| With the rapid development of science and technology,many electronic products or equipments need to work in various harsh environments,such as high temperature,vibration,humidity,salt fog and dust.These external complex environments seriously endanger the performance and service life of electronic components and their products.In order to enable electronic products to resist the erosion of the external environment and prolong their service life,it is necessary to coat a protective coating based on synthetic resin or polymer on the surface of printed circuit boards and their components,which is also called conformal coating or three-proofing coating of PCBA.The coated film can protect the electronic components and circuit board substrates from the harsh external environment,which can significantly improve the life and reliability of electronic products.However,there are many different types of conformal coatings with varying quality in the market.How to choose a conformal coating with excellent performance,good protective effect and adaptability to its own manufacturing process and application environment has become an urgent problem to be solved.At present,although there are mature standard methods for the quality evaluation of the conformal coatings in the industry,critical assessment items affecting the use of the conformal coatings in these methods are lack,such as compatibility,special environmental adaptability,etc.More importantly,according to the existing standard methods to evaluate the conformal coating,it can only meet the basic quality qualified certification of the product,and is unable to help users to select safe and reliable conformal coatings which are suitable for product characteristics.In this work,according to the requirements of environment and working conditions of PCBA and its products,a reasonable evaluation system was designed and established for conformal coatings used for PCBA.Six different types of conformal coatings were selected and tested with the evaluation system,and their quality ranking was obtained.Then different ranking coatings were coated and characterized to confirm the scientific effectiveness of the evaluation system.The main research contents and results are listed as follows:(1)According to the performance characteristics of electronic products and the application requirements of the three-proofing coatings for PCBA in industry,a complete and comprehensive evaluation system was established for the conformal coatings by selecting key performance parameters.The evaluation system included two parts of properties evaluation and cross-compatibility evaluation.The scoring system was adopted,and the parameters of each item in the system was set different weight scores according to their influence on the reliability of electronic products.The total score of all items in the system was 200 points.The weight of the properties evaluation of coatings was 100 points,including 6 aspects:(1)Physical and chemical properties scored 8 points(fluorescence-2,acid value-3,non-volatile content-3).(2)Electrical properties scored 12 points(electrical strength-3,volume resistivity-3,moisture and surface insulation resistance-6).(3)Safety properties scored 24 points(flash point-5,toxicity test-5,six items of Ro HS-5,flammability-6,halide content-3).(4)Quality reliability performance of conformal coating scored 17 points(thermal shock-3,temperature resistance-3,flexibility-2,adhesion-6,water absorption-3).(5)Three-proofing propreties scored 31 points(liquid resistance-5,temperature and humidity aging-6,fungus resistance-6,salt fog test-6,humid sulfur vapor test-8).(6)Process adaptability scored 8 points(dry time of cured film surface-2,heating and dry time of cured film surface-2,repairing test-4).As a relatively independent part of the system,cross compatibility was 100 points,and was mainly evaluated from the appearance and insulation resistance value.Technical requirements were subdivided into six grades,the highest reliability grade of the score was set to 100 points,the grade not meet the requirements set to 0 points,the interval between the intermediate grades was 10 points.(2)Six types of conformal coatings widely used in industry from different manufacturers were named as A,B,C,D,E and F,respectively.The coating A and B were polyurethane modified alkyd resin produced by two different manufacturers,the coating C,D,E and F were organic silicone resin,acrylic resin,styrene-butadiene rubber and polyurethane type coatings,respectively.Six types of conformal coatings were evaluated with the established evaluation system.First,their own properties were evaluated,and the final scores were 68.84,74.67,87.75,78.18,81.85 and 88.51,respectively.According to the technology process,the samples were prepared with the auxiliary materiala and the six conformal coatings,and then the cross-compatibility was scored with the evaluation system.The compatibility scores of these six conformal coatings were 37.20,39.40,53.20,61.40,41.20 and 46.40,respectively.The scores of the two parts were added and the comprehensive scores of the coatings were calculated as follows: 106.04 of A,114.07 of B,140.95 of C,139.58 of D,123.05 of E and 134.91 of F.Final comprehensive quality order of six types of conformal coatings was obtained as C>D>F>E>B>A,here ">" represented "superior ",obviously the coating C comprehensive quality was optimal,followed by D,the worst was A.(3)To verify the applying effectiveness of the evaluation system in short time,humid sulfur vapor test and salt fog & temperature and humidity aging test sensitive to the components and materials of PCBA were performed.To save experiment cost and shorten experiment time,only three ranked conformal coatings were selected,including conformal coating C ranking 1,the coating E ranking 4 and the coating A ranking 6.These conformal coatings were respectively coated on the PCBA surface,and the above environmental tests were carried out after the coating cured.The final test results showed that the PCBA coated with coating C had the highest adaptability to vulcanization environment and salt fog & temperature and humidity environment and the longest life in this harsh environment.Therefore,the conformal coating C had the best protective effect for PCBA.Comparatatively,the conformal coating E showed the worse protective effect,and the conformal coating A had the worst protective effect.The results were completely consistent with the comprehensive and quality ranking of the conformal coating,verifying the effectiveness of the evaluation system. |
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