What are the reasons for the appearance of holes in the plating layer of the circuit board? How to Solve the Hole Wall Plating Hole of the Circuit Board
1. Hole wall plating cavity caused by PTH
The holes in the hole wall plating caused by PTH are mainly point-shaped or ring-shaped holes. The specific reasons are as follows:
(1) Copper content, sodium hydroxide and formaldehyde concentration in copper sink
The solution concentration of the copper tank is the first consideration. Generally speaking, the concentration of copper, sodium hydroxide and formaldehyde are proportional. When any of them is less than 10% of the standard value, the balance of chemical reactions will be destroyed, resulting in poor chemical copper deposition and spotting. The void. So give priority to adjusting the potion parameters of the copper tank.
(2) The temperature of the bath
The temperature of the bath also has an important influence on the activity of the solution. There are generally temperature requirements in each solution, some of which must be strictly controlled. So pay attention to the temperature of the bath at any time.
(3) Control of activation solution
Low divalent tin ions will cause the decomposition of colloidal palladium and affect the adsorption of palladium, but as long as the activation solution is added regularly, it will not cause major problems. The key point of the activation solution control is that it cannot be stirred with air. The oxygen in the air will oxidize the divalent tin ions. At the same time, no water can enter, which will cause the hydrolysis of SnCl2.
(4) Cleaning temperature
The cleaning temperature is often ignored. The best cleaning temperature is above 20°C. If it is lower than 15°C, the cleaning effect will be affected. In winter, the water temperature becomes very low, especially in the north. Due to the low washing temperature, the temperature of the board after cleaning will also become very low. The temperature of the board cannot rise immediately after entering the copper tank, and the deposition effect will be affected by missing the golden time of copper deposition. Therefore, in places where the ambient temperature is low, pay attention to the temperature of the cleaning water.
(5) The use temperature, concentration and time of the pore regulator
The temperature of the liquid medicine has strict requirements. Too high temperature will cause the decomposition of the pore modifier, lower the concentration of the pore modifier, and affect the effect of the pore. The obvious feature is the glass fiber cloth in the hole. Dotted holes appear. Only when the temperature, concentration and time of the liquid medicine are properly matched, can a good hole-regulating effect be obtained, and at the same time it can save costs. The concentration of copper ions continuously accumulated in the liquid medicine must also be strictly controlled.
(6) Use temperature, concentration and time of reducing agent
The effect of reduction is to remove the remaining potassium manganate and potassium permanganate after decontamination. The out-of-control parameters of the chemical solution will affect its effect. Its obvious feature is the appearance of dotted holes in the resin in the hole.
(7) Oscillator and swing
The out-of-control of the oscillator and the swing will cause a ring-shaped cavity, which is mainly due to the failure of the bubbles in the hole to be eliminated, and the small orifice plate with a high aspect ratio is the most obvious. The obvious feature is that the cavities in the hole are symmetrical, and the copper thickness of the part with copper in the hole is normal, and the pattern plating layer (secondary copper) wraps the entire board plating layer (primary copper).
2. Hole wall plating caused by pattern transfer
The holes in the hole wall plating layer caused by pattern transfer are mainly ring-shaped holes and ring-shaped holes in the hole. The specific reasons are as follows:
(1) Pre-treatment brush
The pressure of the brush plate is too large, and the copper layer at the full board copper and the PTH hole is brushed off, so that the subsequent pattern plating cannot be plated with copper, resulting in a ring-shaped hole in the hole. The obvious feature is that the copper layer of the orifice gradually becomes thinner, and the pattern plating layer wraps the entire plate plating layer. Therefore, it is necessary to control the brushing pressure by doing a wear scar test.
(2) Residual glue at orifice
The control of process parameters in the pattern transfer process is very important, because poor pre-treatment drying, improper film temperature, and pressure will cause residual glue at the edge of the orifice, resulting in an annular cavity in the orifice. The obvious feature is that the thickness of the copper layer in the hole is normal, the single-sided or double-faced opening presents a ring-shaped cavity, extending to the pad, the edge of the fault has obvious traces of etching, and the pattern plating layer does not cover the entire board (see image 3).
(3) Pretreatment micro-etching
The amount of micro-etching in the pretreatment should be strictly controlled, especially the number of rework of the dry film board. The main reason is that the thickness of the plating layer in the middle of the hole is too thin due to the problem of electroplating uniformity. Too much rework will cause the copper layer in the full board hole to be thinned, and finally a ring-shaped copper-free in the middle of the hole. The obvious feature is the gradual thinning of the whole plate coating in the hole, and the pattern plating layer wraps the whole plate coating (see Figure 4).
3. Hole wall plating caused by pattern plating
(1) Pattern plating micro-etching
The amount of micro-etching of pattern plating should also be strictly controlled, and the defects produced are basically the same as those of dry-film pre-treatment. In severe cases, the hole wall will be free of copper in a large area, and the thickness of the entire board on the board surface is obviously thinner. Therefore, to measure the micro-etch rate regularly, it is best to optimize the process parameters through DOE experiments.
(2) Tin (lead tin) has poor dispersion
Due to factors such as poor solution performance or insufficient swing, the thickness of the tin plating layer is insufficient, and the tin layer and copper layer in the middle of the hole are etched away during the subsequent film removal and alkaline etching, resulting in ring-shaped voids. The obvious feature is that the thickness of the copper layer in the hole is normal, the edge of the fault has obvious traces of etching, and the pattern plating layer does not cover the entire board. In response to this situation, you can add some tinning brighteners in the pickling before tinning, which can increase the wettability of the board and increase the swing amplitude.
4 Conclusion
There are many factors that cause coating voids, the most common one is PTH coating voids, which can effectively reduce the generation of PTH coating voids by controlling the relevant process parameters of the potion. However, other factors cannot be ignored. Only through careful observation and understanding of the causes of coating voids and the characteristics of defects can the problems be solved in a timely and effective manner and the quality of the product can be maintained. Due to my limited level of experience, here are some practical problems encountered in daily production to share and communicate with colleagues.
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