removing parts from PCBs

[Jeff Walther]

>Date: Mon, 14 Nov 2005 23:48:14 -0800
>From: "Chuck Guzis" <cclist at sydex.com>
>On 11/14/2005 at 11:24 PM Jeff Walther wrote:
>
>>For big surface mount chips I like to use Chip Quik desoldering
>>alloy, which will lower the melting point of the solder on the board.
>>If I preapply Chip Quik, I can usually use the 600F setting for a
>>minute or two to loosen a chip.  I've successfully done this on chips
>>as large as 208 pin QFPs.
>
>Jeff, unless the thermostat on your heat gun is faulty, you should be able
>to go much lower than 600F, shouldn't you?  60/40 solder flows at about
>370F.

You would think so, wouldn't you?  As I recall, I tried lower 
temperatures first and it just took too long for the solder to 
loosen.  My back of a beer soaked envelope explanation would be that 
the heat is conducting away at some rate and so one needs a higher 
temperature source than the target temperature.  But beats me.  The 
600F setting is the one that gets the chips loose in about 2 minutes.

The heat gun could be poorly calibrated.  It was inexpensive but gets 
the job done.

Interestingly, if I'm doing a bunch of boards, e.g. pulling the flash 
off of cheap ATA-66 cards, after the first card the following ones go 
faster.  Either the heat gun takes a while to reach its set 
temperature, or my work area gets hot and helps the boards along or 
something.  I use a 1/16" sheet of sheet metal as a work surface.  I 
figure it's anti-static protection and fairly impervious to 
mechanical, chemical and thermal insult.

>It seems to me that I've also seen special desoldering rigs that use hot
>air and a special nozzle to better focus the airstream.

I've heard of those, but they seem to cost multiple hundreds of 
dollars.  I like the < $40 heat gun + modeling clay solution. 
That's a little bit of hyperbole as I also have dental picks and the 
Chip Quik set for the big chips and a nice bottle of resin.  With 
those and a 15W and a 40W soldering pencil I manage.

>Thanks for the tip on Chip Quik; from the MSDS, it appears to be a
>Tin-Indium alloy.

I thought there was bismuth involved but the memory is moldy.  It 
melts/mixes in with the existing solder and lowers the melting point. 
That's the point behind the stuff.  It's fairly expensive, but very 
little of it is needed per desoldered chip.  And four beads of the 
stuff makes the big QFPs come off soooo much easier.

Jeff Walther