Electronic circuit board components PCBA (Printed Circuit Board Assembly) as the core of the electronic components, should have good resistance to environmental adaptability and electrical parameters. Moisture + dust + salt spray is an important factor in PCBA failure. Electronic circuit board components coated with three anti (anti-salt spray, anti-heat, anti-mold) paint can withstand the impact of harsh environments on the circuit and components, increase the mechanical strength and reliability, to prevent sudden changes in temperature caused by condensation Wire leakage between the short circuit and even breakdown, for working in high voltage and low pressure conditions of the printed circuit board can improve the wire between the creepage, breakdown phenomenon, thereby enhancing the reliability of the product.
The three anti-protective performance
In the actual work, the three anti-protective performance mainly by accelerated aging test method to verify.
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1, PCBA coating wet heat aging test after the failure of the common situation and analysis
PCBA coating hot and humid test failure common phenomenon mainly coating off, bubbles, loss of light, white and so on. Through the collection, finishing in recent years, three anti-PCBA hot and humid test the emergence of various situations, common in the following several.
(1) the coating formula, the number of spraying, spraying thickness, baking temperature and other process parameters are different, the test results vary widely.
(2) in the winter and summer, using the same process parameters, spray the same appearance, but in the hot and humid aging, the summer spray coating prone to fine bubbles, the surface of the coating on the winter often particle phenomenon, the main analysis Is caused by the impact of environmental humidity.
(3) the machine is often purchased PCBA comes with three anti-coating hot and humid test after a serious loss of light, powder or large area whitening phenomenon; also exists part of the outsourcing of power supply module class three anti-stick coating The situation, the exclusion of paint selection factors, the reasons for the occurrence of such cases mainly due to improper handling of three anti-coating caused by the way.
(4) product chassis structure in different forms, the coating failure degree is inconsistent.When the hot and humid test, the sealed chassis or directly placed in the test box in the sample processing is good, some ventilation chassis PCBA coating in varying degrees of bubble phenomenon.
(5) the same printed circuit board solder mask is different, after the hot and humid test coating protective effect is also different. Using PCBA blue solder mask, matte green solder mask polyurethane coating appeared serious dense blistering, from the layer, bright green solder mask no exception.
(6) different coating protection and use of different occasions, resistance to heat and heat are not the same.
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2, by analyzing the above situation, resulting in PCBA coating wet heat test loss of light, powder, foaming failure factors are:
(1) the coating formulation, coating thickness, baking temperature and other process parameters;
(2) the effect of air humidity on the coating performance during the production process;
(3) the effect of the cleaning process before coating on the coating performance of PCBA;
(4) the impact of the chassis structure on the coating performance; PCB solder mask type on the coating performance;
(5) the effect of the coating material on the wet heat resistance of the coating.
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Therefore, in the PCBA three anti-spray before the need to clean the printed circuit board. To achieve cleanliness indicators, commonly used cleaning methods are alcohol, gasoline, trichlorethylene or water cleaning process, recommended the use of water cleaning art, the cleaning process green, the use of the process can completely remove the flux residue. The power module in the power module module is usually encapsulated with silicone oil. It is soaked in alcohol, gasoline and other solvents. It is prone to chemical reaction, oozing silicone oil pollution board, because PCBA is contaminated by organic silicon , The coating will produce discontinuous areas. Resulting in the coating can not even attach the board surface and affect the protective performance, for the power module type device printed circuit board, the actual work is usually used to dip the solvent for local scrubbing way to avoid the power module silicone oil leakage of pollution caused by Non-stick paint affects protection performance. Inspection coating protection effect is actually to study the degree of bonding paint and solder mask, different solder mask film due to the composition, content is different, and the size of the coating adhesion is inconsistent. The degree of adhesion of the coating and the molecular polarity of the printed circuit board are closely related to the molecular polarity of the coating, and the surface polarity of the matte solder mask is not analyzed.
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As the non-polar polymer surface energy is low, difficult to form a low-energy polymer with low-energy combination, so wetting is not good, can not be well bonded. And non-polar polymers and non-polar polymers can produce good adhesion between the similar structure of good mutual solubility, is conducive to diffusion, easy to bond firmly and get excellent adhesion. From the protective performance, the best silicone, followed by polyurethane, acrylic times.
3, the conclusion
Through the above test analysis, to get excellent moisture resistance of the PCBA coating, should be in the following aspects of process control:
(1) process parameters, three anti-paint formulations and spraying methods and thickness, curing temperature is the main factor. At the same time air humidity, compressed air quality, pre-sprayed PCBA pre-baking tungsten should be used as an important parameter control process parameters.
(2) PCBA coating before the PCBA coating must be thoroughly cleaned welding residues, dust, hand sweat and other pollutants to ensure that the varnish coating and PCBA solder resistance or device adhesion between the good.
(3) from the test results, PCB solder mask color should be preferred green bright (GRN) solder mask, recommended PSR-2000KX700G.
(4) chassis structure on the PCBA board has a greater impact on the protective performance, ventilation-type chassis easier to accelerate the aging of the coating.
(5) from the coating process test comparative analysis can be seen, should be based on the use of the product environment selection of paint, acrylic coatings suitable for temperature and humidity control of the indoor environment, polyurethane coating good wet heat resistance, coating hardness, bright, , Stain resistance than silicone paint more advantages, but the silicone can be reusable and harsh environmental adaptability is better than polyurethane.