Using a hot plate for soldering flex
circuits
The idea with using a hot plate when soldering a flex-circuit is to
create a high temperature boundary condition, so that very little
additional heat is required to form a solder joint. Even heating the
substrate up to 100 degrees F, by setting the plate on low and placing
the circuit directly on the plate can significantly reduce the amount
of heat from a soldering iron required to form a solder joint. This is
actually advice that I got from Dave Zephyr of Zephyrtronics.com, and
having tried it, I have to say, IT WORKS! There are also, some
additional benefits such as less thermal shock for electrical
components. In addition, one can leave the flex-board on the plate and
slowly ramp down the temperature.
One more great thing about using a hot plate when using solder paste is
that the small amount of heat can help precisely place solder paste on
contacts. A small amount of heat is enough to wet-out and
activate the flux in the solder paste so that a quick dab of the solder
iron and instant solder joint.
Actually, contrary to my statement on the referencing page, you can use
a hot plate even for flex circuits with mylar substrates, you just need
to be careful. What tends to happen, is if the mylar gets a little too
hot, the residual stresses that are put into the mylar when it's rolled
out get relieved when it gets hot and the substrate contracts, leading
to a curled flex-board.
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