18 Oct
2012
18 Oct
'12
9:21 a.m.
From: David Riley <fraveydank at gmail.com>
To: "General Discussion: On-Topic and Off-Topic Posts"
<cctalk at classiccmp.org>
Subject: Re: An 80386 CPU S-100 Board
I suppose. I've had terrible luck with bridging, but I never
really kept at it long enough to get much good at soldering
fine-pitch parts. My wife got me a pretty decent iron as a
birthday present recently, though (I picked a good one!), so
I may be doing a bit more in the not-too-distant future (more
so if my QBUS board ever gets off the ground, because I don't
really feel like paying for assembly).
I have been doing fine-pitch soldering for a long time. I do stuff as
fine as
0.4mm lead pitch, which is a bit of a pain. The bulk of stuff I now make
with a pick and place machine and a reflow oven (converted toaster oven
with thermocouple ramp-and-soak controller). But, I still do a lot
of prototypes and rework by hand. The MOST important thing is a
stereo zoom microscope with a long working distance. A working
distance of 2-3" is necessary to get your hands and a soldering iron
under it. A ring light can be made from a ring of PC board material
that fits around the snout of the microscope. Carve a ring in the
copper of the PCB so it becomes two concentric rings. Use a 12 V
DC wall-wart power supply and 8 while LEDs, with about 1 K Ohm
series resistors. Make a piece of cardboard with a 1/2" hole in it,
use this as a mask to isolate the light from one LED at a time
and bend the LEDs so they all converge about where the viewing
area is. This mask is for alignment only, store it somewhere as
you will occasionally bump the LEDs and have to realign them.
Solder braid can be used to remove excess solder bridging the
leads, which WILL happen frequently. For big, high-density
chips, this is my procedure: First, put a tiny dab of solder
on 2 corner pads. Align the chip with the pads, and solder
the corners that have the extra solder. Now, use a piece of
solid wire to apply GC brand (or equivalent) liquid flux
across the leads. Just dip the end of the wire into the bottle
and then wipe it across the leads. If the board has enough
solder plating, you may not need to add any solder, just use
the iron to reflow the pad solder onto the leads. I generally
add a little speck of solder every 4th or 5th lead as I go
down the rows. I use .015" solder, the thinnest I seem to
be able to get nowadays. You used to be able to get
.010" fairly easily. I've probably done about a thousand of these
now using this technique. It is probably best to wash off the
flux residue later with solvent.
Jon