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106 SMT007 MAGAZINE I MARCH 2019 issues with this process are how and where parts are stored. Many times, I see warehouse racks that are in the same room as the receiving dock that is fully open to the outside elements. This can create spikes in excessive humidity that can impact parts that aren't marked for moisture-sensitive packaging. For example, if it's a dock in a normally dry climate such as Mexico and it rains for some reason, the air can become saturated with dust and contaminants from the typically extremely dry ground. If a breeze carrying some of the dust and contaminants settles on top of the stored raw goods, this can cause issues with solderability or other reliability-related prob- lems. If partially used packs of bare boards or components are stored in this environment, the risk increases. That is why it's important to have the smarts to separate the receiving dock and storage areas. Moving on to the paste print process, the main thing to be smart about is controlling the humidity in the printer to help reduce solder balls and similar defect types. Beyond that, keeping track of the print life of the paste and the screen clean are the main opportunities for introducing some sort of failure. The reflow process is the first opportunity to do some real damage by not being smart. The top things to think about are oven mainte- nance and proper thermal profiling of assem- blies. Oven maintenance is something that I see scheduled anywhere from weekly to annu- ally with the level being varied. It is essential to keep the chains oiled and the fans exhaust- ing, but it's also important to keep the interior free of excessive residues that collect and rede- posit back on the PCBA surface. I normally see interior cleaning no more than twice a year, and depending on the number of pieces ran through, that may be woefully inadequate. The paste flux residues are normally very concentrated forms of flux activators, and any of this material will be detrimental to reliabil- ity—no question about it. Properly profiling the assemblies is equally important because if the areas with the highest thermal mass aren't monitored, the required temperatures for ren- dering no-clean flux near benign may not be reached and the activators will remain active. If no-clean flux residues aren't properly pro- cessed, they can be as detrimental as leaving a water-soluble flux behind after wash. Not smart. The PTH wave solder process is among the worst offenders of all as there are many ways it can go wrong. It truly takes a smart opera- tor to know how to properly run a wave sol- der process. The main issues are excessive flux penetrating either the top side of the board or under selective pallet keep-out areas. If the pallets aren't properly cleaned, flux residues can build up in the corners, and any part of the board that comes into contact with that is at a greatly increased risk of field failure (if it even makes it that far). If the hold-down springs don't have equal pressure at all points, there is also a risk that the board will raise up on the side that has less pressure. This allows spray flux to be deposit- ed on areas of the assembly that were never in- tended to see flux. Any part of the board under the pallet will be shielded from the full thermal energy and remain active. What I am trying to convey is that it's not smart to leave any active flux residues on an assembly; you don't have to be smart to know that. Next up are any hand solder operations that take place post-SMT or wave. This is where the smartest of operators are needed because there is nothing more dangerous to the reliabil- The reflow process is the first opportunity to do some real damage by not being smart. The top things to think about are oven maintenance and proper thermal profiling of assemblies.

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