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Why does the conformal paint turn white near the PCBA solder joints?

date:2021-01-13 16:20:47


Many smt manufacturers ignore some uncommon problems when doing PCB Assembly. For example, the issue we are going to discuss today: The three-proof paint around the solder joints of PCBA Board turns white, and it will fall off with a slight scratch. The white substance will disappear immediately after heating or cleaning, and it is closely integrated with the PCB. It is easy to be scratched off, how does this cause it?



Bad reasons:

1. Take a small part of the whitening and non-whitening three anti-paint samples, and put them under an infrared microscope for infrared spectroscopy. The number and position of the main characteristic absorption peaks of the two infrared spectra are the same, and similar to the acrylic resin spectrum .

2. Porous tissue can be seen under a high-power microscope.

3. It is preliminarily believed that there is too much solvent in the three-proof paint ratio, which volatilizes to form a porous structure, which will turn white after absorbing moisture, but it does not explain why it is easily scraped off. The reason why it is easy to turn white around the solder joints is because the three-proof paint around the solder joints is thicker.


Bad case:

After using this board for half a year, the three-proof paint film around the solder joints turns white and has poor adhesion to the PCB. It will be removed with a light scrape, as shown in the figure below. But after heating or cleaning, the white color will disappear immediately, and the bonding force with the PCB will be restored, as shown in the figure below.

PCBA solder joints turn white


Solution:

1. Use drying equipment to dry at 110℃ for one hour before PCB processing.

2. The three-proof paint should not be too thick in a single spray, so as to avoid porous paint film


note:

1. The specific reason is still unknown.

2. It is temporarily suspected that no polymerization reaction has occurred.


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