Table 1-39 Potassium Borohydride Plating Solution with Added EDTA and Ethanolamine

Composition and operating conditions	Parameters	Composition and operating conditions	Parameters
Potassium gold cyanide	1.45g/L	EDTA	5g/L
Potassium cyanide	11 g/L	Αιθανολαμίνη	50mL/L
Υδροξείδιο του καλίου	11.2g/L	Θερμοκρασία	72℃
Potassium borohydride	10.8g/L	Plating rate	1. 5μm/h

The gold plating must first be done on metallic nickel by displacement plating, followed by chemical gold plating. Displacement gold plating cannot completely and thoroughly cover the base layer of nickel. Adding a reducing agent can solve the problem of reduced plating solution stability caused by the dissolution of trace amounts of nickel ions. In the DMAB plating solution, a second reducing agent, hydrazine, is added. The composition of the plating solution is shown in Table 1-40. Lead acetate and potassium carbonate, respectively, promote the anodic reaction and the local cathodic reaction, thereby increasing the plating speed.

Table 1-40 Plating Solution with Two Reducing Agents (Hydrazine and DMAB)

Composition and operating conditions	Parameters	Composition and operating conditions	Parameters
Potassium gold cyanide	0. 005mol/L	DMAB	0,05 mol/L
Potassium cyanide	0. 035mol/L	Hydrazine	0,25mol/L
Υδροξείδιο του καλίου	0. 8mol/L	Θερμοκρασία	80℃
Ανθρακικό κάλιο	0. 45mol/L	Plating rate On the Nickel Base Layer (Initial Stage)	2. 6μm/h
Lead acetate	15X10^-6	On the gold (fixed value)	7. 8μm/h

In the plating solution, the initial reaction is the catalytic oxidation of hydrazine by the nickel base layer, which acts as a catalyst. In contrast, hydrazine reduces gold ions to deposit gold. After the nickel base layer is completely covered by gold, DMAB acts as the reducing agent, and the usual autocatalytic gold plating reaction occurs, with gold continuing to deposit. The anodic oxidation curves of hydrazine and DMAB on nickel and gold electrodes show that hydrazine is easily oxidized on the nickel electrode but not on the gold electrode (Figure 1-41). Conversely, DMAB is easily oxidized on the gold electrode but hardly oxidized on the nickel electrode, as shown in Figure 1-42. This difference in catalytic activity between the substrate metal and the deposited gold metal for oxidizing reducing agents is a characteristic of substrate-catalyzed gold plating.

In a plating solution containing only the reducing agent hydrazine, gold can be deposited solely relying on the catalytic effect of the nickel base layer. When the base layer is completely covered by gold, gold deposition stops, and at this point, the thickness of the gold plating layer depends on the concentration of free cyanide in the plating solution. Although the maximum plating thickness of this plating solution is limited (about 2μm), the plating layer is dense enough to meet the requirements of key pressure bonding.

Besides hydrazine, many other available reducing agents are available in the material catalytic gold plating solution; see Table 1-41.

Table 1-41 Other Reducing Agent Plating Solutions

Αναγωγικό μέσο	pH	Θερμοκρασία/℃
Hypophosphite	7 〜 7. 5	93
	7. 5 〜 13. 5	96
	3 〜 4	70 〜 80
Hydrazine	7 〜 7. 5	92 〜 95
	5. 8 〜 5. 9	95
	11. 7	95
Hydroxylamine	2. 3 〜 2. 8	70 〜 85
Sodium cyanoborohydride (NaBH₃CN)	3.5	90
Hydrazinylborane (N₂H₄ ・ BH₃)	>13	60
Θειουρία	6. 5 〜 7. 0	83 〜 90
Θειουρία	4.2	80 ~ 85
Ascorbic acid	8	63
Titanium trichloride	5	75

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5. Chemical Gold Alloy Plating

(1) Gold-silver alloy:

A chemical gold-silver alloy plating can be achieved by continuously adding potassium silver cyanide solution to the borohydride gold plating solution. Ag(CN)₂^– is more easily reduced than Au(CN)₂^–. When preparing a new plating solution, silver will precipitate first if the two are mixed, and the alloy cannot be plated.

By continuously adding small amounts of potassium silver cyanide dropwise into the plating solution, maintaining a relatively low condensation [molar ratio of Au:Ag (26:10~(26:0.5)] A gold-silver alloy plating layer with a silver content of 80%~20% can be precipitated. In this plating solution, the precipitation of silver is a replacement reaction product of the gold that precipitates first and the Ag(CN)₂^–, in solution.

(2) Gold-Copper Alloy:

In conventional coordination-based EDTA and formaldehyde reducing agent chemical copper plating solution, only potassium gold cyanide can plate a gold-copper alloy. By changing the potassium gold cyanide concentration, the copper mass fraction in the gold alloy plating layer can be adjusted from 1%~94%.

(3) Gold-Tin Alloy:

Gold and tin alloy chemical plating solution use the SnCl₂ reducing agent method. The composition and concentration range of the plating solution are shown in Table 1-42.

Table 1-42 Gold-Tin Alloy Plating Solution

Composition and Operating Conditions	Parameters	Composition and Operating Conditions	Parameters
Potassium gold(III) cyanide	4〜10g/L	Triolefin	0. 05 ~ 0. 5g/L
Potassium cyanide	5〜15g/L	Θερμοκρασία	Θερμοκρασία δωματίου
Υδροξείδιο του καλίου	60〜100g/L	Plating rate	1.5μm/h (no triolefin addition)
Stannous chloride	40〜60g/L		5μm/h (triolefin addition 0.5g/L)

The gold-tin alloy is a low-melting-point eutectic alloy. The tin content in the alloy plating layer can be adjusted from 5%~60%. Adding a small triene to the plating solution increases the plating speed. The tin deposition in this plating solution is a disproportionation reaction of divalent tin.

(4) Gold-indium alloy:

The gold-indium alloy plating solution is prepared by adding Ih₂(SO₄)₃ and EDTA to a borohydride solution. The indium content in the plating layer is 1%~4%. At room temperature, a thin gold-indium alloy plating layer deposited on palladium-activated n-GaAs, after 350℃ heat treatment, has a contact resistance superior to pure gold.

(5) Gold-nickel, gold-cobalt alloys:

Uemaki developed 18~22K bright gold-nickel and gold-cobalt alloy plating solutions. pH 3~5, the gold alloy plating layer of cyanide chemical gold plating solution with added reducing agent sodium hypophosphite has physical properties such as hardness and wear resistance inferior to those of gold alloy electroplating layers containing nickel and cobalt.

Section II Cyanide-Free Chemical Gold Plating

1. Overview

Displacement-type gold plating uses the potential difference between the oxidation potential of the base layer metal and the reduction potential of gold ions as the plating reaction driving force. In contrast, reduction-type chemical gold plating uses the potential difference between the oxidation potential of the reducing agent and the reduction potential of gold ions as the plating reaction driving force.

The electrons involved in the reduction reaction of gold ions are provided either by the oxidation reaction of the base layer metal (displacement-type gold plating) or by the oxidation reaction of the reducing agent (reductive gold plating).

Other methods include: ① Adding heavy metal salts to the displacement-type gold plating solution, where the base layer metal surface adsorbs the heavy metals, shifting the potential in the direction that makes gold ions easier to reduce; ② Adding a reducing agent to the displacement-type gold plating solution, allowing redox reactions to occur simultaneously to plate thicker gold; ③ In gold plating solutions containing reducing agents, the base layer noble metal acts as a reducing agent and undergoes oxidation with a catalyst, performing base layer catalytic chemical gold plating.

Ligands in chemical gold plating solutions commonly use cyanide, which forms very stable complexes with gold ions (stability constant K=4×10²⁸). Currently, many semiconductor components have switched to using near-neutral, low-cyanide, or cyanide-free gold plating solutions, such as sulfite or thiosulfate gold plating solutions and cyanide-free chemical gold plating solutions with thiol RSH series ligands.

2. Displacement-Type Chemical Gold Plating

Generally, the bonding strength between the reduced-type thick-layer electroless gold plating and the base layer of base metals is relatively poor. Therefore, displacement-type electroless gold plating must be performed before plating a thick gold layer.

In the displacement-type gold plating solution, the base layer’s base metal dissolves (oxidizes) in the electrolyte solution, releasing electrons. In contrast, the gold ions in the solution accept electrons and deposit (reduce) on the non-metal surface. The formation of gold ligands reduces the concentration of gold ions in the plating solution, shifting the reduction potential in the negative direction, as shown in Table 1-43.

Table 1-43 Ligands and Reduction Potentials of Gold Ion Complexes

Ligands	Ligand	E/V
H₂O	Au(H₂O)₂⁺ Au(H₂O)₄³⁺	1.68 1.50
Cl^-	AuCl₂ ^- AuCl₄ ^-	1.15 0.92
SCN^-	Au(SCN)₂ ^- Au(SCN)₄ ^-	0.67 0.64
I^-	AuI₂^- AuI₄^-	0.58 0.57
Νιου Χάμσαϊρ₃	Au(NH)₄³⁺	0.56
Ω^-	Au(OH)₄ ^-	0.48
Θειουρία	Au(Thu)₂⁺	0.38
S₂O₃^2-	Au(S₂O₃)₂^3-	0.15
SO₃^2-	Au(SO₃)₂^3-	0.06
R-SH	Au(R-S)^2-	-0. 3 〜 - 0. 5
ΣΟ^-	Au(CN)₂ ^-	-0.65

Table 1-43 shows that besides cyanides, other ligands such as thiols, sulfites, and thiosulfates can also form stable complexes with gold ions and exhibit negative potentials.

In the sulfite system displacement-type gold plating solution, besides sulfite ions, ligands such as polyamine polycarboxylic acids and their salts, water-soluble amines, amine salts, ethylenediamine triacetate salts, stabilizers like tetraalkylammonium salts, ethylenediamine tetra (methylene phosphonate), sugars, and thiol compounds can also be used. The concentrations of ammonium ions, chloride ions, sulfate ions, bromide ions, or iodide ions must be maintained within a specific range and not be too high; otherwise, ammonium (complex)–gold ligands may form in the plating solution, whose redox potential is more positive than sodium sulfite. When the plating solution is left standing or during gold plating, sodium sulfite may oxidize and reduce gold, causing instability in the plating solution.

Thiol succinate, acetylcysteine, cysteine, and other thiol series compounds and gold ions can form stable ligands in cyanide-free gold-plating solutions. Thiol succinate [HOOCCH(SH)－CH₂COOH] and the reduction potential of the gold ion ligand, that is, Au(HOOCCHSCH₂COOH)₂^– + e^– ⇌Au(s) + 2 ( HOOCCH – SCH₂COOH)^–, the correct value of the standard electrode potential for the reaction is difficult to obtain. The measured reduction potential of the plating solution is about -0.3~0.5V. The thiol succinate gold ligand exists in the form of [Au(HOOC – CHSCH₂COOH)₂]^–, with gold in the +1 oxidation state.

The reaction of mercapto succinic acid added to chloroplatinic acid (HAuCl₄ ):

HAuCl₄ + 3H₂O → Au(OH)₃ + 4HCl (1-13)

The reaction of Au(OH)₃ with mercapto succinic acid:

Au(OH)₃ + 4[HOOCCH(SH)CH₂ COOH] → [Au(HOOCCH—S—CH₂COOH)₄ ]^– + 3H₂O+H⁺(1-14)

Decomposition of [Au(HOOCCH-S-CH₂COOH)₄]^– :

[Au( HOOCCH—S—CH₂COOH)₄]^– → [Au(HOOCCH—S—CH₂COOH)₂]^– + HOOCH₂CHOOCCH—S—S—CHCOOHCH₂COOH (1-15)

The typical composition and operating conditions of cyanide-free displacement gold plating solution are shown in Table 1-44.

Table 1-44 Cyanide-Free Displacement Gold Plating Solution

Composition and operating conditions	Sulfite system		Mercaptosuccinic acid system
Sodium gold chlorate/(mol/L)			0.05
Sodium thiosulfate/(mol/L)	0.028	0.015
Gold mercaptosuccinate/(mol/L)				0.01
Sodium sulfite/(mol/L)	0.52	0.1
Mercaptosuccinic acid/(mol/L)			0.25	0.25
Trisodium citrate/(mol/L)	0.22
Acetylcysteine/(mol/L)				0.03
Tetramethylammonium chloride/(mol/L)	0.8
EDTA/( mol/L)				0.015
EDTA - 2Na/(mol/L)		0.02
Amino tris(methylenephosphonic acid)/(mol/L)		0.1
Sodium carboxymethyl cellulose/(g/L)	10
pH	7.0	7.0	1.5 (Adjusted with hydrochloric )	7.0
Θερμοκρασία /℃	60	85	80 ~ 90

The base plating layer of the displacement-type gold plating layer uses a chemical nickel plating layer. The deposition process and welding performance of the displacement gold plating layer are affected differently, and defects such as discoloration and porosity may occur in the gold plating layer.

3. Reduced-Type Chemical Thick Gold Plating Layer

In gold plating solutions with thiosulfate as the ligand, sodium sulfite prevents the decomposition of thiosulfate ions. In NaAuCl₄ plating solutions containing trivalent gold salts, monovalent gold is reduced by excess thiosulfate. In weakly alkaline gold plating solutions, pH buffers such as ammonium chloride, sodium tetraborate, and boric acid are commonly added.

Thiol compounds can form ligands with gold ions that have excellent stability and act as reducing agents. These thiol compounds include L-cysteine and 2-ethanamine thiol, among others. Table 1-45 lists the composition and operating conditions of cyanide-free reduced-type chemical gold plating solutions using sodium thiosulfate and thiol compounds as ligands.

Table 1-45 Cyanide-Free Reduction-Type Gold Plating Solution

Composition and operating conditions	Thiosulfate system			Thiol system
Gold chlorate/(g/L)					0.01
Sodium gold chlorate/(g/L)	0.0125		0.0125
Sodium gold sulfite/(g/L)		0.02
Gold mercaptosuccinate/(g/L)				0.01
Mercaptosuccinic acid/(g/L)				0.27
Sodium thiosulfate/(g/L)	0.1	0.17	0.1
Sodium sulfite/(g/L)	0.1		0.4
Ammonium sulfite/(g/L)		0.43
EDTA ・ 2Na/(g/L)		0.19
Triethanolamine/(g/L)		0.034
Ammonium chloride/(g/L)	0.05
Sodium tetraborate/(g/L)			0.13
Potassium dihydrogen phosphate/(g/L)				0.15
Thiourea/(g/L)			0.0033
Hydroquinone/(g/L)			0.002
Ascorbic acid/(g/L)	0.25
Hydrazine/(g/L)		0.3
L-cysteine/(g/L)				0.08
2-aminoethyl mercaptan/(g/L)					0.2
Potassium benzotriazole/(g/L)		0.05
pH	7.5	7.0	8.0	7.0	7.5
Θερμοκρασία/℃	60	80	70	80	80

4. Base Layer Catalytic Chemical Gold Plating

Base layer catalytic chemical gold plating is a reduction-type chemical gold plating method using a reducing agent. The reducing agent only has catalytic activity on the surface of the substrate, the base metal (nickel) layer, and has no catalytic activity on the deposited gold surface. The catalytic chemical gold plating layer is smoother, denser, and has fewer or smaller pores than the displacement-type gold plating layer.

In thiosulfate and sulfite chemical gold plating solutions, gold can be deposited on the nickel surface without adding other reducing agents. On the nickel surface, sulfite only reduces the thiosulfate gold complex and does not act on the gold surface, so it is called a base layer catalytic type of chemical gold plating. A displacement gold plating reaction also occurs simultaneously on the surface of the base nickel plating layer. Therefore, the reducing agent sulfite undergoes different oxidation reactions influenced by the composition of the base nickel plating layer and pretreatment conditions. The concentration of the thiosulfate sodium ligand or the pH of the plating solution affects the different ratios of displacement and reduction reactions, the maximum thickness of the gold plating layer, appearance, porosity, and adhesion, so it is necessary to select an appropriate plating solution composition and gold plating conditions.

5. Stability of Cyanide-Free Chemical Gold Plating Solution

The stability of cyanide-free chemical gold plating solution depends on the ligands and the atomic valence of gold ions in the plating solution, the types and concentrations of impurities, and especially the influence of ammonium ions. As shown in Figure 1-43, the relationship between the gold plating time and the thickness of the plated layer after impregnating pure gold plate at 80℃, 6h with ammonia 0~1.0 mol/L added to the replacement chemical gold plating solution of mercaptosuccinic acid gold salt 0.01 mol/L and mercaptosuccinic acid 0.27 mol/L is shown in Figure 1-43.

Increasing the concentration of ammonia water accelerates the deposition rate of the gold plating layer. In plating solutions with high ammonia concentration, mercapto succinate acts as a reducing agent, and a reduction reaction occurs simultaneously with the displacement reaction.

The contamination of copper or iron ions is the main cause of instability in cyanide-free chemical gold plating solutions. Copper dissolved from the nickel plating layer’s pores and iron from the substrate easily promote the instability of the chemical gold plating solution.

Reasons: ① Gold ions accept electrons released when these metals oxidize, increasing the reduction rate. ② Copper ions or iron ions catalyze the oxidation reaction of sulfite or thiol, accelerating the reaction rate, which causes defects on the surface of the chemical gold plating layer and reduces welding performance and bonding strength. In chemical gold plating solutions, the dissolution and contamination of these metals should be suppressed by adding ligands that form stable complexes with these dissolved and contaminated metal ions.

6. Cyanide-Free Chemical Gold Plating Solution

(1) Tetrachloroaurate (III) salts and weakly reducing amine borane gold plating solutions

Tetrachloroaurate (III) salts (NaAuCl₄) are easily reduced to deposit gold. Ether-substituted tertiary amine borane reducing agents can be used with NaAuCl₄ to form an autocatalytic plating solution, or reducing agents such as trimethylamine borane, methylmorpholine borane, and diisopropylamine borane, along with stabilizers like thiols and iodide compounds.

(2) Gold plating solution with gold sulfite salt:

Currently a large number of monovalent gold sulfite plating solutions are used, with hypophosphites, formaldehyde, hydrazine, tetrahydroborate and DMAB as reducing agents. Gold sulfite salt [Na₃Au(SO₃)₂] is unstable in water and requires the addition of stabilizers such as 1,2-diaminoethane and potassium bromide.

(3) Thiosulfate gold plating solution

① Gold plating solution with thiourea and its derivatives as reducing agents:

The combination of monovalent gold thiosulfate and thiourea plating solution has good stability, does not produce hydrogen gas around neutral pH, and has no porosity. The composition of the plating solution is shown in Table 1-46 (Plating Solution A).

Table 1-46 Thiosulfate Gold Plating Solution

Composition and Operating Conditions	Plating Solution A	Plating Solution B
NaAuCl4 / (mol/L)	0.1	0.0125
Na2S2O3/(mol/L)	0.08	0.1
Na2SO3 /(mol/L)	0.4	0.1
Na2B4O7/( mol/L)	0.1	-
NH4Cl/(mol/L)	-	0.05
Thiourea/(mol/L)	0.1	-
Sodium L-ascorbate/(mol/L)	-	0.25
pH	9.0	6.0
Θερμοκρασία/℃	80	60
Plating rate/(μm/h)	1. 9 〜 2. 3	1. 5 〜 2. 0

Monovalent thiosulfate ion Au(SO₃)₂ ^3- is generated by the reaction of NaAuCl₄ salt with excess sodium thiosulfate solution. Sodium sulfite can prevent the decomposition of thiosulfate ions, and the addition of thiourea or thiourea derivatives such as methyl thiourea and ethyl thiourea also has a significant effect.

② Gold plating solution with ascorbic acid as a reducing agent:

In the thiosulfate plating solution with L-ascorbic acid sodium as the reducing agent, sodium sulfite is present, which can stably plate gold. The composition of the plating solution is shown in Table 1-46 (Plating Solution B).

Effective reducing agents in sodium thiosulfate plating solutions, besides thiourea and sodium ascorbate, include sodium tartrate, glycolic acid, and hypophosphorous acid.

③ Reaction mechanism of thiosulfate plating solution:

Gold salt reacts with thiosulfate to produce Au(S₂O₃)₂^3-. When there is no thiosulfate in the solution, only sulfite is present and formed Au(SO₃)₂^3-. The reaction equation is as follows:

Au³⁺+ 2S₂O₃ ^2- + Η₂O ⇌ Au(S₂O₃)₂^3- + SO₄ ^2- + 2 ώρες⁺ (1-16)

Au³⁺ + 3SO₃^2- + Η₂O ⇌ Au(SO₃ )₂^3- + SO₄^2- + 2 ώρες⁺ (1-17)

The cathodic polarization curves of gold deposition reactions in sulfite and thiosulfate gold plating solutions are shown in Figure 1-44. Figure 1-44 indicates: a. The three polarization curves of trivalent gold are relatively close. b. In the curves represented by “●” and “○,” only sodium thiosulfate concentration differs, while other components are the same. The results show that sodium thiosulfate concentration has little effect on the deposition of trivalent gold. c. “△” is a solution without sodium thiosulfate, indicating that the presence or absence of sodium thiosulfate has little effect on the deposition of trivalent gold. d. “▽” is is the polarization curve of 1-valent gold precipitation, compared with the previous three polarization curves, the polarization increases, indicating that 3-valent gold is easier to be reduced than 1-valent gold. e. Comparing the stability constants of monovalent gold complex ions formed in sulfite and thiosulfate solutions, as in equations (1-18) and (1-19):

Au⁺ + 2SO₃^2- ⇌ Au(SO₃)₂^3- K=10¹⁰ (1-18)

Au⁺ + 2S₂O₃^2- ⇌ Au(S₂O₃)₂^3- K = 10²⁶ (1-19)

Thiosulfate has a stronger complexing ability with monovalent gold than sulfite, so monovalent gold is more difficult to deposit from plating solutions containing sodium thiosulfate than those containing sulfite.

In the sulfite gold plating solution with the reducing agent ascorbate, due to the difference in cathodic polarization curves between gold sulfite complex ions and gold thiosulfate complex ions, the gold deposition rate is only 1/10 of that in the sodium thiosulfate plating solution. Figure 1-45 shows ascorbate sulfite and thiosulfate gold’s cathodic and anodic polarization curves.

The reaction principle of thiosulfate electroless gold plating solution refers to the oxidation state of the reducing agent, and the local reactions are as follows.

Cathodic reaction:

Au(S₂O₃)₂^3- + 2e^– → Au + 2S₂O₃^2- (1-20)

Anodic reaction:

Thiourea plating solution

CS(NH₂)₂+ 5H₂O → CO(NH₂)₂ + Η₂SO₄+ 8H⁺+ 8e^– (1-21)

The reaction product CO(NH₂)₂ is urea.

Ascorbic acid plating solution

C₆H₈O₆ → C₆H₆O₆ + 2 ώρες⁺ + 2e^–

The reaction product C₆H₈O₆ is dehydroascorbic acid.

7. Gold Plating Solution of Sugar Additive with Gold Sulfite Salt

After adding saccharide compounds to the gold sulfite plating solution, it can remain stable for a long time, and the gold plating layer is good. The composition of the plating solution is as follows.

Gold sulfite salts: potassium gold sulfite, sodium gold sulfite, ammonium gold sulfite, etc.

Sulfites: potassium sulfite, sodium sulfite, ammonium sulfite, etc.

pH adjuster: Adjust to pH 6~9 with various buffers.

Stabilizers: water-soluble amine compounds, ethylenediamine, diethylenetriamine, triethylenetetramine, etc.; Water-soluble amino acids or salts, ethylenediamine tetraacetic acid, triethylenetetramine hexaacetic acid, trans-1,2-cyclohexane diamine tetraacetic acid or salts, etc.; water-soluble organophosphates or salts, amino tris (methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra(methylene phosphonic acid), diethanolamine, pentakis(methylene phosphonic acid) or salts, etc.; water-soluble aromatic nitro compounds can also be added, such as mono-, di-, and tri-nitrobenzol acid, mono- and di-nitrosalicylic acid, nitrobenzene dicarboxylic acid, mono-, di-, and tri-nitrophenol, dinitroaminophenol, mono-, di-, and tri-nitrobenzene, etc.

Sugars other than starch can also include hexoses, glucose, mannose, galactose and other monosaccharides, erythritol, pentitol, hexanol and other sugar alcohols, glucaric acid and other aldaric acids, gluconic acid, hexatonic acid and other aldonic acids, oligosaccharides, etc. These sugar compounds can increase the stability of the plating solution and expand the current density range of the bright plating area.

The plating results are shown in Table 1-47.

Table 1-47 Various Sulfite Gold Plating Baths, Operating Conditions, and Effects of Additives

Serial Number	Various Sulfite Gold Plating Baths and Operating Conditions	Additive Effect
Νο. 1	Gold sodium sulfite 10g/L Sodium sulfite 65g/L Trisodium citrate 65g/L Ethylenediaminetetramethylenephosphonic acid (EDTMP) 85g/L pH 7 Temperature 60℃ Total current 0.2A Specimen substrate:42Fe-Ni alloy Ideal current density method with strong stirring	Electrolysis 1080 ℃, electroplating solution decomposition, producing black particles. Appearance of plating layer with different current densities produces spots and scorching.
No.1 Add sodium carboxymethyl cellulose (CMC) 10g/L to the plating solution. Other conditions are the same as No.1		Electrodeposition 1740℃, plating solution decomposition, black particles. The appearance of plating layer is improved, and spots and scorching are obviously reduced.
Αρ. 2	Gold sodium sulfite 10g/L Sodium sulfite 130g/L Trisodium citrate 65g/L Triammonium citrate 65g/L p-Nitro(benzene)phenol 1g/L pH 7 Temperature 40℃ Total current 0.2A Specimen substrate: copper Strong stirring	High current density areas burnt, plating layer appearance is very poor, appear diffuse spots
No.2 plating solution with starch 5g/L, other conditions are the same as No.2.		No burning in high current density area, good appearance of plating layer in wide current density range, stable plating solution.
Αρ. 3	Sodium gold sulfite 10g/L Sodium sulfite 100g/L Disodium borate 50g/L Boric acid 100g/L p-Nitro(benzene)phenol 1g/L pH 7 Temperature 40℃ Total current 0.2A Specimen substrate: copper Strong stirring	Poor appearance of plating layer
Add 5g/L starch to No.3 plating solution, other conditions are the same as No.3.		Good appearance of plating layer in wide range of current density, stable plating solution.
Αρ. 4	Sodium gold sulfite 12g/L Sodium sulfite 100g/L Phosphite 3g/L Ethylenediamine hydrate 30g/L pH 7 Temperature 60°C Total current 0.2A Specimen substrate:42Fe-Ni alloy Strong stirring	High current density area burnt, plating layer appearance is extremely poor
No.4 add starch 5g/L to the plating solution, other conditions are the same as No.4.		Good appearance of plating layer and stability of plating solution in wide range of current density.

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Αυτός ο οδηγός σας βοηθά να συνδυάσετε τα κοσμήματα με το σχήμα του προσώπου, τον τύπο του σώματος και το ντύσιμο. Μάθετε ποια σκουλαρίκια, κολιέ και βραχιόλια ταιριάζουν καλύτερα με διαφορετικές εμφανίσεις. Είναι ιδανικό για καταστήματα κοσμημάτων, σχεδιαστές και όσους πωλούν online. Χρησιμοποιήστε το για να βρείτε τα κατάλληλα κομμάτια για κάθε στυλ ή περίσταση.

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Εικόνα 3-7-23 Κόκκινο κοράλλι Akoya γυαλισμένο αρχικό κλαδί

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Εξερευνήστε τον μαγευτικό κόσμο των κοραλλιογενών κοσμημάτων - μάθετε για το ιστορικό παρελθόν τους, την πολιτιστική τους αξία και τις συμβουλές φροντίδας τους. Μάθετε ποιοι τύποι, όπως τα Akoya, Sardinian και Angel Skin, είναι πιο πολύτιμοι και γιατί το χρώμα και η υφή τους έχουν σημασία. Είτε βρίσκεστε σε ένα κοσμηματοπωλείο, είτε διευθύνετε ένα στούντιο, είτε κατασκευάζετε κομμάτια κατά παραγγελία, αυτός ο οδηγός σας βοηθά να πιστοποιήσετε και να διατηρήσετε τους κοραλλιογενείς πολύτιμους λίθους σας.

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Τα συμπληρωματικά χρώματα αναφέρονται σε δύο χρώματα που απέχουν μεταξύ τους 12°~180° στον χρωματικό κύκλο των 24 χρωμάτων και συνήθως εμφανίζονται σε ζεύγη όπως κίτρινο και μπλε, κόκκινο και κυανό, κ.λπ. Επιπλέον, τα σκούρα και τα ανοιχτά χρώματα και τα ψυχρά και τα θερμά χρώματα μπορούν επίσης να παρουσιάσουν συμπληρωματικές χρωματικές σχέσεις. Παρακάτω παρατίθενται παραδείγματα χρήσης συμπληρωματικών χρωμάτων για την παρουσίαση εφέ κοσμήματος.

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Τι είναι η χημική επιχρύσωση και πώς λειτουργεί;

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Μάθετε για τη χημική επιχρύσωση κοσμημάτων. Χρησιμοποιεί ειδικά διαλύματα για να τοποθετήσει ένα λεπτό στρώμα χρυσού στα αντικείμενα. Ορισμένες μέθοδοι χρειάζονται κυάνιο, άλλες όχι. Η διαδικασία μπορεί να είναι αργή και περίπλοκη, αλλά κάνει τα κοσμήματα να φαίνονται υπέροχα. Ιδανική για κατασκευαστές και πωλητές κοσμημάτων που θέλουν να προσθέσουν μια πινελιά χρυσού.

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ΠΡΟΒΛΕΠΟΜΕΝΑ προϊόντα

κατηγορία προϊόντων

κατηγορία υλικών

Τι είναι η χημική επιχρύσωση και πώς λειτουργεί;

Τι είναι η χημική επιχρύσωση και πώς λειτουργεί;

Chemical Gold Plating: Techniques, Solutions, and Applications for Jewelry

Εισαγωγή:

Πίνακας περιεχομένων

Section I Cyanide Chemical Gold Plating

1. Overview

Table 1-24 Types of Gold Coordination Salt and Reducing Agents in Chemical Gold Plating Solution

Table 1-25 Functions of Components in Chemical Gold Plating Solution

2. Hypophosphite Gold Plating Solution

Table 1-26 Composition and Conditions of Hypophosphite Chemical Gold Plating Solution

Table 1-27 Composition and Operating Conditions of Hypophosphite Gold Plating Solution

3. Borohydride Salt Plating Solution

Table 1-28 Composition and Operating Conditions of Potassium Borohydride Chemical Gold Plating Solution

Table 1-29 Properties of Chemical Gold Plating Solution with Potassium Borohydride Reducing Agent

Table 1-30 Functions of Components in Potassium Borohydride Chemical Gold Plating Solution

Table 1-31 Composition and Operating Conditions of Potassium Borohydride Gold Plating Solution

Table 1-32 Composition of BMAB Gold Plating Solution

Table 1-33 Composition of Potassium Gold(III) Cyanide Chemical Gold Plating Solution

4. Reaction Mechanism of Borohydride Salt Chemical Gold Plating Solution

Table 1-34 Potassium Borohydride and DMAB Chemical Plating Solutions

Table 1-35 Potassium Borohydride Plating Solution with Added Pb2+

Table 1-36 Potassium Borohydride Plating Solution with Added T1+

Table 1-37 Potassium Gold(III) Cyanide Potassium Borohydride Plating Solution

Table 1-38 DMAB Plating Solution (Gold Hydroxide Dissolved in Potassium Hydroxide Solution)

Table 1-39 Potassium Borohydride Plating Solution with Added EDTA and Ethanolamine

Table 1-40 Plating Solution with Two Reducing Agents (Hydrazine and DMAB)

Table 1-41 Other Reducing Agent Plating Solutions

5. Chemical Gold Alloy Plating

Table 1-42 Gold-Tin Alloy Plating Solution

Section II Cyanide-Free Chemical Gold Plating

1. Overview

2. Displacement-Type Chemical Gold Plating

Table 1-43 Ligands and Reduction Potentials of Gold Ion Complexes

Table 1-44 Cyanide-Free Displacement Gold Plating Solution

3. Reduced-Type Chemical Thick Gold Plating Layer

Table 1-45 Cyanide-Free Reduction-Type Gold Plating Solution

4. Base Layer Catalytic Chemical Gold Plating

5. Stability of Cyanide-Free Chemical Gold Plating Solution

6. Cyanide-Free Chemical Gold Plating Solution

Table 1-46 Thiosulfate Gold Plating Solution

7. Gold Plating Solution of Sugar Additive with Gold Sulfite Salt

Table 1-47 Various Sulfite Gold Plating Baths, Operating Conditions, and Effects of Additives

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Table 1-35 Potassium Borohydride Plating Solution with Added Pb²⁺

Table 1-36 Potassium Borohydride Plating Solution with Added T1⁺