24 Jan
2002
24 Jan
'02
11:08 a.m.
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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