SMT007 Magazine

SMT007-July2019

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58 SMT007 MAGAZINE I JULY 2019 test and should continue. At a minimum, the next test should be an analysis of the stackup using cross-section. Many PC fabrication shops will send a cross-section puck sample with each lot. However, most of the time, I see them tucked away in a drawer in a bag stapled to a CofC, but they have not been examined by the customer. Believe it or not, a file cabinet does not take any measurements. One of the main issues with simply filing away a CofC and a puck is building product under the assumption that what the PC fabrication shop sent aligns with the print requirement. Only when a fail - ure is detected does anyone go back and exam- ine what was sent. Plenty of CMs do the work and examine the puck, but I can also say it cer- tainly isn't all of them. You know who you are. Another very important test for bare boards is cleanliness testing of some sort with histori- cal data or objective evidence to back up your acceptance criteria. This process is now clearly defined in the IPC J-STD-001 Amendment 1. This is especially important if your product is built with no-clean flux as there is not an end- of-assembly wash process to make up for sins upstream. If you start with a dirty bare board, you may experience issues related to electrical leakage or electrochemical migration that have nothing to do with your assembly process, so it's best to know up front if you're fighting an uphill battle. There are many other tests you can perform on bare panels, which can be found in the IPC 4000 and 6000 series documents with IPC-A- 600 as a visual reference guide. Depending on the end-use environment, there may be a much larger suite of test you need to perform. This needs to be discussed with the PCB supplier and customer. Once you are satisfied that you are getting what you asked for regarding the bare boards, it's time to move on to your assembly. Material selections are often done using test coupons and some sort of elevated heat and humidity testing, which is a great idea. Surface insulation resistance (SIR) testing per IPC TM-650 2.3.6.7 has become a go-to test for determining how well your equipment can process the fluxes and what effect elevated heat and humidity will have, The idea is to either leave behind res - idues that are near benign or fully removed in the case of water-soluble flux types. There are also material compatibility tests for conformal coating, potting, adhesives, underfill, etc. It's important to know what effect different mate - rials will have on the product. Not all materi- als play well together, so if you plan to coat or use some other type of surface encapsulation, these tests can be very useful and eliminate a potential issue before it manifests in a failure. After all of the materials have been selected, it is time to move on to building your PCBAs and even more testing to see what your reli - ability risk is based on all of the choices and testing done up to this point. Of course, there's more testing; there's always more testing if you care about reliability. Again, you can look to IPC for a myriad of testing and need to consider the end-use environment when determining exactly what test you need, but I will cover a few of what we consider to be the most impor - tant tests are. The IPC documents you need to review are J-STD-001 and the companion IPC- A-610 for visual acceptance. These documents are a great reference tool if you have any ques- tions about the assembly process. And no, I don't work for IPC, but I do spend time in task group meetings making a small contribution to help make the decisions that ultimately go into the assembly documents. One of the first tests I recommend is ion chromatography to determine the level of cleanliness after assembly. IC will tell you the exact type and amount of ionic contamination, which is crucial when determining the risk for Many times, the only verification testing I see being done on bare boards is dimensional measurements.

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