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Analysis on the process control of Multilayer PCB gold plate
Addtime: 2020-11-11

1、 Brief introduction of Technology

The purpose of gold deposition process is to deposit nickel gold coating with stable color, good brightness, flat coating and good solderability on the surface of printed circuit. It can be divided into four stages: pretreatment (degreasing, micro etching, activation and post leaching), nickel precipitation, gold precipitation and post-treatment (waste gold washing, di washing and drying).

2、 Pretreatment

The pretreatment of gold precipitation generally includes the following steps: degreasing (30% ad-482), micro etching (60g / inaps, 2% H2SO4), activation (10% act-354-2), and post leaching (1% h2s04). In order to remove the oxide on the copper surface, palladium is deposited on the copper surface to act as the activation center of nickel deposition. If one of the links is not handled properly, it will affect the subsequent nickel and gold deposition, and lead to batch scrap. In the production process, all kinds of liquid medicine must be analyzed and added regularly, and controlled within the required range. For example, the micro etching rate should be controlled at "25u-40u". When the copper content of the activating solution is greater than 800ppm, a new cylinder must be opened. The cleaning and maintenance of the chemical solution cylinder also has a great impact on the quality of the PCB. In addition to the oil cylinder, the micro etching cylinder and the post immersion cylinder should be changed every week, and each water washing cylinder should also be cleaned every week.

3、 Nickel precipitation

The main components of nickel plating solution are Ni2 + (5.1-5.8g / 1), reducing agent sodium hypophosphite (25-30g / 1) and stabilizer. Due to the strict requirements of chemical nickel on the composition range of the solution, the chemical nickel must be analyzed twice per shift in the production process, and the Ni2 reducing agent shall be added according to the bare copper area of the production plate or experience. The principle of small amount, dispersing and multiple feeding shall be followed when adding materials to prevent local corrosion The pH value and bath temperature have great influence on the thickness of nickel. The temperature of nickel solution is controlled between 85 ℃ and 90 ℃. When pH is 5.3-5.7 and nickel cylinder is not in production, the temperature of nickel cylinder should be reduced to about 70 ℃ to slow down the aging of plating bath. Electroless nickel plating bath is sensitive to impurities, and many chemical components are harmful to electroless nickel, which can be divided into the following categories: inhibitors: including Pb.SnHg.Ti Bi (heavy metal with low melting point), organic impurities including S2, nitric acid and anionic wetting agent. All of these substances will reduce the activity of electroless plating and lead to the decrease of electroless plating speed and the leakage of plating. When severely punished, the electroless nickel plating process will be completely stopped.

Organic impurities: including: in addition to the organic stabilizer mentioned above, there are also plastic agents and impurities from equipment and solder. Although some impurities can be removed by continuous plating, they cannot be completely removed.

Unstabilizers: including PD and a small amount of copper. These two components are caused by the unstability of chemical nickel, which makes the coating rough and excessively plated on the tank wall and heater. Solid impurities: including calcium sulfate or calcium phosphate and other insoluble substances into or into the solution. Filtration can remove solid particles.

In short: in the production process, effective measures should be taken to reduce such impurities mixed into the bath.

4、 Sinking gold

Gold precipitation process is a kind of gold leaching process, the main component of gold precipitation tank; Au (1.5-3.5g / L), binder (ec0.06-0.16mol / L), can replace the pure gold on the nickel phosphorus alloy layer, making the coating smooth, crystal fine, pH value of plating solution is generally between 4-5, the control temperature is 85 ℃ - 90 ℃.

5、 Post treatment

The post-treatment of gold deposition is also an important link. For printed circuit boards, it generally includes the steps of waste gold washing, di washing and drying. If possible, the horizontal washing product can be used to further wash and dry the gold deposition board. The horizontal washing machine can set the process according to the sequence of chemical washing (sulfuric acid 10%, hydrogen peroxide 30g / L), high pressure DI water washing (30 ~ 50psi), DI water washing, blow drying and drying, so as to completely remove the chemical solution and water stains in and on the hole surface of printed circuit board, and obtain the gold plate with uniform coating and good brightness.

6、 Control in production process

In the process of nickel and gold deposition, the problems often occur, such as the imbalance of bath composition, poor quality of additives and excessive content of bath. To prevent and improve this problem plays a great role in process control. The following factors should be paid attention to in the production process:

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

Between micro etchant and palladium activator

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

The relationship 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 solution. The concentration of palladium is very low, but it should be washed well before entering the electroless plating bath. It is suggested to use two courses of water washing with air stirring.

Between chemical nickel and gold leaching

Between these two steps, the transfer time is easy to passivate the nickel layer, resulting in uneven gold immersion and poor adhesion. This is easy to cause throwing gold and tin.

After immersion in gold

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

Nickel tank pH, temperature

The pH of nickel tank should be increased, adjusted with less than 50% ammonia water, and reduced with 10% v / V sulfuric acid. All the additives should be slowly injected and continuously stirred. The pH value measurement should be carried out with sufficient stirring to ensure a balanced bath concentration. The higher the temperature, the faster the plating speed. When plating thick layer, low temperature is used to slow down the appearance of needle. When not operating, do not keep the temperature at the operating temperature, which will lead to the decomposition of reductant and stabilizer components.

In the process of gold precipitation, we should focus on the amount of gold precipitation, the solution of gold precipitation, the formula of additives, the quality of ingredients and so on. With the improvement of the requirements of production technology, some traditional process control methods can not meet the quality requirements. Circuit board companies should continue to explore advanced technology, strictly control all parameters, and strengthen management in order to continuously improve product quality.

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