SMT007 Magazine

SMT-July2018

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48 SMT007 MAGAZINE I JULY 2018 ical migration (also known as dendrite growth). These are hard shorts and solid failures that won't necessarily leave. When looking at an electrical leakage path, three ingredients must be present: conductive residues, voltage differ- ential, and available atmospheric moisture. If you can remove any one of these three pieces, the likelihood of experiencing issues is greatly reduced. In the real world, removing either of the last two ingredients is not a good plan to make money, so let's assume that power will be applied and the product will indeed be used in some type of atmosphere. Thus, the focus shifts to the level of cleanliness. This is the one thing that can be controlled and monitored before shipping prod- uct to the field. Let's start at the begin- ning and look at each processing step related to each material choice. The base for most assemblies is a stan- dard laminate printed circuit board. The old saying is, "Garbage in, garbage out." This is especially true when processing with a no-clean flux type because there is not a final wash process that can make up for the sins commit- ted upstream, so if you start with a dirty PCB, it will surely get worse before the product is complete. When the topic of PCB cleanliness comes up, a traditional bulk ionic test might be required or supplied. However, without corre- sponding data from a more sensitive analyti- cal test, such as ion chromatography or surface insulation resistance, the number may supply little more than a false sense of security. Too many OEMs and CMs look to others to decide what is acceptable and simply want to check a box to show due diligence, but if we have learned anything from our time analyz- ing field failures, it's that each manufacturer needs to own their process to build in qual- ity based on the use of a specific product. The main problem is there isn't a one-size-fits-all number to use because each product has its own set of variables, like voltage, spacing, metallization, and end-use environment. This has led to many companies seeing issues in the field on product that passed the industry accepted test. They don't know where to turn next, but I digress back to bare board manu- facturing. Many times, PCB fabrication suppliers cut costs by using less than ideal water quality for washing and rinsing the etch and plating residues. This could mean anything from local river water (not a joke, I have seen this) to tap water with unknown amounts of dissolved solids and ionic content. Tap, or river, water has an increased surface tension that can prevent effec- tive rinsing of vias around 8 mil or less on bare boards. This will leave behind processing chemis- tries that easily facil- itate electrical leakage and corrosion. Most often, the only testing performed to supply a certificate of compliance is dimensional infor- mation. While this is equally important to the cleanliness data, it only tells you half of the story when looking at incoming bare board quality. Components go through similar process- ing steps as bare boards. Both have metalli- zation over a base metal and cleaning steps that include harsh chemistries that can induce issues in the field. When large enough amounts of the plating chemistries are left on the metalized finish, corrosion can propagate without additional moisture or voltage. These two factors can expedite the corrosion but are certainly not required. After the bare board and components are chosen, they are assembled together using harsh processes that include aggressive flux chemistries and soldering temperatures, some-

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