how to avoid the bubble on PCB Green solder mask ink

how to avoid the bubble on PCB Green solder mask ink

The green film on the surface of the circuit board is actually the solder mask ink of the circuit board. It is printed on the PCB mainly to prevent welding. When processing the PCB, it is often encountered that the green solder mask oil of the circuit board falls off.

There are three main reasons for this:

1. The pre-treatment of the PCB was not done properly when printing the ink. For example, there are stains, dust or impurities on the surface of the PCB board, or some areas are oxidized. There are foreign objects on the board surface that are not cleaned up, and the adhesion of the ink on the board surface is affected, resulting in blistering on the board surface. After the grinding plate is treated, ask the time to print the board in the purification car. It is best to print the board within 2 hours to avoid oxidation of the board surface. The line speed of the solder mask pre-treatment is too fast, the temperature of the drying section is not within the specification, and the absorbent sponge is dry and cannot absorb the moisture in the hole, resulting in residual water vapor in the hole. When the board is baked, the water vapor evaporates to the hole mouth, causing the ink to bubble. This abnormality is also prone to oxidation of the board surface and bubbles. The brush width of the solder mask pre-treatment is abnormal, and the copper surface roughness is not enough to affect the adhesion of the ink. At this time, it is necessary to do the pre-treatment again, and strive to clean the stains, impurities or oxide layer on the surface of the circuit board. This is an operational error by the production staff and is also caused by lax management.

2. Short baking time or insufficient temperature. Insufficient pre-baking conditions and insufficient exposure energy lead to severe side erosion during solder mask development, and surface blistering caused by the attack of the liquid during surface treatment. After the circuit board is printed with thermosetting ink, it must be baked at high temperature. Insufficient baking temperature or time will lead to insufficient strength of the ink on the board. The board is then processed according to the process. During the patch processing, the high temperature of the tin furnace causes the ink on the circuit board to fall off.

3. Ink quality problems or expired ink. The ink has not been used up for a long time, and its color will be different after repeated use, and its performance will be greatly reduced, making it more likely for the ink to fall off.

The solder paste used does not match the solder mask ink. During the reflow soldering process, the flux in the solder paste attacks the solder mask ink, causing the solder mask to turn white and bubble, and then cause the solder mask to fall off.

Formation Mechanism

The poor matching between solder paste and solder mask ink is mainly reflected in the following aspects.

1. The influence of the activity of solder paste flux: the stronger the flux activity, the more organic acid content, so the more corrosive it is to the ink.

2. Since the solder paste flux on the solder mask at the bottom of the component is not easy to evaporate and decompose, the attack on the solder mask ink will be greater under the high temperature of reflow soldering.

3. After reflow soldering, the undecomposed active substances in the flux residues remaining on the solder mask will form a long-term chronic attack on the solder mask ink in the area covered by it.

4. When the solder mask ink particles are too large, the attack on the solder mask ink is also greater, and the probability of blistering is also greater.