SMT007 Magazine

SMT007-Sept2020

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26 SMT007 MAGAZINE I SEPTEMBER 2020 plan needs to include monitoring the approved assembly process, they choose test coupons that are most representative of their final prod- uct based on the mix of SMT and PTH com- ponents. They then assemble boards using the proposed combination of materials and equip- ment to be used for the final product. Along with two bare reference samples, the assem- bled test boards are tested per IPC 2.6.3.7. If they pass that test, they are tested with ion chromatography to determine the average lev- els of specific anions, cations, and WOA to cre- ate baseline data. Next, they build a set of 20 samples of the actual product. A set of 10 boards are tested using ion chromatography with global extrac- tion. The second set of 10 is tested in a ROSE tester. The average of the ROSE test results is the acceptance criteria used on a per shift basis. Remember that the number is being derived from your ROSE tester and can differ from another machine of the same make and model. It doesn't really matter if that num- ber is 1 or 101 µg NaCl equivalence per square centimeter. That number has been verified with other testing. Often, IC testing is done on a quarterly basis for further evidence of process control. The quarterly test results are compared to the baseline. Some customers will also perform elevated heat and humidity exposure testing with nor- mal operating voltages to further validate the acceptance criteria. This is known as temper- ature-humidity-bias (THB) testing and is simi- lar to SIR testing. THB testing is done on actual products using normal operating voltages and duty cycles. This is one of the most important tests to consider because while the test cou- pons are considered predictors of performance, a lot of things change when it's the real deal. A large percentage of reliability failures we see are tied to cleanliness. In this column, I have addressed bare board, raw component, and test board assembly cleanliness, but those are only three sources for contamination out of a much larger number of options. Anything that can come into contact with the PCBA, either directly or indirectly, is a possible source of contamination. You must consider testing everything around the PCBAs, such as hous- ings, large connector bodies, and any other number of materials. We see a lot of failures that have good objec- tive evidence of their assembly process, but because they were only testing the PCBAs, they don't see the full picture. Materials like mold release on metal and plastic housings can be very ionic. If enough atmospheric moisture is available, it will collect at a low point and drip down on the board. That moisture can contain high levels of ionic content from the housing interior surface. We have seen vibration dampening foam be extremely high in ionic content that was pressed directly against the surface of the PCBA without doing any cleanliness testing on the material (Figure 9). This was used in an under-hood application and not hermetically sealed. This was done on purpose by someone getting paid to make those types of decisions. It can happen to the best of us. With any luck, someone reading this right now will start to think about every part of their product outside of just PCBA manufacturing that can impact their product reliability. I have barely scratched the surface on reli- ability testing, as so many are product-spe- cific. Some products require a lot of vibration or extreme temperature exposure testing, but what I have covered applies to every product foundation. The title of my column this month Figure 9: Impact of vibe dampening foam.

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