remove all components from a board with oven/heater?

Question

I've seen a couple of comments on the 'net that it's possible to mass-remove components from a circuit board by heating the whole board (in an oven, or over a heater) enough that the solder will melt, and the components will fall out if the board gets a sharp tap. This sounds like both a very bad idea, and a very bad idea, at the same time.

Questions:

1. How hot would the air around the board need to be?
2. At what temperature would components start getting damaged?
3. What components would get damaged first?
4. Is there a reasonably reliable and safe way to do this? (I.e. one that doesn't require melting lead in an oven that must later be used for cooking)

Answer 1

1. The solder used in for much of consumer electronics melts at temperatures of about 180°C. So temperature-wise there is no problem using a normal oven to do the job.
2. Even at 180°C components will unavoidably get damaged. Different components react to heat differently.
3. The first to suffer are typically connectors with plastic housings (e.g. to interface with a PC etc.). They are extremely difficult to remove without damaging the plastic. Other components tolerate heat better. The lifetime of aluminum electrolyte capacitors however is likely to be reduced by repeated heating. With ICs, there is less of a risk of classic thermal damage but rather due to mechanical stresses resulting from too fast heating or cooling. - On the whole however, components will not suffer too much if you don't keep them heated for more than a few minutes.
4. If you want to avoid using an oven for heating, you can fix the board horizontally in a vise and heat it from below with a heat-gun until the solder liquifies. Then take the board with pliers and tap it vertically on the table. The components will fall off quite easily.

Attention: with a heat-gun it is easy to over-heat the board or heating the board too rapidly. Solder in vias may be ejected outwards due to the rapid heating: wear eye-protection if you really want to do this.

Generally however it is not worth recovering components en masse from assys: What you recover will typically be proprietary chips you cannot really use, tiny smd capacitors too small to handle, and unmarked smd capacitors and inductors. - I propose you only remove those components that you really want/need.

Answer 2

1. That would depend on the type of solder used, different alloys have different melting points.
2. That depends on the components, connectors, or other components that have plastic bodies tend to deform if the temperature gets to high. Usually component datasheets have the maximum temperature allowed for reflow soldering.
3. Usually the components that will fall first are the heaviest, surface tension of the solder tend to "grab" the lighter components. Here solder flux would help.
4. A hot air gun could solve the problem.

Answer 3

For various values of safe, I'd also suggest the heat gun method:

http://www.youtube.com/watch?v=vBjPH3BQWrU

Answer 4

My personal way for mostly THT PCB is:

a) lock the board vertically, from a side, in a bench vise.

b) use an air gun to heat the part of the board farthest from the vise.

c) when the solder melts, bend with a finger (gloved!) the board and let it go. Almost every component will be "launched" far from the board at once.

d) rotate the board and repeat.

Obviously, kids don't try this at home :-)