-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On Mon, Jun 09, 2008 at 03:07:39PM -0700, James Newton wrote:
> Were does one get good solder paste in a syringe these days? 
> 
> I found this:
> http://www.mouser.com/Search/ProductDetail.aspx?qs=wT7LY0lnAe2r5UPJoeJJnQ%3d
> %3d has anyone used this stuff?

I use the Kester brand stuff from digikey myself. I've used both the
No-Clean and Water Soluble stuff, the former leaves a bit of clear but
visible residue, while the latter really does wash off with water.

That said... In my last job I got access to a microscope for the first
time, and also for the first time noticed a big gotcha with solder paste
and soldering irons. Basically, the stuff comes from the tube as
thousands of tiny little balls. Problem is, on an actual board they
don't all melt... Granted, these are some very tiny balls, maybe 1/10th
the size of a typical SOIC lead, if even that. But get some clumping
together and migrating to the wrong place and I'm sure they could end up
shorting out something. Baking the board in an oven doesn't seem to have
this problem to the same degree, as they all tend to melt and migrate to
bulk solder globs. All my personal stuff has been to date done without
solder masks, which would only make the problem worse, though at least
my feature sizes are always pretty big. (15mil trace widths/clearances
minimum usually)


I ended up having to trash three of my Alternate Pace clocks due to this
problem, or at least a varient. I was soldering down a lot of SOICs, 14x
per board, by first running a thin strip of solder paste along the pads,
placing them, and then using an iron to melt the paste. (The boards were
a too big to simply place in my toaster oven and have them evenly heat.)

What happened was that over time some of the leds would come on
unexpectedly, from current sneaking it's way accross adjacent pins.
Applying heat to the pins fixed some of the boards, but it was obvious
that unmelted paste was causing shorts at the back of the pins. I've
probably had, or nearly had the problem before, but it was only really
aparent here due to how many connections were involved, and the
relatively large number of Alternate Pace clocks I've made.

Now where I'm working I've been doing a decent amount of soldering by
putting a little dab of solder down, applying flux, positioning the chip
and melting that solder to hold things down. Haven't had a problem
since, but the boards I've working with are probably tougher than what
you describe.

> I'm just doing some SMT re-work (without real SMT tools or experience, of
> course ;) ) and I'm hoping this will help simplify the process, especially
> the part where I'm holding the iron in one hand, the part down with a pair
> of tweezers in the other, and holding the solder to it with my third hand
> (mouth). 
> 
> I know I could make a two legged, plus a pick, weighted thingy to hold the
> part down, but I'd rather have the control.
> 
> And I could solder blob the pads, then flux, place the part and solder
> down... but these boards are really weak and I've already killed a couple
> heating the pads twice. I'm thinking a bit of solder paste, hold the part
> on, touch it quickly with the iron on each pad and done.

- -- 
http://petertodd.org 'peter'[:-1]@petertodd.org 
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.6 (GNU/Linux)

iD8DBQFITdsW3bMhDbI9xWQRAojyAJ0YXpzX/psZ2F+413EuWBrYGR6e0ACfU+K5
q5Cnh38KMYBtCquLwai++H8=
=UMPY
-----END PGP SIGNATURE-----
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist