QFP soldering

From: Clint Wolff <vaxman_at_earthlink.net>
Date: Thu Jan 24 22:10:15 2002


You'll end up with major problems getting the QFP to sit down,
leading to unsoldered pins!

If you have a microscope, use it!!!

Gently place the QFP on the board. They sell special vacumm
wands that work good, but gently setting down then sliding
it into position works ok (put these chips on first).

Get the part into the approximate position, then finely
align one corner. Tack one pin down.

Align the other corner. You can twist the part one or two
pads without damage, but more is bad. Unsolder the first
pad if it is too far out of alignment. Use solder wick to
clean all traces of solder off the pad and start over.

Once you get two opposite corners tacked, make sure your
pin 1 is correct, and all the remaining pins are over the
correct pads. It's pretty easy to shift the whole package
by a little bit, especially if you don't have a microscope.

Apply a water soluble liquid flux to one entire side of
the chip. Really goop it on. It doesn't hurt, and you can
wash off the excess.

I've tried soldering one pin at a time, and never had any
luck with it. The way I was shown was to use a pretty fine
tip with a 45 degree angle about 1/2 inch from the tip.
If you lay the 1/2 inch part of the tip in the crotch formed
by the pin legs, you heat a number of pins, with the pin at
the tip getting the most heat. Put a smallish blob of solder
on the first 3 pins furthest away from you (yes, solder bridge),
and using the tip drag the blob towards you and onto more

If you have the heat right, the surface tension of the solder
will leave perfectly soldered joints on the pins you are
moving off of, and the blob will solder the pins you are
starting to cover. You'll have to back up occasionally to
pick the blob back up. After practice, I can solder a 208 pin
QFP in about 2 minutes start to finish.

If you screw up, DONT try to remove the part. You'll ruin
the board, and the part too. If you must remove the part,
use a dremel tool to cut the pins loose from the package,
very carefully so you don't rip the traces off the board.
Then desolder each pin individually (pretty easy). You
can also heat one pin at a time, then bend it out of the
way with tweezers. You'll ruin the part, but it's better
than ruining the board. You can save the part by heating
the underside of the board with a plumbing torch, then
tapping the edge gently to dislodge the parts when the
solder melts.

Good luck!

PS If I was you, I'd try and find someone that does
electronic assembly for a living, and give them
twenty bucks to solder the parts on. For twenty
bucks, they'd probably be willing to build the
entire board for you, but you might enjoy that part
of the work...

On Thu, 24 Jan 2002, Peter C. Wallace wrote:

> On Thu, 24 Jan 2002, Davison, Lee wrote:
> > Richard Cini wrote...
> >
> > So, here's the stupid question...how do I solder
> > these things? Do I hold the iron parallel or perpendicular
> > to the package leads? The board is pre-tinned, but I
> > should I also tin the QFP leads?
> >
> > Neither. The best way to home solder these things is to use
> > solder paste and a toaster oven, there's an article on
> > www.seattlerobotics.org on how to do this.
> Actually you dont even need solderpaste, you can just tin the pads
> (_NOT_ the qfp, you'll bend the pins and have a real mess if you try that)
> after tha pads have been tinned, carefully align the QFP to the card and tack
> solder the corner pins, using a excess of rosin flux.
> Now you put a lot of rosin flux on the leads and solder the QFP by heating the
> card with a hot air gun from the back. once the solder melts, surface tension
> will pull the chip into accurate alignment. a _gentle_ tap in the board with a
> screwdriver will often help "settle" the part. It is easy to burn the card ,
> so I would suggest practicing on some old ISA cards with QFP parts. (you can
> use the hot air gun to remove the parts also)
> You notice I mentioned an excess of flux. That is the key: Surface tension is
> your friend here.
> Its even possible to solder BGA's this way, but a mistake with a BGA fatal to
> the chip (unless you have a re-baller)
> >
> > If you must use a soldering iron use a large bit (4.5 to 6 mm)
> > and run along the edge of the tinned section, not quite touching
> > the pins, and allow the solder to wick into the joints. Don't
> > worry about solder bridges, these can be removed later with
> > solder wick and a good flux.
> I use this method also and it works quite well, you can get much better
> quality that by individual pin soldering. You can also remouve mos of the
> extra solder by tilting the board, appling lots of flux, and gently running
> the iron along the QFP edges, with the iron pointing up (so the solder runs
> down from the board to the iron)
> >
> > If you make a complete mess of it remove the chip using a
> > hot air gun then clean the board with solder wick removing
> > any left over flux with solvent and try again.
> >
> > Also use a soldering iron of at least 45W with a temperature
> > controlled bit.
> >
> > The easiest way to practice is with the chips on scrap PC
> > motherboards ... see they do have a use 8^)=
> >
> > Lee.
> >
> >
> >
> >
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> Mesa Electronics
Received on Thu Jan 24 2002 - 22:10:15 GMT

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