Electroless plating
Electroless plating plays an important role in surface treatment technology. Electroless plating is a chemical treatment method in which metal ions in a solution are selectively reduced to a metal plating layer on a catalyst-activated surface using a suitable reducing agent. The following formula can be used:
M2++2e (provided by reducing agent)--->M
In electroless plating, the metal ions in the solution are reduced to the corresponding metals by obtaining the desired electrons. For example, using hypophosphite as a reducing agent in an acidic electroless nickel plating solution, its redox reaction process is as follows:
Ni2++2e--->Ni (reduction)
(H2PO2)-+H2O--->(H2PO3)-+2e+2H+(oxidation)
Addition of the two formulas results in a total reduction oxidation reaction:
Ni2++(H2PO2)-+H2O--->(H2PO3)-+Ni+2H+
The degree of effectiveness of the reducing agent can be inferred from its standard oxidation potential. From the above, it can be seen that hypophosphite is a strong reducing agent that produces a positive standard oxidation-reduction potential. However, the value of E° should not be overestimated, because in practical applications, due to the influence of different ions in solution, activity, overpotential, and the like, the E° value will be greatly different. However, the calculation of oxidation and reduction potential still helps to estimate the effectiveness of different reducing agents in advance. If all standard oxidation-reduction potentials are too small or negative, metal reduction will hardly occur.
The composition of the electroless plating solution and its corresponding working conditions must be that the reaction is limited to the surface of the catalyzed part, and the solution itself should not spontaneously undergo reduction and oxidation to prevent the solution from decomposing naturally and causing the solution to fail quickly. If the metal to be plated (such as nickel, palladium) itself is a catalyst for the reaction, the process of electroless plating has an autocatalytic effect, so that the above reaction proceeds continuously. At this time, the thickness of the plating layer also gradually increases to obtain a certain thickness. In addition to nickel, cobalt, rhodium, palladium, etc. have an autocatalytic effect.
For parts that do not have an autocatalytic surface, such as plastics, glass, ceramics, and other non-metals, usually require special pretreatment to activate the surface and have a catalytic effect in order to perform electroless plating.
Electroless plating and electroplating have the following advantages:
1 does not require external DC power supply equipment.
2 dense plating, less porosity.
3 There is no influence of nonuniform distribution of power lines, and uniform thickness of the coating can be obtained for the plating of complex geometry;
4 It can be plated on various different substrates such as metal, non-metal, and semiconductor.
Electroless plating is inferior to electroplating in that the solution used has poor stability, and the maintenance, adjustment, and regeneration of the solution are bothersome and the material cost is high.
Electroless plating process has an important position in the electronics industry. Due to the different types of reducing agents used, there are significant differences in the properties of the plating obtained by electroless plating. Therefore, when selecting the formulation of the plating solution, the economics of the plating solution and the properties of the resulting coating must be carefully considered.
Currently, electroless nickel, copper, silver, gold, cobalt, palladium, platinum, tin, and electroless alloys and chemical composite coatings have been used in industrial production.
How to make electroless nickel
Electroless nickel plating is the most widely used method for electroless plating. The reducing agents used include hypophosphite, hydrazine, sodium borohydride and dimethylamine borane.
At present, sodium hypophosphite is used as a reducing agent in most domestic production, and sodium borohydride and dimethylamine borane are used at a relatively low price due to their relatively high price.
1. Use of plating
The electroless nickel plating layer has fine crystals, low porosity, high hardness, uniform plating, good solderability, good plating capability, and high chemical stability. It has been widely used in electronics, aviation, aerospace, machinery, and precision instruments. , daily hardware, electrical appliances and chemical industry.
The use of electroless nickel plating on non-metallic materials is increasing. In particular, after electroless nickel plating, plastic products can be plated with the desired metal plating according to conventional plating methods to obtain the same appearance as metal. Plastic electroplating products have been widely used in electronic components, household appliances, and daily-use industrial products.
Electroless nickel plating has been used in the atomic energy industry, such as the production of parts and containers in nuclear fuel systems and rocket, missile, and jet engine components.
The components of the compressor in chemical equipment are corrosion and abrasion resistant, and the use of electroless nickel plating is very advantageous.
The electroless nickel plating layer can also improve the welding performance of aluminum, copper and stainless steel materials, reduce wear of rotating parts, and reduce stress corrosion of stainless steel and titanium alloys.
The depth of the hole, the blind hole, and the inner surface of the cavity, which are required for the precise size of the plating layer for precision parts and geometrically complex parts, can be coated with the same thickness as the outer surface by electroless nickel plating.
For parts requiring high hardness and wear resistance, electroless nickel plating can be used instead of hard chromium plating.
2. Plating composition and characteristics
<1> Composition of plating
The electroless nickel plating solution using hypophosphite as a reducing agent is plated with 4% to 15% of phosphorus and is a nickel-phosphorus alloy. The coating obtained by the use of borohydride or amine borane as a reducing agent is a pure nickel layer, and the nickel content can be more than 99.5%. The newly deposited electroless nickel plating layer is amorphous and has an amorphous lamellar structure.
Phosphorus content in the plating is mainly determined by the pH of the solution. As the pH value decreases, the phosphorus content increases. The phosphorus content of the deposit deposited in the conventional acidic electroless nickel plating solution is 7% to 12%, while the phosphorus content of the deposited nickel layer in the alkaline solution is 4% to 7%. In addition, the composition of the solution, the content of each component and their relative ratios, and the working temperature of the solution all have a certain influence on the phosphorus content.
<2> Characteristics of plating
1 hardness
Electroless nickel plating is much harder than electroplated nickel and is more wear resistant. The hardness of the electroplated nickel layer is only HV160-180, and the hardness of the electroless nickel plating layer is generally HV300-500.
The heat treatment method can greatly improve the hardness of the electroless nickel plating layer. After heating at 400° C. for 1 hour, the highest hardness value can reach about HV1000. If the temperature of the heat treatment is further increased, if the temperature is increased to 600°C, the hardness is reduced to HV700 instead.
The electroless nickel plating layer before the heat treatment is an amorphous amorphous structure, and after heat treatment, it transforms into a crystalline structure, and a Ni3P phase is formed in the plating layer. The precipitation amount of Ni3P phase increases with the increase of heat treatment temperature, and the maximum precipitation amount depends on the phosphorus content of the coating.
In order to increase the hardness of the plating layer, a suitable heat treatment rule is: the temperature is 380 to 400°C and the time is 1 hour. To prevent discoloration of the coating, it is preferable to have a protective atmosphere or vacuum heat treatment. In the absence of protective atmosphere conditions, the appropriate reduction in heat treatment temperature (eg, 280° C.) and prolonged treatment time can also increase the hardness value.
When the coating has the maximum hardness, the brittleness also increases and it is not suitable for use under high load or impact conditions. Selecting appropriate heat treatment conditions allows the coating to have both hardness and ductility.
Sodium Acetate 5 15
Sodium citrate 5 15
Succinic acid 5 16
Lactic acid 80% (ml/l) 25 25
Glycine 5-15
Malic acid 24
Boric acid 10
Sodium fluoride 1
(Pb2+) (added as lead acetate) 0.001 0.003
pH 4-5 3.5-5.4 4.4-4.8 4.4-4.8 5.8-6
Temperature (°C) 80-90 85-95 90-94 90-92 90-93
Deposition rate (μm/h) 10 12-15 10-13 15-22 48
Loading capacity (dm2/L) 1 1 1 1 1
Phosphorus content in plating (%) 8-10 7-11 8-9 8-9 8-11
Formulation formula 1 is prepared as follows:
Sodium citrate and acetic acid are dissolved in a container with hot distilled water at 60 to 70°C, and nickel sulfate is dissolved in hot distilled water in another container. After dissolution, the solution is poured into the solution with constant stirring, and the resulting mixture is filtered into the bath. For electroless plating, pre-dissolved and filtered sodium hypophosphite solution is added to the tank. After mixing, distilled water is added to the required volume. Finally, the pH is adjusted to the specified range with 10% dilute sulfuric acid or sodium hydroxide solution. Upper limit.
Formulas 2, 3, 4 and 5 can be prepared by referring to the above method.
However, the lactic acid solutions in Formulations 3 and 4 were pre-neutralized with sodium bicarbonate solution to a pH of about 4.6 before they could be mixed with the other components.
The composition and process conditions of the alkaline electroless nickel plating solution are shown in Table 4-26 below.
Table 4-26 Composition and Process Conditions of Alkaline Electroless Nickel Plating Solution
Bath composition (g/l) and process conditions 1 2 3 4 5
Nickel sulfate 10-20 33 30 25 30
Hypophosphite 5-15 15 25 25 30
Sodium Citrate 30-60 50
Sodium pyrophosphate 60-70 50 60
Lactic acid 80% (ml/l) 1-5
Triethanolamine 100
pH 7.5-8.5 8 10-10.5 10-11 10
Temperature (°C) 40-45 90 70-75 65-75 30-35
Deposition rate (μm/h) 20-30 15 10
Phosphorus content in plating (%) 7-8 about 5 about 4
Formulations 1 and 5 are suitable for the metallization of plastic products, usually about 10 minutes.
The electroless nickel plating layer of a general steel workpiece is treated at a temperature of 200° C. for 2 hours to increase the plating adhesion and relieve stress. The aluminum workpiece is preferably held at 150 to 180°C for 1 hour.
2 magnetic properties
The magnetic properties of the electroless nickel plating layer are determined by the phosphorus content and the heat treatment temperature. Phosphorus content over 8% of the coating is weakly magnetic; phosphorus content is above 11.4%, completely non-magnetic; phosphorus content of less than 8% of the coating is only magnetic, but its magnetic properties than the nickel plating layer is small, After heat treatment, the magnetic properties have been significantly improved.
For example, in the alkaline electroless nickel plating solution, the magnetic properties when the heat treatment without a heat treatment is a coercive force H0 = 160 A/m, and H0 = 8800 A/m after heat treatment at 350C for 1 hour.
3 Resistivity
The resistivity of the electroless nickel plating layer is related to the phosphorus content. The higher the general phosphorus content, the greater the resistivity. The electroless nickel plating layer obtained in the alkaline solution has a resistivity of about 28 to 34 μΩ·cm. The electroless nickel plated layer obtained in the acidic solution has a resistivity of about 51 to 58 μΩ·cm, which is higher than the electroplating nickel. The layer height is several times (the resistivity of pure nickel is 9.5 μΩ·cm). The resistivity of the electroless nickel coating decreases significantly after heat treatment. For example, the electroless nickel plating layer containing 7% of phosphorus has a resistivity of 72 μΩ·cm reduced to 20 μΩ·cm after heat treatment at 600°C. The electroless nickel-boron plating layer containing boron in an amount of 1.3% to 4.7% has a resistivity of 13 to 15 μΩ·cm. When the nickel plating is reduced with dimethylamine borane, the boron content is 0.6%. The resistivity is 5.3 μΩ·cm, which is lower than that of pure nickel.
4 coefficient of thermal expansion and density
The thermal expansion coefficient of the electroless nickel plating layer is generally 13×10-6°C-1.
The density of the electroless nickel plating layer is generally about 7.9 g/cm3, and the density of the electroless nickel plating layer decreases as the phosphorus content increases.
The comprehensive performance of electroless nickel plating is shown in Table 4-24:
Table 4-24 Comprehensive Properties of Electroless Nickel Plating Comprehensive Properties of Electroless Nickel Plating Nickel Phosphorus Alloy Layer (8% Phosphorus Content-10%)
Hardness (HV) before heat treatment 500
400 °C after heat treatment 1000
Density (g/cm3) 7.9
Melting point (°C)890
Resistivity (μΩ·cm) 60 to 75
Thermal expansion coefficient (°C-1) 13×10-6
Thermal conductivity [W/(m·k)] 5.02
Elongation (%) 3 to 6
Reflection coefficient (%) 50 (approximate)
3. Process Conditions and Plating Solution Electroless nickel plating with sodium hypophosphite as the reducing agent is the most widely used technology at home and abroad. It is divided into two major categories: acidic plating solution and alkaline plating solution. The composition and process conditions of the acidic electroless nickel plating solution are shown in Table 4-25:
Table 4-25 Composition and Process Conditions of Acidic Electroless Nickel Plating Solution
Bath composition (g/l) and process conditions 1 2 3 4 5
Nickel sulfate 25-30 30 20 25 25
Sodium Hypophosphite 20-25 15-25 24 20 24
Formula 3, with addition of triethanolamine, can adjust the pH value in addition to the complexation, so that the plating solution can still have a high deposition rate at low temperatures. When nickel salt is added, it must first be complexed with triethanolamine and then added to the plating tank, otherwise precipitation will occur. When preparing, the ratio of nickel sulfate to sodium hypophosphite or sodium pyrophosphate should be roughly controlled at 1:2, which ensures that nickel is in a complex state.
Formulation 2 is suitable for electroless nickel plating on aluminum and aluminum alloys.
Formulation 4 can work over a wide range of concentrations. The pH value is preferably greater than 10, otherwise the nickel pyrophosphate complex will decompose. When adding nickel sulfate, it should also be dissolved in ammonia water before adding the plating tank.
4. The composition and process conditions of electroless nickel plating solution
<1>Effect of nickel salt concentration on deposition rate
1 The increase of nickel ion concentration in the acidic electroless nickel plating solution can increase the deposition rate of nickel. Especially when the concentration of nickel salt is below 10g/L, the concentration of nickel salt increases, and the deposition rate of nickel increases. For example, when the plating solution contains 20 g/L of sodium hypophosphite, 20 g/L of sodium acetate, a temperature of 82-84° C., and pH of 5.5, the nickel salt concentration changes from 5 g/L to 60 g/L, and deposition occurs. The effect of speed is shown in Table 4-27:
Table 4-27 Effect of nickel salt on deposition rate
Nickel sulfate (g/l) 5 10 20 30 40 50 60
Stacking speed (μm/h) 12 19 24 21 20 20 20
When the concentration of nickel salt reaches 30 g/L, the concentration will continue to increase, and the deposition rate of the coating will no longer increase or even decrease. When the nickel salt concentration is too high, the stability of the plating solution may decrease, and rough plating may easily occur.
2 In the alkaline electroless nickel plating bath, when the concentration of nickel salt is below 20g/L, the nickel salt concentration is increased so that the chemical deposition rate is significantly increased; but when the nickel salt concentration is higher than 25g/L, By increasing the nickel salt content, the deposition rate tends to be stable.
Fig. 4-15 Effect of Sodium Phosphate Concentration on Deposition Rate <2> Effect of Sodium Hypophosphite on Deposition Rate
Increasing the sodium hypophosphite concentration increases the deposition rate, as shown in Figure 4-15. However, the increase of sodium hypophosphite concentration does not increase the deposition rate of nickel indefinitely. The concentration of sodium hypophosphite in different baths
Electroless plating plays an important role in surface treatment technology. Electroless plating is a chemical treatment method in which metal ions in a solution are selectively reduced to a metal plating layer on a catalyst-activated surface using a suitable reducing agent. The following formula can be used:
M2++2e (provided by reducing agent)--->M
In electroless plating, the metal ions in the solution are reduced to the corresponding metals by obtaining the desired electrons. For example, using hypophosphite as a reducing agent in an acidic electroless nickel plating solution, its redox reaction process is as follows:
Ni2++2e--->Ni (reduction)
(H2PO2)-+H2O--->(H2PO3)-+2e+2H+(oxidation)
Addition of the two formulas results in a total reduction oxidation reaction:
Ni2++(H2PO2)-+H2O--->(H2PO3)-+Ni+2H+
The degree of effectiveness of the reducing agent can be inferred from its standard oxidation potential. From the above, it can be seen that hypophosphite is a strong reducing agent that produces a positive standard oxidation-reduction potential. However, the value of E° should not be overestimated, because in practical applications, due to the influence of different ions in solution, activity, overpotential, and the like, the E° value will be greatly different. However, the calculation of oxidation and reduction potential still helps to estimate the effectiveness of different reducing agents in advance. If all standard oxidation-reduction potentials are too small or negative, metal reduction will hardly occur.
The composition of the electroless plating solution and its corresponding working conditions must be that the reaction is limited to the surface of the catalyzed part, and the solution itself should not spontaneously undergo reduction and oxidation to prevent the solution from decomposing naturally and causing the solution to fail quickly. If the metal to be plated (such as nickel, palladium) itself is a catalyst for the reaction, the process of electroless plating has an autocatalytic effect, so that the above reaction proceeds continuously. At this time, the thickness of the plating layer also gradually increases to obtain a certain thickness. In addition to nickel, cobalt, rhodium, palladium, etc. have an autocatalytic effect.
For parts that do not have an autocatalytic surface, such as plastics, glass, ceramics, and other non-metals, usually require special pretreatment to activate the surface and have a catalytic effect in order to perform electroless plating.
Electroless plating and electroplating have the following advantages:
1 does not require external DC power supply equipment.
2 dense plating, less porosity.
3 There is no influence of nonuniform distribution of power lines, and uniform thickness of the coating can be obtained for the plating of complex geometry;
4 It can be plated on various different substrates such as metal, non-metal, and semiconductor.
Electroless plating is inferior to electroplating in that the solution used has poor stability, and the maintenance, adjustment, and regeneration of the solution are bothersome and the material cost is high.
Electroless plating process has an important position in the electronics industry. Due to the different types of reducing agents used, there are significant differences in the properties of the plating obtained by electroless plating. Therefore, when selecting the formulation of the plating solution, the economics of the plating solution and the properties of the resulting coating must be carefully considered.
Currently, electroless nickel, copper, silver, gold, cobalt, palladium, platinum, tin, and electroless alloys and chemical composite coatings have been used in industrial production.
How to make electroless nickel
Electroless nickel plating is the most widely used method for electroless plating. The reducing agents used include hypophosphite, hydrazine, sodium borohydride and dimethylamine borane.
At present, sodium hypophosphite is used as a reducing agent in most domestic production, and sodium borohydride and dimethylamine borane are used at a relatively low price due to their relatively high price.
1. Use of plating
The electroless nickel plating layer has fine crystals, low porosity, high hardness, uniform plating, good solderability, good plating capability, and high chemical stability. It has been widely used in electronics, aviation, aerospace, machinery, and precision instruments. , daily hardware, electrical appliances and chemical industry.
The use of electroless nickel plating on non-metallic materials is increasing. In particular, after electroless nickel plating, plastic products can be plated with the desired metal plating according to conventional plating methods to obtain the same appearance as metal. Plastic electroplating products have been widely used in electronic components, household appliances, and daily-use industrial products.
Electroless nickel plating has been used in the atomic energy industry, such as the production of parts and containers in nuclear fuel systems and rocket, missile, and jet engine components.
The components of the compressor in chemical equipment are corrosion and abrasion resistant, and the use of electroless nickel plating is very advantageous.
The electroless nickel plating layer can also improve the welding performance of aluminum, copper and stainless steel materials, reduce wear of rotating parts, and reduce stress corrosion of stainless steel and titanium alloys.
The depth of the hole, the blind hole, and the inner surface of the cavity, which are required for the precise size of the plating layer for precision parts and geometrically complex parts, can be coated with the same thickness as the outer surface by electroless nickel plating.
For parts requiring high hardness and wear resistance, electroless nickel plating can be used instead of hard chromium plating.
2. Plating composition and characteristics
<1> Composition of plating
The electroless nickel plating solution using hypophosphite as a reducing agent is plated with 4% to 15% of phosphorus and is a nickel-phosphorus alloy. The coating obtained by the use of borohydride or amine borane as a reducing agent is a pure nickel layer, and the nickel content can be more than 99.5%. The newly deposited electroless nickel plating layer is amorphous and has an amorphous lamellar structure.
Phosphorus content in the plating is mainly determined by the pH of the solution. As the pH value decreases, the phosphorus content increases. The phosphorus content of the deposit deposited in the conventional acidic electroless nickel plating solution is 7% to 12%, while the phosphorus content of the deposited nickel layer in the alkaline solution is 4% to 7%. In addition, the composition of the solution, the content of each component and their relative ratios, and the working temperature of the solution all have a certain influence on the phosphorus content.
<2> Characteristics of plating
1 hardness
Electroless nickel plating is much harder than electroplated nickel and is more wear resistant. The hardness of the electroplated nickel layer is only HV160-180, and the hardness of the electroless nickel plating layer is generally HV300-500.
The heat treatment method can greatly improve the hardness of the electroless nickel plating layer. After heating at 400° C. for 1 hour, the highest hardness value can reach about HV1000. If the temperature of the heat treatment is further increased, if the temperature is increased to 600°C, the hardness is reduced to HV700 instead.
The electroless nickel plating layer before the heat treatment is an amorphous amorphous structure, and after heat treatment, it transforms into a crystalline structure, and a Ni3P phase is formed in the plating layer. The precipitation amount of Ni3P phase increases with the increase of heat treatment temperature, and the maximum precipitation amount depends on the phosphorus content of the coating.
In order to increase the hardness of the plating layer, a suitable heat treatment rule is: the temperature is 380 to 400°C and the time is 1 hour. To prevent discoloration of the coating, it is preferable to have a protective atmosphere or vacuum heat treatment. In the absence of protective atmosphere conditions, the appropriate reduction in heat treatment temperature (eg, 280° C.) and prolonged treatment time can also increase the hardness value.
When the coating has the maximum hardness, the brittleness also increases and it is not suitable for use under high load or impact conditions. Selecting appropriate heat treatment conditions allows the coating to have both hardness and ductility.
Sodium Acetate 5 15
Sodium citrate 5 15
Succinic acid 5 16
Lactic acid 80% (ml/l) 25 25
Glycine 5-15
Malic acid 24
Boric acid 10
Sodium fluoride 1
(Pb2+) (added as lead acetate) 0.001 0.003
pH 4-5 3.5-5.4 4.4-4.8 4.4-4.8 5.8-6
Temperature (°C) 80-90 85-95 90-94 90-92 90-93
Deposition rate (μm/h) 10 12-15 10-13 15-22 48
Loading capacity (dm2/L) 1 1 1 1 1
Phosphorus content in plating (%) 8-10 7-11 8-9 8-9 8-11
Formulation formula 1 is prepared as follows:
Sodium citrate and acetic acid are dissolved in a container with hot distilled water at 60 to 70°C, and nickel sulfate is dissolved in hot distilled water in another container. After dissolution, the solution is poured into the solution with constant stirring, and the resulting mixture is filtered into the bath. For electroless plating, pre-dissolved and filtered sodium hypophosphite solution is added to the tank. After mixing, distilled water is added to the required volume. Finally, the pH is adjusted to the specified range with 10% dilute sulfuric acid or sodium hydroxide solution. Upper limit.
Formulas 2, 3, 4 and 5 can be prepared by referring to the above method.
However, the lactic acid solutions in Formulations 3 and 4 were pre-neutralized with sodium bicarbonate solution to a pH of about 4.6 before they could be mixed with the other components.
The composition and process conditions of the alkaline electroless nickel plating solution are shown in Table 4-26 below.
Table 4-26 Composition and Process Conditions of Alkaline Electroless Nickel Plating Solution
Bath composition (g/l) and process conditions 1 2 3 4 5
Nickel sulfate 10-20 33 30 25 30
Hypophosphite 5-15 15 25 25 30
Sodium Citrate 30-60 50
Sodium pyrophosphate 60-70 50 60
Lactic acid 80% (ml/l) 1-5
Triethanolamine 100
pH 7.5-8.5 8 10-10.5 10-11 10
Temperature (°C) 40-45 90 70-75 65-75 30-35
Deposition rate (μm/h) 20-30 15 10
Phosphorus content in plating (%) 7-8 about 5 about 4
Formulations 1 and 5 are suitable for the metallization of plastic products, usually about 10 minutes.
The electroless nickel plating layer of a general steel workpiece is treated at a temperature of 200° C. for 2 hours to increase the plating adhesion and relieve stress. The aluminum workpiece is preferably held at 150 to 180°C for 1 hour.
2 magnetic properties
The magnetic properties of the electroless nickel plating layer are determined by the phosphorus content and the heat treatment temperature. Phosphorus content over 8% of the coating is weakly magnetic; phosphorus content is above 11.4%, completely non-magnetic; phosphorus content of less than 8% of the coating is only magnetic, but its magnetic properties than the nickel plating layer is small, After heat treatment, the magnetic properties have been significantly improved.
For example, in the alkaline electroless nickel plating solution, the magnetic properties when the heat treatment without a heat treatment is a coercive force H0 = 160 A/m, and H0 = 8800 A/m after heat treatment at 350C for 1 hour.
3 Resistivity
The resistivity of the electroless nickel plating layer is related to the phosphorus content. The higher the general phosphorus content, the greater the resistivity. The electroless nickel plating layer obtained in the alkaline solution has a resistivity of about 28 to 34 μΩ·cm. The electroless nickel plated layer obtained in the acidic solution has a resistivity of about 51 to 58 μΩ·cm, which is higher than the electroplating nickel. The layer height is several times (the resistivity of pure nickel is 9.5 μΩ·cm). The resistivity of the electroless nickel coating decreases significantly after heat treatment. For example, the electroless nickel plating layer containing 7% of phosphorus has a resistivity of 72 μΩ·cm reduced to 20 μΩ·cm after heat treatment at 600°C. The electroless nickel-boron plating layer containing boron in an amount of 1.3% to 4.7% has a resistivity of 13 to 15 μΩ·cm. When the nickel plating is reduced with dimethylamine borane, the boron content is 0.6%. The resistivity is 5.3 μΩ·cm, which is lower than that of pure nickel.
4 coefficient of thermal expansion and density
The thermal expansion coefficient of the electroless nickel plating layer is generally 13×10-6°C-1.
The density of the electroless nickel plating layer is generally about 7.9 g/cm3, and the density of the electroless nickel plating layer decreases as the phosphorus content increases.
The comprehensive performance of electroless nickel plating is shown in Table 4-24:
Table 4-24 Comprehensive Properties of Electroless Nickel Plating Comprehensive Properties of Electroless Nickel Plating Nickel Phosphorus Alloy Layer (8% Phosphorus Content-10%)
Hardness (HV) before heat treatment 500
400 °C after heat treatment 1000
Density (g/cm3) 7.9
Melting point (°C)890
Resistivity (μΩ·cm) 60 to 75
Thermal expansion coefficient (°C-1) 13×10-6
Thermal conductivity [W/(m·k)] 5.02
Elongation (%) 3 to 6
Reflection coefficient (%) 50 (approximate)
3. Process Conditions and Plating Solution Electroless nickel plating with sodium hypophosphite as the reducing agent is the most widely used technology at home and abroad. It is divided into two major categories: acidic plating solution and alkaline plating solution. The composition and process conditions of the acidic electroless nickel plating solution are shown in Table 4-25:
Table 4-25 Composition and Process Conditions of Acidic Electroless Nickel Plating Solution
Bath composition (g/l) and process conditions 1 2 3 4 5
Nickel sulfate 25-30 30 20 25 25
Sodium Hypophosphite 20-25 15-25 24 20 24
Formula 3, with addition of triethanolamine, can adjust the pH value in addition to the complexation, so that the plating solution can still have a high deposition rate at low temperatures. When nickel salt is added, it must first be complexed with triethanolamine and then added to the plating tank, otherwise precipitation will occur. When preparing, the ratio of nickel sulfate to sodium hypophosphite or sodium pyrophosphate should be roughly controlled at 1:2, which ensures that nickel is in a complex state.
Formulation 2 is suitable for electroless nickel plating on aluminum and aluminum alloys.
Formulation 4 can work over a wide range of concentrations. The pH value is preferably greater than 10, otherwise the nickel pyrophosphate complex will decompose. When adding nickel sulfate, it should also be dissolved in ammonia water before adding the plating tank.
4. The composition and process conditions of electroless nickel plating solution
<1>Effect of nickel salt concentration on deposition rate
1 The increase of nickel ion concentration in the acidic electroless nickel plating solution can increase the deposition rate of nickel. Especially when the concentration of nickel salt is below 10g/L, the concentration of nickel salt increases, and the deposition rate of nickel increases. For example, when the plating solution contains 20 g/L of sodium hypophosphite, 20 g/L of sodium acetate, a temperature of 82-84° C., and pH of 5.5, the nickel salt concentration changes from 5 g/L to 60 g/L, and deposition occurs. The effect of speed is shown in Table 4-27:
Table 4-27 Effect of nickel salt on deposition rate
Nickel sulfate (g/l) 5 10 20 30 40 50 60
Stacking speed (μm/h) 12 19 24 21 20 20 20
When the concentration of nickel salt reaches 30 g/L, the concentration will continue to increase, and the deposition rate of the coating will no longer increase or even decrease. When the nickel salt concentration is too high, the stability of the plating solution may decrease, and rough plating may easily occur.
2 In the alkaline electroless nickel plating bath, when the concentration of nickel salt is below 20g/L, the nickel salt concentration is increased so that the chemical deposition rate is significantly increased; but when the nickel salt concentration is higher than 25g/L, By increasing the nickel salt content, the deposition rate tends to be stable.
Fig. 4-15 Effect of Sodium Phosphate Concentration on Deposition Rate <2> Effect of Sodium Hypophosphite on Deposition Rate
Increasing the sodium hypophosphite concentration increases the deposition rate, as shown in Figure 4-15. However, the increase of sodium hypophosphite concentration does not increase the deposition rate of nickel indefinitely. The concentration of sodium hypophosphite in different baths