Analysis of gold deposition process control of the

With the rapid development of surface mount technology, the requirements for the surface flatness of printed boards are becoming higher and higher, which directly leads to the check of whether the piston travel limit switch works by referring to the ruler on the main column The rapid development of gold precipitation technology in recent years was introduced. Based on my experience, this paper analyzes the control method of gold sinking process

I. process introduction

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

II. Pretreatment

the pretreatment of gold precipitation generally includes the following steps: oil removal (30%ad-482), micro etching (60g/inaps, 2%h2so4), activation (10%act-), and post leaching (1%h2s04). To remove the oxide on the copper surface, and precipitate palladium on the copper surface as the activation center of nickel precipitation. If one of the links is not handled well, it will affect the subsequent nickel and gold precipitation, and lead to batch scrapping. During the production process, all kinds of potions must be regularly analyzed and supplemented to control within the required range. More importantly, for example, the micro etching rate should be controlled at "25u-40u". When the copper content of the activation liquid medicine is greater than 800ppm, a new cylinder must be opened. The cleaning and maintenance of the liquid medicine cylinder also has a great impact on the quality of the PCB. In addition to the oil cylinder, the micro etching cylinder, the post immersion cylinder should be replaced every week, and each water washing cylinder should also be cleaned every week

III. nickel precipitation

the main components of nickel precipitation potion are Ni + (5..8g/1) and reducing agent sodium hypophosphite (g/1) and stabilizer. Because chemical nickel has strict requirements on the composition range of potion, it must be analyzed and tested twice a shift in the production process, and the Ni reducing agent should be added according to the bare copper surface area of the production board or experience. When adding materials,

the principle of small amount, dispersing and adding materials for many times should be followed to prevent local plating solution from reacting violently, Lead to accelerated aging of plating solution, pH value and plating solution temperature have a great impact on Nickel thickness, and the temperature of nickel solution is controlled at 85 ℃ -90 ℃. PH at 5 7. When the nickel cylinder is not produced, the temperature of the nickel cylinder should be reduced to about 70 ℃, and the steps are as follows to slow down the aging of the plating solution. The chemical nickel plating solution is sensitive to impurities, and many chemical components are harmful to chemical nickel, which can be divided into the following categories: inhibitors: including (heavy metals with low melting point),

organic impurities include s, nitric acid and anionic wetting agent. All these substances will reduce the activity, resulting in the reduction of electroless plating speed and missing plating. When severely punished, it will lead to the complete stop of the electroless nickel plating process

organic impurities: including: in addition to the organic stabilizers 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

non stabilizer: including PD and a small amount of copper, which are caused by the instability of chemical nickel, making the coating rough, and too much plating on the tank wall and heater. Solid impurities: including calcium sulfate or calcium phosphate and other insoluble substances sinking into or brought into the solution. Filtration can remove solid particles

in short: effective measures should be taken to reduce the mixing of such impurities into the plating solution in the production process

IV. gold precipitation

the gold precipitation process is a gold leaching process, and the main component of the gold precipitation tank; Au (1..5g/l), the binding agent is (ec0..16mol/l), which can replace pure gold plating on the nickel phosphorus alloy layer, making the coating smooth and fine crystallization. The pH value of the plating solution is generally between, and the control temperature is 85 ℃ -90 ℃

v. post treatment

post treatment of gold precipitation is also an important link. For printed circuit boards, it generally includes: waste gold washing, di washing, drying and other steps. If conditions permit, the gold precipitation board can be further washed and dried with horizontal washing product. The horizontal plate washing machine can set the process according to the sequence of liquid medicine washing (sulfuric acid 10%, hydrogen peroxide 30g/l), high-pressure DI water washing (30~50psi), DI water washing, drying and drying, so as to completely remove the liquid medicine and water stains in the holes and on the surface of the printed circuit board, and get the gold plated board with uniform coating and good brightness

VI. control in the production process

the problems that often occur in the production process of nickel plating gold are often the imbalance of bath composition, the poor quality of additives and the excessive content of bath magazines. Preventing and improving this problem plays a great role in process control. Now the following factors should be paid attention to in the production process:

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

2). Between the micro etchant and the palladium activator

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

3). 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 liquid. The concentration of palladium is very low, but it should also be well washed before entering the electroless plating tank. It is recommended to use two washes with air stirring

4). Between chemical nickel and gold leaching

between these two steps, the transfer time is easy to passivate the nickel layer, resulting in uneven gold leaching and poor adhesion. This is easy to cause gold and tin to be dumped

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

6). PH and temperature of nickel sink cylinder

the nickel sink cylinder should increase pH, adjust with less than 50% ammonia, reduce pH, and adjust with 10% v/v sulfuric acid. All additions should be injected slowly and stirred continuously. PH value measurement should be carried out when fully stirred to ensure a balanced bath concentration. The higher the temperature, the faster the plating speed. When plating a thick layer, low temperature is used to slow down the appearance of the needle. Do not keep the temperature at the operating temperature when comparing the new and old national standard test conditions in Table 3, which will lead to the decomposition of reductant and stabilizer components. See the following table:

fault 1: missing plating: the edge of the road board is thin or not plated with chemical nickel

reason: 1.1 heavy metal pollution 1.2 excessive stabilizer 1.3 too intense stirring 1.4 improper copper activation

improvement methods: 1.1 reduce the source of impurities 1.2 check the maintenance method and improve it if necessary 1.3 evenly stir, check the outlet of the pump 1.4 check the activation process

fault 2: bridging: chemical nickel is also plated between them

reasons: 2.1 the activation time with palladium activator is too long, 2.2 the palladium concentration in the activator is too high 2.3 the concentration of hydrochloric acid in the activator is too low. 2.4 the chemical nickel is too active. 2.5 the copper and the wire are not completely slightly corroded. 2.6 the water washing is not sufficient.

improvement methods: 2.1 shorten the activation time. 2.2 dilute the activator and adjust the concentration of hydrochloric acid. 2.3 adjust the concentration of hydrochloric acid. 2.4 adjust the operating conditions 2.5 improve the micro etching. It will be more beneficial if the micro etching time is a little longer

fault 3:3. The gold is too thin

reason: 3.1 the temperature of gold leaching is too low 3.2 the leaching time is too short 3.3 the pH value of gold exceeds the range

improvement method: 3.1 improve after inspection 3.2 extend the gold leaching time 3.3 improve after inspection

fault 4:4. Circuit board deformation

reason: 4.1 the temperature of chemical nickel or gold leaching is too high

improvement method: 4.1 reduce the temperature

fault 5:5. Poor weldability

cause: 5.1 incorrect thickness of gold 5.2 impurities caused by chemical nickel or gold leaching process 5.3 the process itself is not wrong 5.4 improper storage of circuit board 5.5 the effect of the last washing is not good

improvement method: 5.1 the best thickness of gold is: 0.05~0.1um5.2. See 1.15.3 reasons from equipment and operators. 5.4 the circuit board should be stored in a dry and cool place, preferably in a closed plastic bag. 5.5 change the water, increase the water flow speed

fault 6:6. Uneven gold layer

reason: 6.1 the transfer time is too long 6.2 the plating solution is aged or contaminated 6.3 the gold concentration is too low 6.4 in the gold leaching process, the organic impurities from the equipment

improvement method: 6.1 optimize the water washing and shorten the transfer time 6.2 chemical nickel restart 6.3 improve after inspection, check whether the tank and filter and raw materials are suitable, and filter and wash before use, simple carbon treatment to restore the gold leaching process. First, carry out the test

fault 7:7, poor adhesion of nickel on copper

reason: 7.1 insufficient micro corrosion 7.2 too long activation time 7.3 activation time is too high 7.4 insufficient water washing

improvement method: 7.1 extend the micro etching time or increase the temperature 7.2 reduce activation time 7.3 reduce the temperature 7.4 optimize the washing conditions

fault 8:8, the appearance of the gold layer is gray

cause: 8.1 the nickel layer is gray 8.2 the gold is too thick

improvement method: 8.1 shorten the time of micro etching or reduce the temperature 8.2 reduce the gold leaching temperature and shorten the gold leaching time

fault 9:9, the coating is rough

cause: 9.1 the chemical nickel solution is unstable 9.2 there are solid particles in the chemical nickel solution

improvement method: 9.1 adjust the temperature 9.2 reduce source of impurities, Improve filtration

fault 10:10, uneven micro etching

cause: 10.1 insufficient cleaning 10.2 aging of cleaning agent or impurities 10.3 aging of micro etching solution (greater than 30g/l) 10.4 over corrosion

improvement method: 10.1 increase the time in cleaning agent or add cleaning agent 10.2 replace the cleaning agent 10.3 replacement, micro etching 10.4 shortening corrosion time

VII. Problems and improvement

now, the causes and solutions of some typical gold deposition problems are listed in Table 1 above for reference:

VIII. Precautions:

during gold deposition, employees should pay attention to safety during operation, wear protective clothing and goggles, and must use ventilation equipment. In case of leakage, rags, towels or other absorbent materials should be used to absorb the leakage, Put it in a plastic lined container to recover gold, and store the solution in a plastic lined barrel to recover gold

conclusion

the control of the whole gold deposition process should be considered for the gold deposition quality of printed wiring board, including surface treatment before gold deposition, nickel deposition, gold deposition, and post-treatment of gold deposition. In the gold precipitation process, we should discuss the gold precipitation amount, gold precipitation solution, additive formula, quality of ingredients, etc. the requirements for production process are becoming higher and higher. Some traditional process control methods can no longer meet the quality requirements. Circuit board companies should continue to explore advanced processes, strictly control all parameters, and strengthen management in order to continuously improve product quality. (end)