PCB Technical - Try to analyze PCB and rigid-flex board hole wall plating

Electroless copper is a very important step in the process of PCB and rigid-flex board hole metallization. Its purpose is to form a very thin conductive copper layer on the hole wall and copper surface to prepare for the subsequent electroplating. Hole wall plating is one of the common defects of PCB board, soft and hard board hole metallization, and it is also one of the items that easily cause printed circuit boards to be scrapped in batches. Therefore, it is the focus of PCB manufacturers to solve the problem of printed circuit board plating holes. A content of control, but due to the various reasons that cause its defects, only by accurately judging the characteristics of its defects can an effective solution be found.

1. Hole wall plating cavity caused by PTH

Hole wall plating cavities caused by PTH are mainly point-shaped or ring-shaped cavities. The specific reasons are as follows:

(1) 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, and some of them must be strictly controlled. So pay attention to the temperature of the bath at any time.

(2) 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.

(3) Cleaning temperature

The cleaning temperature is often overlooked. 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, which will affect the deposition effect because the golden time for copper deposition is missed. Therefore, in places where the ambient temperature is low, pay attention to the temperature of the cleaning water.

(4) The use temperature, concentration and time of the pore modifier

The temperature of the chemical liquid 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. Punctate voids 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.

(5) Use temperature, concentration and time of reducing agent

The role 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 voids at the resin in the hole.

(6) 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. 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 in the orifice and ring-shaped holes in the hole. The specific reasons are as follows:

(1) Pre-treatment brush plate

The pressure of the brush plate is too large, and the copper layer of the whole plate copper and the PTH hole is brushed away, so that the subsequent pattern electroplating 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, and there is a ring-shaped cavity at the single or double face opening, extending to the pad, and there are obvious traces of etching on the edge of the fault, and the pattern plating layer does not cover the entire board.

(3) Pretreatment micro-etching

The amount of micro-etching in the pre-treatment 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 result in the thinning of the copper layer in the full-board hole, and finally a ring-shaped copper-free in the middle of the hole. Its obvious feature is that the plating layer of the whole plate in the hole gradually becomes thinner, and the pattern plating layer wraps the plating layer of the whole plate.

3. Hole wall plating caused by pattern plating

(1) Micro-etching of pattern plating

The amount of micro-etching of pattern plating should also be strictly controlled, and the defects it produces are basically the same as those of dry-film pre-treatment micro-etching. 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, it is necessary to regularly measure the micro-etching rate, and it is best to optimize the process parameters through DOE experiments.

(2) Poor dispersion of tin plating (lead tin)

Due to factors such as poor solution performance or insufficient swing, the thickness of the tin plating layer is insufficient. During the subsequent film removal and alkaline etching, the tin layer and copper layer in the middle of the hole are etched away, resulting in a ring-shaped cavity. The obvious feature is that the thickness of the copper layer in the hole is normal, there are obvious traces of etching on the edge of the fault, and the pattern plating layer does not cover the entire board. In view of this situation, you can add some tinning brightener in the pickling before tinning, which can increase the wettability of the board and increase the swing amplitude at the same time.

4 Conclusion

There are many factors that cause coating voids, the most common one is PTH coating voids. By controlling the relevant PCB process parameters of the potion, the generation of PTH coating voids can be effectively reduced. 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 products can be maintained.