SMT007 Magazine

SMT007-Oct2018

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52 SMT007 MAGAZINE I OCTOBER 2018 joint. However, the problem with this particu- lar solder joint is the fact it ties to a large ther- mal mass, so a little wire won't do the job. The operator can fix this by grabbing a bottle of flux from the bottom drawer to help that solder to do its job. The problem is this becomes a differ- ent rework process than what is in the work instruction. While the solder joint is now acceptable, the amount of flux residue could be drastically different than what is typically present after using only a cored wire solder. When a bottle of flux is used to help speed up the normal rework process, you will see an increase in the amount of active flux residues. Figure 1 shows how liquid flux migrates into the neighboring component, and if there is no contact with the soldering iron, it will remain conductive and corrosive. Further, Table 1 shows the high levels of ions on the surface of the PCBA, which significantly increases the risk for electrical leakage and/or electrochemi- cal migration. The secondary risk is increased contact time with the solder iron to the PCB. This can cause internal delamination within the fiberglass and if your board is multilayer—which it probably is—delamination can cause connectivity issues between the internal interconnects, such as vias and ground pads. If the deviation from the standard touch-up process isn't communicated in some form, the failure mode will most likely remain a mystery upon the return of the hard- ware to the failure analysis (FA) lab. The next issue you might see on a semiregular basis is called the "third shift issue" that occurs when you see a group of boards with perfor- mance issues processed within a single shift. In general, the third shift has less oversight from production managers than the first or second shift, so the chances of deviations from the normal process occurring and not being docu - mented are higher. I am not saying the oper- ators on the third shift are any less qualified to build production assemblies, but the lack of oversight sometimes leads to a lack of report- ing issues and "creative" resolutions. Don't get me wrong—creativity is important—but can be devastating to the reliability of your hardware. The documentation of any deviation from the normal assembly process is paramount to track any returns and root causes of failure. One other issue more often seen on the third shift versus other shifts is ramping up the belt speed to meet an expected production number. This is sometimes done when the expected numbers of boards to be produced isn't lowered when the number of operators is reduced. I wrote about this in a previous column (so I won't go into the full details now), but when you increase the belt speed of any piece of assembly equipment, the quality will suffer Table 1: Ion chromatography data showing active flux residues. Figure 1: Liquid flux used for rework.

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