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SMT007-Sept2020

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22 SMT007 MAGAZINE I SEPTEMBER 2020 are much lower (debatable) but most likely still could have been rooted out with proper upfront reliability testing. This month, I plan to share some testing rec- ommendations based on failure analysis, as well as lessons learned from a few of our cus- tomers over the years using case studies and data on failed units. Make no mistake, I will focus a lot on cleanliness and how it relates to reliability. "Write what you know," they said, so that's my plan. What does reliability even mean? According to the all-knowing internet, reliability is "the qual- ity of being trustworthy or of performing consis- tently well." I think that pretty much sums it up from the 50,000-foot view. When we get a little closer to the ground, we need to expand that to refer more specifically to the class of prod- uct being manufactured. Reliability and Class 1 don't really overlap in the big Venn diagram of quality; it will most likely work when it goes out the door. That's about it in a lot of cases. When looking at Class 2 and Class 3 hard- ware, there is most certainly a need to focus on reliability. According to IPC-A-610, "Class 2 Dedicated Service Electronic Products include products where continued performance and extended life is required and for which unin- terrupted service is desired but not critical. Typically, the end-use environment would not cause failures" and "Class 3 High-Perfor- mance Electronic Products include products where continued high performance or perfor- mance-on-demand is critical, equipment down- time cannot be tolerated, end-use environment may be uncommonly harsh, and the equipment must function when required, such as life sup- port or other critical systems." What this tells me is that not all reliability is equal. When it comes down to it, there are minor differences between these two classes of electronics. Outside of some high-end exotic assemblies, most parts and assembly processes are used for both classes. The biggest difference is what happens if it fails. It's literally a matter of life and death in some cases. Sorry, I didn't mean to bring you down there, but it is impor- tant to remember that. The good news is that most companies building those types of elec- tronics are on top of it with testing that would not be required for many Class 2 assemblies. Enough of the pseudo-philosophical elec- tronics talk; let's get down to it. Approving a new supplier for any part of your process is a major key to reliability because you need to know that the bare board and components aren't going to also supply a surprise down the road. Let's start at the bare board level. When it comes to guidance, anything that is agreed to between the user and the supplier will dom- inate any requirement from any other source. In lieu of any internal guidance, most com- panies lean on IPC-6012: Qualification and Per- formance Specification for Rigid Printed Boards. Looking at the applicable documents specific to PCB manufacturing, there are 23 test methods within the TM-650, 35 related documents, and another 18 joint industry and other association documents. That is a lot of information for those who need it and should cover pretty much every conceivable combination of materials. In no way am I suggesting you need to review each and every one of these documents, but they are there either way. If you start with IPC- 6012, you can go pretty much anywhere in the testing realm, but not all tests are required— or even necessary—for new supplier approval. Some of the parameters to test for include plat- ing thickness on PTH barrels and pads, solder mask cure, conductive anodic filament (CAF) resistance, and cleanliness, among others. Let's look at what some of those tests are looking for and the possible reliability issues tied to those. I'm going to start at layer one of the PCB fab process. Quality really does start there, and What does reliability even mean? According to the all-knowing internet, reliability is "the quality of being trustworthy or of performing consistently well."

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