For a long time now, I had a fear, that when you solder a SMD IC to a PCB, that you won't be able to desolder and resolder it again without destroying it. Recently, I did just that, but it didnt damage the chip, so I feel a bit better. I used a hot air gun at my college. I did it because not all pins were soldered proper before.
Q:
What is the rule of thumb, how many times can I desolder and again solder the chip, to save the cost on testing?
Can it be infinite or will it start producing magic smoke after a while?
Answer
The rule of thumb is 'assume a chip you've desoldered from a board is damaged, until you've proved otherwise'. In other words, don't solder an SMD IC down to test it.
There's a significant difference between automated SMD soldering, in a temperature controlled oven with a calibrated warm-up and cool-down profile, and eye-balling the melting of solder with a hot-air gun. With the latter, it's so easy to get it a few 10s of degrees too hot, or dwell there for a few seconds too long.
The junctions made in an IC, often by thermal diffusion, can also be unmade by over-diffusion. The damage an IC gets from over-temperature is cummulative. The amount of damage an IC gets is also exponentially related to the temperature. It's impossible to give you a figure for how many times you can heat it to above its (often recommended) 150C limit, without being very precise about the temperature.
Whether it 'works' is a poor indicator of how much damage you've done after one solder/desolder cycle. If you instead measure the input leakage current, power supply quiescent current, noise level, gains before and after the thermal abuse, then you'll have an idea of how much shift in parameters that cycle caused, and how many more cycles it can tolerate before the parameter shifts amount to 'not working'.
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