The phenomenon of depositing hole cavity in PCB hole and its countermeasures

- Nov 03, 2018-

Chemical sinking copper is a very important step in the process of metallization of PCB hole, which aims to form an extremely thin conductive copper layer on the hole wall and copper surface to prepare for electroplating at the back.And hole wall coating hole is one of the common defects of PCB hole metallization, easy cause batch printed circuit boards scrap is one of the project, therefore solve the problem of PCB plating cavity is one of the key control PCB manufacturer, but due to various causes of the defects, only accurately judge the defect characteristics can effectively find out the solutions.

1. Hole wall plating cavity caused by PTH

PTH induced hole wall coating cavity is mainly point or ring hole. The specific reasons are as follows:

(1) copper content in the copper cylinder, sodium hydroxide and formaldehyde concentration

The solution concentration of the copper cylinder is the first consideration.Generally speaking, the concentration of copper content, sodium hydroxide and formaldehyde is proportional. When the content of any one of them is less than 10% of the standard value, the balance of chemical reaction will be destroyed, resulting in poor chemical copper deposition and a punctiform cavity.Therefore, priority should be given to adjusting the parameters of the copper cylinder.

(2) temperature of tank liquid

The temperature of the bath also has an important effect on the activity of the solution.There are usually temperature requirements in each solution, some of which are strictly controlled.So you have to pay attention to the temperature of the tank.

(3) control of activated liquid

The low bivalent tin ions will cause the decomposition of colloid palladium, which will affect the adsorption of palladium. However, as long as the activated liquid is regularly added, it will not cause big problems.The key point of the activation solution control is that the air cannot be stirred. Oxygen in the air will oxidize tin divalent ions, and water cannot enter into the air, resulting in the hydrolysis of SnCl2.

(4) the cleaning temperature

Cleaning temperature often ignored, cleaning is the best temperature above 20 ℃, if below 15 ℃ can affect the effect of cleaning.During the winter, the water temperature can be very low, especially in the north.Due to the low temperature of washing, the temperature of the plates will be very low after washing. The temperature of the plates cannot be raised immediately after entering the copper cylinder, which will affect the effect of the deposition due to missing the golden time of copper deposition.So in places where the ambient temperature is lower, also pay attention to the temperature of the cleaning water.

(5) temperature, concentration and time of use of the pore filling agent

The temperature of the liquid medicine has strict requirements, too high temperature will cause the decomposition of the pore-forming agent, so that the concentration of the pore-forming agent becomes low, which affects the effect of the whole hole, and its obvious feature is the point hole in the glass fiber cloth inside the hole.Only when the temperature, concentration and time of the liquid are properly matched, can we get a good hole effect and save cost at the same time.The concentration of copper ions in the liquid must also be strictly controlled.

(6) temperature, concentration and time of use of reducing agent

The effect of reduction is to remove the residual potassium manganate and potassium permanganate after the contamination is removed. The loss of control of relevant parameters of the liquid will affect the effect, and its obvious feature is that the puncture hole appears in the resin.

(7) oscillator and swing

The loss of control of the oscillators and the swing can lead to annular voids, which are mainly due to the failure of the bubbles in the voids, and the voids with high aspect ratio are the most obvious.The obvious feature is that the cavity inside the hole is symmetrical, while the copper inside the hole is normal, and the figure electroplating layer (secondary copper) wraps the whole plate coating layer (primary copper).

2. Hole wall plating hole caused by graph transfer

The hole wall coating hole caused by the graph transfer is mainly the hole hole ring and the hole ring in the hole. The specific reasons are as follows:

(1) pretreatment brush plate

When the pressure of the brush plate is too high, the copper layer at the mouth of the PTH hole and the whole plate is brushed away, so that the subsequent figure electroplating cannot be plated with copper, resulting in the annular hole.Its obvious characteristic is that the copper layer of the hole mouth gradually becomes thin, the figure electroplating layer wraps the whole plate coating layer.Therefore, the pressure of brush plate should be controlled through the grinding mark test.

(2) orifice residues

It is very important to control the process parameters in the graph transfer process, because poor drying before treatment, inappropriate temperature of the film patch, and improper pressure will lead to the residual glue at the edge of the orifice and lead to the annular hole of the orifice.Its obvious feature is that the copper layer inside the hole is normal in thickness, the hole on one side or both sides presents a ring hole, which extends all the way to the welding plate, the fault edge has obvious etched traces, and the graph electroplating layer does not cover the whole plate.

(3) pretreatment microerosion

The microerosion amount of pretreatment should be strictly controlled, especially the rework number of dry film plate.The main problem 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 whole plate hole to thin, and finally the ring in the middle of the hole will be produced without copper.Its obvious characteristic is that the plate coating inside the hole gradually thin, the figure electroplating coating covering the plate coating.

3. Hole wall coating hole caused by graph electroplating

(1) pattern electroplating microcorrosion

The microerosion of graph electroplating should be strictly controlled, and the defect is basically the same as that of dry film pretreatment.When serious, the hole wall will be large area without copper, the thickness of the whole plate layer on the surface of the plate is obviously thin.Therefore, it is better to optimize the process parameters through DOE experiment to measure the microerosion rate regularly.

(2) tin plating (lead tin) has poor dispersion

Due to factors such as poor solution performance or insufficient sway, the thickness of tin coating is insufficient. The tin layer and copper layer in the middle of the hole are corroded in the subsequent defilm and alkaline etching, resulting in ring cavity.Its obvious feature is that the copper layer inside the hole is normal in thickness, the fault edge has obvious etching traces, and the graph electroplating layer does not cover the whole plate (see figure 5).In this case, some tinning gloss can be added in the acid before tin plating, which can increase the wettability of the plate and increase the swing range.

4, the conclusion

There are many factors causing PTH coating cavities. The most common one is PTH coating cavities.However, other factors cannot be neglected. Only through careful observation and understanding of the cause of the coating hole and the characteristics of defects can we solve problems and maintain product quality in a timely and effective manner.