Shenzhen Xinmaolong Precision Circuit Co., Ltd. Analysis of Multilayer PCB Immersion Gold Process Control

Shenzhen xinmaolong precision circuit Co,.LtdRelease Time:2022-01-13

1. Introduction to the process


The purpose of the immersion gold process is to deposit a nickel-gold coating with stable color, good brightness, smooth coating and good solderability on the surface of the printed circuit. Basically it can be divided into four stages: pre-treatment (oil removal, micro-etching, activation, post-dipping), nickel immersion, gold immersion, post-treatment (waste gold washing, DI washing, drying).


2. Pretreatment


Pre-treatment of immersion gold generally has the following steps: degreasing (30% AD-482), micro-etching (60g/InaPS, 2% H2SO4), activation (10% Act-354-2), post-dipping (1% H2S04) ). To remove the copper surface oxide, and immerse palladium on the copper surface to activate the center of nickel deposition. If one of the links is not handled properly, it will affect the subsequent immersion nickel and immersion gold, and lead to batch scrapping. In the production process, all kinds of potions must be regularly analyzed and supplemented, and controlled within the required range. For example, the micro-erosion rate should be controlled at "25U-40U". When the copper content of the activation potion is greater than 800PPM, a new tank must be opened. The cleaning and maintenance of the potion tank also has a great impact on the quality of the PCB. The post-dipping tank should be changed every week, and the washing tanks should also be cleaned every week.


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3. Immersion nickel


The main components of immersion nickel syrup are Ni2+ (5.1-5.8g/1), reducing agent sodium hypophosphite (25-30g/1) and stabilizer. Because chemical nickel has strict requirements on the composition range of syrup, it must be Class analysis and testing are carried out twice, and Ni2 reducing agent is added according to the bare copper area of the production board or experience.


The principle of small amount and multiple feedings should be followed to prevent the local plating solution from reacting violently, resulting in accelerated aging of the plating solution. The pH value and the temperature of the plating solution have a great influence on the nickel thickness. The temperature of the nickel solution should be controlled at 85℃-90℃. . When the PH is 5.3-5.7 and the nickel cylinder is not in production, the temperature of the nickel cylinder should be lowered to about 70 °C to slow down the aging of the plating solution. The electroless nickel plating solution is more sensitive to impurities, and many chemical components are harmful to chemical nickel, which can be divided into the following Several categories: Inhibitors: including Pb.SnHg.TI.Bi (low melting point heavy metals),


Organic impurities include S2, nitric acid and anionic wetting agents. All of these substances reduce activity, result in reduced electroless plating rates and underplating, and when severely punished, can cause the electroless nickel plating process to stop completely. string


Organic impurities: Including: in addition to the above-mentioned organic stabilizers, there are plastics and impurities from equipment and solder. Although some impurities can be removed by continuous plating, they cannot be completely removed.


Unstable agent: including Pd and a small amount of copper, these two components cause the instability of chemical nickel, make the coating rough, and excessively plated on the tank wall and heater. Solid impurities: including calcium sulfate or calcium phosphate and other insoluble substances settled or brought into the solution. Filtration removes solid particles.


In a word: effective measures should be taken to reduce the mixing of such impurities into the plating solution during the production process.


4. Shen Jin


The immersion gold process is a immersion gold process. The main component of the immersion gold cylinder; Au (1.5-3.5g/l), the binder is (Ec0.06-0.16mol/L), which can be replaced on the nickel-phosphorus alloy layer. Pure gold plating makes the coating smooth and crystallized. The pH value of the plating solution is generally between 4-5, and the control temperature is 85℃-90℃.


5. Post-processing


Immersion gold post-processing is also an important link. For printed circuit boards, it generally includes: waste gold washing, DI water washing, drying and other steps. drying. The horizontal surface plate washer can be set up in the sequence of medicinal water washing (sulfuric acid 10%, hydrogen peroxide 30g/L), high pressure DI water washing (30~50PSI), DI water washing, blow drying, and drying sequence, so as to completely remove the holes in the printed circuit board and The surface potion and water stains can be obtained to obtain an immersion gold plate with a uniform coating and good brightness.


6. Control in the production process


The problems that often occur in the production process of immersion nickel gold are often the imbalance of the composition of the plating solution, the poor quality of the additives and the excessive content of the magazine in the plating solution. Preventing and improving this problem plays a great role in process control. The production process is now Factors to be aware of are the following:


In the chemical nickel-gold process, because of the small holes, water washing between each step is necessary, and special attention should be paid.


Between microetchant and palladium activator


After micro-etching, copper is easy to fade, and in severe cases, the palladium coating is uneven, which leads to the failure of the nickel layer. If the circuit board is not washed well, the oxidant from the micro-etching will prevent the deposition of palladium, which will affect the effect of immersion gold. affect the quality of the board.


Between palladium activator and chemical nickel


Palladium is the most dangerous impurity in the chemical nickel process, and a very small amount of palladium will naturally decompose the bath. Although the concentration of palladium is very low, it should be washed with water well before entering the electroless plating tank. It is recommended to use two water washes with air agitation.


Between chemical nickel and immersion gold


Between these two steps, the transfer time will easily passivate the nickel layer, resulting in uneven gold immersion and poor bonding. This is easy to cause gold and tin to be thrown away.


After gold immersion


In order to maintain the solderability and ductility, fully wash with water after gold plating (the last wash is best to use distilled water), and completely dry, especially in the hole completely.


Immersion nickel tank PH, temperature


The PH of the nickel sinking tank should be raised and adjusted with less than 50% ammonia water, and the PH should be lowered with 10% V/V sulfuric acid. All additions are poured in slowly, stirring continuously. The pH measurement should be carried out with sufficient agitation to ensure a balanced bath concentration. The higher the temperature, the faster the plating rate. Low temperatures are used to slow needle emergence when thick layers are plated. Do not maintain the temperature at operating temperature when not in operation, as this will cause the reducing agent and stabilizer components to decompose.


In the immersion gold process, the amount of immersion gold, the immersion gold solution, the additive formula, the quality of the ingredients, etc. should be discussed. With the improvement of the production process requirements, some traditional process control methods can no longer meet the quality requirements. We should continue to explore advanced technology, strictly control all parameters, and strengthen management to continuously improve product quality.


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