Chemical copper deposition is a very important step in the metallization process of PCB holes. Its purpose is to form a very thin conductive copper layer on the hole wall and copper surface, which is ready for electroplating later. 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.
Hole wall coating hole caused by PTH
The hole wall coating cavity caused by PTH is mainly point-like or annular cavity, and the specific reasons are as follows:
(1) Copper content, sodium hydroxide and formaldehyde concentration of copper sink cylinder
The solution concentration of the copper cylinder is the first consideration. Generally speaking, the copper content, sodium hydroxide and formaldehyde concentration is proportional, when any one of the content is less than 10% of the standard value will destroy the balance of the chemical reaction, resulting in bad chemical copper deposition, the appearance of point-like void. So the priority is to adjust the copper cylinder of each potion parameters.
(2) The temperature of the tank liquid
The temperature of the tank 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 keep an eye on the temperature of the tank. (3) Control of activation fluid
Low tin ions will cause the decomposition of colloidal palladium and affect the adsorption of palladium, but as long as the activation liquid is added regularly, it will not cause a big problem. The key point of activation solution control is not to use air stirring, oxygen in the air will oxidize tin divalent ions, and at the same time, no water can enter, which will cause the hydrolysis of SnCl2.
(4) Cleaning temperature
Cleaning temperature is often ignored, the best cleaning temperature is above 20℃, if less than 15℃ will affect the cleaning effect. In winter, the water temperature can get very cold, especially in the north. Due to the low temperature of washing, the temperature of the board will also become very low after cleaning. After entering the copper cylinder, the temperature of the board cannot rise immediately, which will affect the deposition effect because of missing the golden time of copper deposition. So in the lower ambient temperature, also pay attention to the temperature of the cleaning water. (5) the use of temperature, concentration and time of holing agent
The temperature of the liquid medicine has strict requirements, too high temperature will cause the decomposition of the holing agent, so that the concentration of the holing agent becomes low, affecting the effect of the holing, its obvious characteristic is that there is a dot cavity in the glass fiber cloth in the hole. Only when the temperature, concentration and time of liquid medicine are properly coordinated, can good holing effect be achieved and cost can be saved. The concentration of copper ions accumulated in the solution must also be strictly controlled.
(6) The use of reducing agent temperature, concentration and time
The role of reduction is to remove the residue of potassium manganate and potassium permanganate after drilling, and the effect will be affected by the loss of the relevant parameters of liquid medicine. The obvious feature is that the spot-like cavity appears in the resin of the hole.
(7) Oscillators and swing
Oscillators and sway out of control will cause annular cavity, which is mainly due to the bubble in the hole failed to remove, the most obvious to the high thickness to diameter ratio of the orifice plate.Its obvious characteristic is that the cavity in the hole is symmetrical, and the copper in the hole part of the copper thickness is normal, graphic electroplating (secondary copper) wrapped whole plate plating (primary copper).
Graphics transfer caused by the hole wall coating hole
The hole wall coating holes caused by graphic transfer are mainly looped holes at the orifice and looped holes in the hole. The specific reasons are as follows:
(1) Pretreatment brush plate
The pressure of the brush plate is too large, and the copper layer at the mouth of the whole plate and PTH is brushed away, so that the graphic electroplating behind cannot be plated with copper, resulting in the annular cavity at the mouth. The obvious feature is that the copper layer at the orifice gradually becomes thinner, and the graphic electroplating coating wraps the whole plate coating. So we should control the pressure of the brush plate by doing abrasion test.
(2) residual glue at the orifice
It is very important to control the process parameters in the graphic transfer process, because the poor drying before treatment, improper temperature and pressure of the film will cause gum residue at the edge of the hole and lead to the annular cavity of the hole. Its obvious characteristics are that the thickness of the copper layer in the hole is normal, the single or double orifice shows a annular cavity, extending to the pad, the edge of the fault has obvious etching traces, the graphic electroplating layer does not wrap the whole plate.
(3) pre-treatment corrosion
The amount of microerosion before treatment should be strictly controlled, especially the rework times of dry film plate. It is mainly due to the thin coating thickness in the middle of the hole due to the uniformity of electroplating, and too much rework will cause the thin copper layer in the hole of the whole plate, and finally produce the ring without copper in the middle of the hole. The obvious characteristic is that the whole plate coating in the hole gradually becomes thinner, and the graphic electroplating coating wraps the whole plate coating.
Hole wall coating hole caused by graphic electroplating
(1) Graphic electroplating corrosion
The etching quantity of graphic electroplating should also be strictly controlled, and the defects produced are basically the same as those of dry film pretreatment. When the hole wall is serious, there will be no copper in a large area, and the thickness of the whole plate layer on the board is obviously thin. Therefore, it is better to optimize the process parameters through DOE test to measure the micro-erosion rate at a regular time.
(2) Tin plating (lead tin) dispersion is poor
Due to poor solution performance or insufficient swing factors such as insufficient tin plating thickness, in the back of the film and alkaline etching of the hole in the middle of the tin layer and copper layer erosion, the annular cavity. The obvious characteristics are that the thickness of the copper layer in the hole is normal, there are obvious etching traces at the fault edge, and the graphic electroplating layer does not wrap the whole plate (see Figure 5). In view of this situation, you can add some tinplating light agent in the leaching acid before tin plating, can increase the wettability of the board, and increase the amplitude of swing.
The conclusion
There are many factors causing the coating hole, the most common is PTH coating hole, through the control of the related technological parameters of the potion can effectively reduce the generation of PTH coating hole. But other factors can not be ignored, only through careful observation, to understand the cause of the coating hole and the characteristics of defects, in order to solve the problem in time and effectively, maintain the quality of the product.