Electroless copper deposition is a very important step in the metallization process of PCB circuit boards. Its purpose is to form an extremely thin conductive copper layer on the walls of the holes and on the copper surface to prepare for electroplating. Holes in the hole wall plating are one of the common defects in PCB hole metallization, and they are also one of the items that can easily lead to the mass scrapping of printed circuit boards. Therefore, solving the void problem of the PCB plating is a key control of the PCB manufacturers. Content, but due to the variety of causes of its defects, only the accurate judgment of the characteristics of its defects can effectively find a solution.
1, PTH caused by hole wall plating voids
The holes in the plating layer caused by PTH are mainly punctiform or ring-shaped voids. The specific causes are as follows:
(1) Copper content in the copper sink, concentration of sodium hydroxide and formaldehyde
Copper tank solution concentration is the first consideration. In general, the copper content, the concentration of sodium hydroxide and formaldehyde are proportional. When any one of them is lower than 10% of the standard value, the balance of the chemical reaction will be destroyed, causing the chemical copper to be deposited poorly and appear spotted. Empty hole. Therefore, priority is given to adjusting the potion parameters of the copper cylinder.
(2) bath temperature
The bath temperature also has an important effect on the activity of the solution. There are generally temperature requirements in each solution, some of which are strictly controlled. So always pay attention to the bath temperature.
(3) Control of activation fluid
Low bivalent tin ions will cause the decomposition of colloidal palladium, affect the adsorption of palladium, but as long as the timing of the activation solution to add supplements, will not cause major problems. The focus of the activation fluid control is that it cannot be stirred with air. Oxygen in the air will oxidize the divalent tin ions and it will not allow water to enter, which will cause the hydrolysis of SnCl2.
(4) Cleaning temperature
The cleaning temperature is often overlooked. The optimum temperature for cleaning is above 20°C. If the temperature is lower than 15°C, the cleaning effect will be affected. In winter, the water temperature will become very low, especially in the north. Because the temperature of washing is low, the temperature of the plate after washing will also become very low. After entering the copper cylinder, the temperature of the plate cannot be raised immediately, and the deposition time will be affected because of the missed golden time of copper deposition. Therefore, in places where the ambient temperature is lower, the temperature of the cleaning water should also be taken into consideration.
(5) Operating temperature, concentration and time of the pore modifying agent
The temperature of the liquid has strict requirements. Excessive temperature will cause the decomposition of the pore-forming agent to lower the concentration of the pore-forming agent and affect the effect of the entire pore. The obvious feature is the glass fiber cloth inside the hole. Spotted holes appear. Only when the temperature, concentration, and time of the liquid medicine are properly coordinated can a good overall pore effect be obtained while at the same time saving costs. The constant concentration of copper ions in the drug solution must also be strictly controlled.
(6) Use temperature, concentration and time of reducing agent
The effect of reduction is to remove the residual potassium manganate and potassium permanganate after de-drilling, and the out-of-control of the liquid-liquid related parameters will affect its effect. The obvious feature is that there are spot-shaped voids in the resin in the hole.
(7) Oscillator and swing
Loss of control of the oscillator and sway will cause a ring-shaped void, which is mainly due to the fact that bubbles in the hole cannot be eliminated, and the high-thickness-diameter ratio of the orifice is most obvious. The obvious feature is that the hole inside the hole is symmetric, and the part of the hole where the copper is copper is normal. The pattern plating layer (secondary copper) is wrapped around the whole plate layer (one copper).
2. Hole wall coating void caused by pattern transfer
The holes in the hole wall caused by the pattern transfer are mainly holes in the hole and holes in the holes in the hole. The reasons are as follows:
(1) Pre-treatment brush plate
Excessive pressure on the brush plate will brush off the copper layer at the full plate copper and PTH orifices so that the subsequent pattern plating will not be plated with copper, creating an annular cavity. The obvious feature is that the copper layer of the orifice gradually becomes thin, and the plating of the pattern covers the entire plate coating. Therefore, we must control the brush plate pressure by doing the wear scar test.
(2) hole residue
The control of the process parameters in the pattern transfer process is very important, because poor drying of the pretreatment, improper temperature and pressure of the film can result in residual glue at the edge of the orifice and result in an annular cavity in the orifice. The obvious characteristic is that the thickness of the copper layer in the hole is normal, and a ring-shaped void appears on the single-sided or double-faced mouth, extending to the pad, and the edge of the fault is obviously etched. The pattern plating layer does not cover the entire plate.
(3) Pre-treatment microetching
The amount of pre-treated microetching should be strictly controlled, and in particular, the number of rework times for dry film should be controlled. It is mainly because the plating thickness is thin due to the uniformity of plating in the middle of the hole. Excessive rework will result in thinning of the copper layer in the hole of the entire plate, and eventually produce a ring-shaped copper in the middle of the hole. The obvious feature is that the plating in the hole becomes gradually thinner, and the plating of the pattern covers the entire plating.
3, plating holes caused by plating plating holes
(1) Pattern plating microetch
The amount of microetching of the pattern plating must also be strictly controlled, and the resulting defects are essentially the same as the dry film pre-treatment microetching. In severe cases, the hole wall will have a large area without copper, and the thickness of the full plate layer on the surface of the board is obviously thin. Therefore, to measure the microetching rate regularly, it is best to optimize the process parameters by performing DOE experiments.
(2) Tin (Pb-Sn) poor dispersion
The thickness of the tin plating is insufficient due to poor solution performance or insufficient sway, and the tin and copper layers in the middle of the hole are etched away during the subsequent film removal and alkaline etching to produce a ring-shaped void. The obvious feature is that the thickness of the copper layer in the hole is normal, and the edges of the fault are clearly etched. The pattern plating does not cover the entire board (see Figure 5). In view of this situation, some tin-plating photo-agents can be added to the pickling before tin plating, which can increase the wettability of the board and at the same time increase the swing amplitude.
There are many factors that cause voids in the plating layer, the most common being PTH coating voids. By controlling the relevant process parameters of the potions, the generation of PTH coating voids can be effectively reduced. However, other factors can not be ignored. Only through careful observation and understanding of the causes of the voids in the plating and the characteristics of defects, can we solve problems in a timely and effective manner and maintain the quality of the products.