Re: gEDA-user: Soldering fine pitch chips

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

On Fri, Mar 10, 2006 at 09:41:49AM +0100, Karel Kulhavy wrote:
> I once replaced a 1.27mm SMD chip on a motherboard and had only
> transformer soldering gun. So I cut a piece of copper tin with a hole
> in the middle. I made sure that the long edge is straight. Then I
> bolted the tin to the loop of the transformer gun and used it to heat
> all pins on one side at a time. It was easy to solder and desolder.
> After rinsing with isopropanol the soldering looked like from a factory.
> 
> Do you think this method could be used to solder down fined pitch chips
> like 1mm 0.7mm or whatever is common today?

I have been assembling quite a number of boards with 0.5mm pitch chips
on both sides, plus some other really small stuff like 5-pin SOTs (not
sure exactly what the pitch is on those -- pretty close to 0.5mm
though).  I reflow solder them in a toaster oven, but without any
fancy controller -- just myself and a flashlight to tell when the
solder melts.  It works quite well, though I often have to fix solder
bridges with desoldering braid after they come out.

I have also soldered 0.5mm chips by hand, one lead at a time, in a
couple of cases where I needed to replace a chip after the board had
been reflowed.  Actually, I ended up doing two leads at a time, using
a chisel-shaped tip that's about 1mm across, which works fine.  In my
case, there was enough solder left on the pads from the original chip
that I didn't need to add solder to most of the pads, rather I just
got each lead hot enough to make the joint.  0.5mm pitch leads really
only need a tiny amount of solder.  The greatest difficulty is to
avoid applying too much pressure with the tip, which will easily bend
and distort the leads.

Overall, reflow soldering is much faster.  You spend a long time
placing each component, but then the soldering itself happens for the
whole board at once, in just a few minutes.  The process is remarkably
forgiving of placement errors with these fine-pitch chips -- the
surface tension of the solder will pull the chip into alignment, so
long as it is approximately correct.  0603 components actually give me
much more problems on those boards, since they have little motivation
to stay put once I've placed them, and are more likely to pull out of
alignment during soldering if one end melts before the other.  I'm not
brave enough to have tried hand placing 0402 chips yet.