There can be a dark side to engineering that is rarely talked about, and mostly covered up if it comes to light. I am going to call it Temporary Solution Syndrome, or TSS. This happens when an engineer runs into a problem, most frequently with an existing product design, and the ‘right’ solution is something that will be done at some undefined point in the future. Hence, the temporary solution is born.
Let me share an example. I worked on a design a while ago that had been shipping for a couple years. It was an audio circuit that had been thought to be working flawlessly, but for some unknown reason, started picking up some high frequency noise when idle. When troubleshooting the problem, putting my finger on top of some of the ICs, I could reduce the noise to imperceivable levels. This was an op-amp design, so the usual suspect is instability. Apparently this was overlooked in the design stage and it had not come up until two years into shipments when a customer complaint and subsequent product return revealed the issue. Sure enough, adding a capacitor to the negative feedback loop created a low pass filter effect, and solved the problem. The only issue is that there was not a pad to install this part, so the solution for production was not going to be easy.
The proper way to handle this would be to add the cap in the schematic, add it into the PCB layout, complete an engineering change order, and release the revised design to production. Production would then scrap the material on hand, and wait several weeks for the new PCBs to be fabricated, and build new boards. Problem solved, right? Well, not sure management will tolerate the $2K in scrap, the re-tooling fees, plus a 6 week shut down on the product. So we needed a Temporary Solution! Since we were using surface mount parts, we could just stack the cap right on top of the feedback resistor. Surely we can deal with that for a couple months of production. We can even satisfy IPC-610 “Acceptability of Electronic Assemblies” with SMT stacking, and even ISO9000 by documenting what we do. Great! Then of course we will follow up when we have a free moment with a revised design, and let it naturally work it’s way through the supply chain, creating zero scrappage.