The cause of electroplate cavity in circuit board and how to prevent it

Electroplated through holes are holes in a printed circuit board with a copper coating. These holes allow the circuit to pass from one side of the board through the copper in the holes to the other side of the board. For any printed circuit board design with two or more circuit layers, electroplated through-holes form an electrical interconnection between the different layers. To create a plated through hole during PCB manufacturing, the manufacturer drills holes into the board laminate and foils present on either side. The wall of the hole is then electroplated so that it conducts signals from one layer to another. To prepare the board for electroplating, the PCB manufacturer must conduct the board from top to bottom through a chemically bonded thin layer of non-electroplated copper that adheres to the inside of the hole and the edge of the board. This step is called copper deposition. After deposition, the circuit image is applied and developed. The area where the circuit is present is then electroplated with a thicker layer of copper, which coats the hole and circuit to the final desired thickness (usually about.001in /.025mm). From this point on, the board will continue the manufacturing process until it is completed.

First, deposition problem

Deposition problems can affect the interconnections within the hole walls and can lead to PCB failure. The most common deposition defect is the presence of electroplating void in the pore wall of copper lining. If the wall of the hole is not smooth and fully coated, the current cannot pass through. The image above shows a cross section of the through-hole where the copper on the wall is too thin, most likely due to deposition and poor plating. During deposition, when the copper is not uniformly coated, plating voids in plating through holes occur, thus preventing proper plating. This may be due to contamination, bubbles on the side of the hole and/or rough drilling. All of this can create an uneven surface on the walls of the through-hole, which makes it difficult to apply smooth continuous copper wire.

Second, the holes in the pore-wall coating caused by PTH are mainly spot-like or annular holes, and the specific reasons are as follows:

1. Copper content of copper cylinder, concentration of sodium hydroxide and formaldehyde

The concentration of solution in the copper tank is the first thing to consider. Generally speaking, the copper content, sodium hydroxide and formaldehyde concentration is proportional, when any of the content is less than 10% of the standard value will destroy the balance of the chemical reaction, resulting in poor deposition of chemical copper, the appearance of spot-like holes. So the priority is to adjust the copper cylinder of the potion parameters.

2. Temperature of the tank

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 the temperature of the tank should also be paid attention to at any time.

3. Control of activation fluid

Low tin ion 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 big problems. The key point of the control of the activation solution is not to stir with air. The oxygen in the air will oxidize the tin bivalent ion. 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 temperature for cleaning is above 20℃. If it is below 15℃, the cleaning effect will be affected. 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 become very low after cleaning. The temperature of the board cannot rise immediately after entering the copper cylinder, which will affect the effect of deposition because the golden time of copper deposition is missed. So in the place where the ambient temperature is low, you should also pay attention to the temperature of the cleaning water.

5. Use temperature, concentration and time of filling agent

The temperature of the liquid medicine has strict requirements. Too high temperature will cause the decomposition of the pore-filling agent, make the concentration of the pore-filling agent lower, and affect the effect of the whole hole. Its obvious feature is the appearance of dotted holes in the glass fiber cloth in the hole. Only when the temperature, concentration and time of the liquid are properly coordinated, can we get good effect of the whole hole and save the cost at the same time. The concentration of copper ions accumulating in liquid medicine must also be strictly controlled.

6. Use temperature, concentration and time of reducing agent

The role of reduction is to remove the residual potassium manganate and permanganate after drilling. The loss of relevant parameters of the liquid medicine will affect its effect, and its obvious feature is that the resin in the hole appears punctate cavity.

7. Oscillators and wobbles

The loss of control of oscillators and oscillations will result in annular cavities, which are mainly due to the failure of air bubbles to be removed from the holes, most notably the orifice plates with the ratio of height to diameter. Its obvious characteristics are the holes in the hole symmetry, and the hole in the copper part of the copper thickness is normal, graphic electroplating (secondary copper) coating (primary copper).

Third, the holes in the hole wall coating caused by pattern transfer are mainly holes in the hole ring and hole ring, the specific reasons are as follows:

1. Pretreatment brush plate

The pressure of the brush plate is too large, the copper layer of the whole plate and the copper layer of the PTH orifice is brushed off, so that the following graphic plating cannot be plated with copper, resulting in the orifice annular cavity. The obvious feature is that the copper layer of the orifice becomes thinner and the graphic electroplating coating covers the whole plate coating. So by doing the abrasion test, control the brush pressure.

2. Orifice residue glue

It is very important to control the process parameters in the graphic transfer process, because the pretreatment drying is poor, the improper temperature and pressure of the film will cause the residual glue at the edge of the orifice and lead to the annular cavity of the orifice. Its obvious characteristics are that the thickness of the copper layer in the hole is normal, the annular cavity at the single or double-sided orifice is extended to the welding pad, the fault edge is obviously etched traces, the graphic electroplating did not wrap the whole plate.

3. Pretreatment micro-etching

The amount of micro-corrosion before treatment should be strictly controlled, especially the rework times of dry film plate. The main reason is that the thickness of the coating is thin due to the problem of electroplating uniformity in the middle of the hole. Excessive rework will cause the thinning of the copper layer in the hole of the whole plate, and eventually produce a ring without copper in the middle of the hole. The obvious feature is that the full plate coating in the hole becomes thinner and the graphic electroplating coating covers the full plate coating.

Four, graphic plating caused by hole wall coating cavity

1. Graphic electroplating micro-etching

The amount of micro-etching in graphic electroplating should be strictly controlled, and the defects produced are basically the same as the micro-etching in dry film pretreatment. When the hole wall will be large area without copper, the whole plate thickness on the surface is obviously thin. Therefore, DOE experiment is the best way to optimize the process parameters to measure the micro-corrosion rate regularly.

2. Poor dispersion of tin plating (lead tin)

Due to the poor performance of the solution or insufficient swing factors such as insufficient tin plating coating thickness, in the back of the film and alkaline etching the hole in the middle of the tin layer and copper layer corrosion, resulting in annular cavity. The obvious characteristics are that the thickness of the copper layer in the hole is normal, the fault edge is obviously etched, and the graphic electroplating coating does not cover the whole plate. In view of this situation, you can add some tinning varnish in acid leaching before tin plating, can increase the wettability of the PCB board, at the same time increase the amplitude of swing.