PCB + ASSEMBLY Acrylic
PCB + ASSEMBLY Acrylic Coatings Urethane Coatings Strengths Weaknesses Strengths Weaknesses Air dry VOC bearing solvents Chemical resistant High stress during temp. cycling Easy solvent rework Poor chemical resistance Humidity resistant Rework Good moisture barrier Flammable Abrasion resistant Long cure rate Ease of use Solvent in high temperature Dielectric properties Heath Risks/Solvent An example of adhesion test results. areas with the soldering performance being the most critical issue. This includes solder joint attributes such as wetting, voiding, and joint strength. Flux residue also receives a great deal of focus because the only materials on the substrate/PCB after soldering are the solder joint and the flux residue (more on residue later). Other considerations are paste print functioning which includes transfer efficiency, stencil life, shelf life, etc . The coating and cleaning attributes of no-clean solder residue can only be addressed after these more critical functions are satisfied. Therefore, there are paste and flux products that exhibit poor compatibility with coatings due to developmental priorities. Source: AIM Solder Test methods and pass/fail criteria Without an industry specification to determine compatibility between coatings and flux residues, designers and assemblers have to develop their own test methods and accompanying pass/fail criteria or rely on the experience and recommendations of their vendor base. AIM has collaborated with multiple major coating suppliers on a variety of customer applications to understand how different material sets will perform. The test approach has been to merge the pertinent specifications from the IPC J-STD-004 A/B and IPC CC-830 conformal coating specifications to provide insight as to how the materials will perform in the field. Solder paste, liquid flux, and cored wire residues were all subjected to the same battery of tests. The first test performed was visual inspection. B-24 SIR test coupons were selected as the test vehicle because the coating thickness is consistent, they do not introduce topographical variability, they are inexpensive and all three tests could be executed on the same style test coupon. After applying flux on coupons, they were coated and cured per the coating manufacturers recommendations. Adhesion issues were usually readily apparent at the time of coating with non-wetting, mealing, or blistering appearing right away. Liquid fluxes were more prone to fail at this point in the process which was attributed to the wetting agents used in the flux and the fact that the residue was present globally, whereas paste and wire residues are location specific. Materials that failed visual inspection after coating was applied were deemed incompatible and did not receive further consideration. The second test performed was surface insulation resistance (SIR) Results of solder flux SIR test; any value under 100 MΩ is a fail condition. Source: AIM Solder 46 EPP EUROPE November 2017
PCB + ASSEMBLY Silicone Coatings Epoxy Coatings Strengths Weaknesses Strengths Weaknesses Humidity Resistant Abrasion Humidity resistant Two-part High dielectric strength Workplace contamination Moisture resistant Rework Flexibility/Low modulus Sulfur absorption Abrasion resistant Pot life Temperature tolerant Dielectric properties testing per J-STD-004 A/B. This test is common to flux manufacturers and establishes the insulative properties of the flux residue under temperature and humidity conditions that promote electrochemical interactions/migrations (40° C/90 % Rh). An apparent failure is any reading under 100 MΩ. Dozens of flux and coating combinations were subjected to these tests and patterns emerged between different classes of coatings with different types of residues. Some flux/coating combinations failed outright, due to what was hypothesized to be either an inhibition of curing not detected at the time of coating or a third product forming when the two materials combined. Some material sets showed little change versus uncoated residues while others showed significant changes with coating applied. Thermal shock testing under a wide range of parameters was also performed to understand how the coating durability was affected when applied over flux residue. Again, dozens of material sets were tested and patterns emerged. In general, a liquid flux that passed the visual test also passed thermal shock adhesion testing, regardless of coating technology. Paste results were much different, low modulus coatings such as silicone easily passed thermal shock adhesion tests, but more rigid coatings such as urethane acrylate, exhibited fracture and delamination much earlier and in a narrower temperature range. Surprisingly, exposure to lower temperatures (
