Re: gEDA-user: best ways to do SMT assembly

[Dylan Smith <dyls@xxxxxxxxxx>]

On Thu, 14 Aug 2008, Robert Butts wrote:

> I got pcbs back and now want to assemble them.  What is the best way, i.e.
> solder paste to use, heating method, solder bridge removal...?  Most of the
> components are small SMT parts.  The through hole parts I'm fine with.

It depends what I'm doing. For discrete components (like 0603 sized Rs and
Cs), just a fine tip soldering iron and fine solder wire - what I do is
melt a small amount of solder on one pad, get the part with the tweezers,
re-melt it and stick the end of the part into the molten blob. Then just
solder the other end as normal. I can do this quite quickly.

Where I have more components, I find it easiest to use solder paste. Small
blobs (you need less than you think!) on each pad. For fine pitch SMD
chips, I just put a bead of solder paste where the IC's pins are going to
land, rather than attempt to put a blob of solder on each one. I then use
a hot air gun. I've found that the air stream won't blow away even 0603
parts - the solder paste prior to melting will hold it, and when it melts,
the surface tension is sufficient to stop the part from blowing away.
Usually, I need to mop up a little excess solder from fine pitch parts - I
just use fine desoldering wick to do this.

I use Edsyn CR44 solder paste (available from Farnell if you live in
Rightpondia). It has a very good shelf life. The drawback of this
particular solder paste is it does tend to spread a little when it's
heated, but I have found once it finally melts, it's not an issue (due to
the soldermask, and surface tension pulling it back together in a better
shaped blob).

Generally once you've done a few SMD boards, you'll find you can often do
it faster than the equivalent through hole - because you're no longer
having to cut off excess bits of wire or turn the board over for each
component.



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