Being able to UN-solder any component without pulling pads off the board is an art in itself. I've worked at places where engineers were not allowed to solder unless they first proved their skills to the boss, and with good reason! Will never forget the young eng who butchered about 10 prototype boards replacing "bad" DRAM controllers before finally concluding that maybe there was an error in his design.
Another consideration is the handling of components before soldering. Everything has a shelf life. Sometimes it will be documented and sometimes not. This holds for components, PC boards and the materials used to join them.
Parts and PC boards absorb moisture over time and the metal surfaces collect impurities or react to impurities over time. Boards and parts can be baked to mitigate the moisture issue, but sometimes the only solution for old parts is to discard them.
Silver finish PC boards are great when fresh, but can and will tarnish over time, especially if not stored properly (sealed in a cool, dark place). PCB manufacturers tend to say that silver boards don't tarnish, but empirical evidence says that they do.
ENIG (electroless nickle immersion gold) is a great surface, but can be degraded too. I've seen fingerprints etched into a component land on gold boards.
Part leads can oxidize, collect dust or react to contaminants in the air. And, of course, rework is best prevented before it's needed. As Jim says, each heat cycle takes a toll on the component and PCB.
Engineers may not perform much actual soldering but they do choose the fluxes and component surfaces that the actual soldering personnel must use. Engineers evaluate the performance of soldering personnel. Engineers lay out the assemblies. Most soldering problems begin with engineers who don't understand the consequences of their actions. That's why engineers need to understand soldering.
Jim Smith President Electronics Manufacturing Sciences, Inc. www.emsciences.com
I graduated electrical engineering in 2011. In all my 5 years of study they never showed or mentioned soldering. Luckily I got some practical soldering skills during my high school years and got a good mentor once I started working.
After reading this article I think I should inform myself more about soldering.