PCB Tech - Common problems of PCBA coating damp heat test failure

Electronic circuit board assembly PCBA (printed circuit board assembly), as the core component of electronics, should have good environmental resistance and electrical parameters. Moisture + dust + salt spray are important factors for PCBA failure. The electronic circuit board components are coated with three-proof (anti-salt spray, heat-proof, anti-mildew) coatings, which can withstand the influence of harsh environments on circuits and components, increase mechanical strength and reliability, and prevent sudden temperature changes from short-circuiting and uniform breakdown through condensation lines The leakage between the wires can improve the leakage and breakdown between the wires for printed circuit boards working under high and low voltage conditions, thereby improving the reliability of the product.

Common problems of PCBA coating damp heat test failure PCB proofing

In actual work, the three protective properties are mainly verified by accelerated aging test methods.

1. The common situation and analysis of PCBA coating after the failure of the same damp and heat aging test

Common phenomena of PCBA coating failure in damp and heat test mainly include coating peeling, air bubbles, light loss, white light, etc. Through the collection and sorting in recent years, the emergence of various situations of three-proof PCB, damp and heat test, are common in the following types. (1) The coating formula, spraying times, spraying thickness, baking temperature and other process parameters are different, and the test results vary greatly. (2) In winter and summer, the same process parameters are used to spray the same appearance, but in the hot and humid aging, the spray coating in summer is prone to produce small bubbles, and the surface often has particle phenomenon on the winter coating. The main analysis is Caused by the influence of environmental humidity. (3) The machine often buys PCBA with three anti-coatings after damp and heat test, which causes serious loss of light, powder or large-area whitening; there are also cases where some power modules outsource some third-level anti-adhesive coatings, and coating selection factors are excluded. The reason for the situation is mainly due to improper handling of the three-proof coating. (4) The product chassis structure is different, and the coating failure degree is inconsistent. During the damp heat test, the sealed chassis or the samples directly placed in the test box are processed well. Some ventilated chassis PCBA coatings have different degrees of air bubbles. (5) The solder mask of the same printed circuit board is different, and the protective effect of the coating after the damp heat test is also different. Using PCBA blue solder mask, the matt green solder resist polyurethane coating has serious dense blistering and layer formation, and the bright green solder mask is no exception. (6) Different coatings protect and use in different occasions, and the heat resistance and heat are different.

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2 The same, by analyzing the above situation, the PCBA coating damp heat test loses light, powder, and foaming failure factors include: (1) coating formula, coating thickness, baking temperature and other process parameters; (2) the influence of air humidity on coating performance Affect the production process;

(3) The effect of the cleaning process before coating on the performance of PCBA coating;

(4) The influence of chassis structure on coating performance; the influence of PCB solder mask type on coating performance;

(5) The influence of the coating material on the moisture and heat resistance of the coating.

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The following is a low-energy polymer with "non-polar polymer with low surface energy, and it is difficult to form a low-energy combination, so it has poor wettability and cannot bond well." Non-polar polymer and non-polar polymer Good adhesion can be produced between them, similar structures have good mutual solubility, which is conducive to diffusion, easy to bond firmly, and obtain excellent adhesion. From the protection performance, the best silicone, followed by polyurethane and acrylic time.

PCBA protection

3. Conclusion

Through the above-mentioned test analysis, in order to obtain the excellent moisture resistance of PCBA coating, the following process control aspects should be adopted:

(1) Process parameters, three anti-paint formulations and spraying ng method and thickness, curing temperature is the main factor. At the same time, air humidity, compressed air quality, pre-sprayed PCBA and pre-baked tungsten should be used as important parameters to control process parameters.

(2) PCBA coating must thoroughly clean PCBA coating welding residues, dust, hand sweat and other contaminants to ensure good adhesion between the varnish coating and PCBA solder resistors or devices.