This article mainly analyzes the two most commonly used processes in the PCB surface treatment process: chemical nickel gold and OSP process steps and characteristics.
1. Chemical nickel gold
1.1 Basic steps
Degreasing → water washing → neutralization → water washing → micro-etching → water washing → pre-soaking → palladium activation → blowing and stirring water washing → electroless nickel → hot water washing → electroless gold → recycling water washing → post-treatment water washing → drying
1.2 Electroless nickel
A. Generally, electroless nickel is divided into “displacement” and “self-catalyzed” types. There are many formulas, but no matter which one, the high-temperature coating quality is better.
B. Nickel Chloride (Nickel Chloride) is generally used as nickel salt
C. Commonly used reducing agents are Hypophosphite/Formaldehyde/Hydrazine/Borohydride/Amine Borane
D. Citrate is the most common chelating agent.
E. The pH of the bath solution needs to be adjusted and controlled. Traditionally, ammonia (Amonia) is used, but there are also formulas that use triethanol ammonia (Triethanol Amine). In addition to the adjustable pH and the stability of ammonia at high temperatures, it also combines with sodium citrate to form a total of nickel metal. Chelating agent, so that nickel can be deposited on the plated parts smoothly and effectively.
F. In addition to reducing pollution problems, the use of sodium hypophosphite also has a great influence on the quality of the coating.
G. This is one of the formulas for chemical nickel tanks.
Formulation characteristic analysis:
A. PH value influence: turbidity will occur when the pH is lower than 8, and decomposition will occur when the pH is higher than 10. It has no obvious effect on the phosphorus content, deposition rate and phosphorus content.
B. Temperature influence: temperature has a great influence on the precipitation rate, the reaction is slow below 70°C, and the rate is fast above 95°C and cannot be controlled. 90°C is the best.
C. In the composition concentration, the sodium citrate content is high, the chelating agent concentration increases, the deposition rate decreases, and the phosphorus content increases with the chelating agent concentration. The phosphorus content of the triethanolamine system can even be as high as 15.5%.
D. As the concentration of the reducing agent sodium dihydrogen hypophosphite increases, the deposition rate increases, but the bath solution decomposes when it exceeds 0.37M, so the concentration should not be too high, too high is harmful. There is no clear relationship between the phosphorus content and the reducing agent, so it is generally appropriate to control the concentration at about 0.1M.
E. The concentration of triethanolamine will affect the phosphorus content of the coating and the deposition rate. The higher the concentration, the lower the phosphorus content and the slower the deposition, so it is better to keep the concentration at about 0.15M. In addition to adjusting the pH, it can also be used as a metal chelator.
F. From the discussion, it is known that the sodium citrate concentration can be adjusted effectively to effectively change the phosphorus content of the coating
H. General reducing agents are divided into two categories:
The copper surface is mostly non-activated surface in order to make it generate negative electricity to achieve the goal of “open plating”. The copper surface adopts the first electroless palladium method. Therefore, there is phosphorus eutectosis in the reaction, and 4-12% phosphorus content is common. Therefore, when the amount of nickel is large, the coating loses its elasticity and magnetism, and the brittle gloss increases, which is good for rust prevention and bad for wire bonding and welding.
1.3 no electricity gold
A. Electroless gold is divided into “displacement gold” and “electroless gold”. The former is the so-called “immersion gold” (lmmersion Gold plaTIng). The plating layer is thin and the bottom surface is fully plated and stops. The latter accepts the reducing agent to supply electrons so that the plating layer can continue to thicken the electroless nickel.
B. The characteristic formula of the reduction reaction is: reduction half reaction: Au e- Au0 oxidation half reaction formula: Reda Ox e- full reaction formula: Au Red aAu0 Ox.
C. In addition to providing gold source complexes and reducing reducing agents, the electroless gold plating formula must also be used in combination with chelating agents, stabilizers, buffers and swelling agents to be effective.
D. Some research reports show that the efficiency and quality of chemical gold are improved. The selection of reducing agents is the key. From early formaldehyde to recent borohydride compounds, potassium borohydride has the most common effect. It is more effective if used in combination with other reducing agents.
E. The deposition rate of the coating increases with the increase of potassium hydroxide and reducing agent concentration and bath temperature, but decreases with the increase of potassium cyanide concentration.
F. The operating temperature of commercialized processes is mostly around 90°C, which is a big test for material stability.
G. If lateral growth occurs on the thin circuit substrate, it may cause a short circuit hazard.
H. Thin gold is prone to porosity and easy to form Galvanic Cell Corrosion K. The porosity problem of the thin gold layer can be solved by post-processing passivation containing phosphorus.