Re: [uCsimm] Soldering, was: RTL Port, actually RTAI.

From: Alexander Langer (eedala@eed.ericsson.se)
Date: Wed Jan 19 2000 - 03:55:21 EST


Hi,

[soldering SMD chips]
> > A friend of mine has a rather novel approch to these
> > parts, first he solders them to the board (without regard to solder bridges), then
> > he uses solder wick to pick up the exess and removes any bridges!
>
> This is the best way if you don't have a hot air tool. It works just fine
> with parts of _any_ pitch (including the TQFP100 parts used on the uCsimm).

I also did that first with a QFP-100 chip.
Then I found out that the tin on the pads is already sufficient.
I put the chip onto the pads, still holding it with my non-soldering
hand (otherwise it could move before it's fixated).
I check with a magnifying glass that the pins and pads are aligned.
Then I press the soldering iron onto two pins on opposite corners to
fixate the chip on the PCB.
Then I check again with the magnifying glass.
After that I put wet flux over one row of pins and pads. I have one
of these pencil/brush-type wet fluxes.
For the actual soldering, I put the soldering iron onto the pads, touching
the pins from the side:
                                     |-------
soldering========= ,--| chip body
iron ========= / |-------
           ========= pin /
            =========----------'
      ---------------------------- pad

and then sliding the soldering iron slowly along the pins. It makes
a small noise when arriving at a new pin and then you see on top of
the pin a small shiny convex surface coming up. That's the time
to move to the next pin. Probably around a second or so per pin.
(probably less than a second)

So far I soldered 15-20 QFP-100 this way and didn't mess up a single
one (which happened with the solder-wick technique).
When it doesn't work, just put on wet flux again and slide along
a second time (was necessary twice so far).
I never had a bridge between pins this way.

I use a Metcal soldering iron for that and are very satisfied with it.
The tip is shaped as if you'd write with a pencil on sanding paper.

If you go the solder wick technique, I found that sometimes it's better
to add even more tin before removing it.
Also, to clear some bridges, I used a scalpel to cut between pins.

No matter which way you go, the pads Protel has per default are
sub-optimal (at least for the chip I'm using). It's a lot easier
to solder when you move the pads slightly outwards:
(grossly exaggerated)

Top view:
    original modified
                            |||||||||
    ||||||| |||||||||
    |||||||
 --- --- --- ---
 --- --- --- ---
 --- --- --- ---
    |||||||
    ||||||| |||||||||
                            |||||||||
  

Side view:
              |--- |---
             ,| ,|
            / |--- / |---
      -----' -----'
    ------------- -------------

The original version left me with bridges under the chip which was quite
nasty.

Alex.

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