7 May
2011
7 May
'11
6:26 a.m.
Date: Sat, 7 May 2011 10:40:55 +0100
From: pete at dunnington.plus.com
To:
Subject: Re: Desoldering a chopper transistor
On 07/05/2011 10:04, Rob Jarratt wrote:
Hi
I thought I'd mention that you always need to refill with solder if it misses
getting it all out. Try heating both sides with two irons( tough with only
two hands but I've gotten good with holding the pump and iron at one
side with just the one hand. You need something to clamp the board.
Also, when first melting the solder, make sure the pin is free and
centered in the hole. I push the lead around with the iron and solder melted
until it feels right( it is a feel thing ).
It seems that no matter how well it is cleared, some small bridge seems
to be there. Often this can be easily broken loose with a little pressure
I need to desolder a chopper transistor from a
PCB in the PSU of my PDP11 so
that I can test it and if necessary replace it. The trouble is that I am not
experienced with soldering and desoldering and I am having a lot of trouble
desoldering it. I have made several attempts, damaging one of the PCB tracks
in the process (should be repairable by adding a wire). I have a cheap
Weller 40W soldering iron and I have been using some narrow tips. I have one
of those pumps for sucking out the molten solder. I also have 2mm solder
wick (braid?).
You don't normally desolder more than one pin at a time but you do need
a good iron and a good pump. On DEC PSUs particularly, I've found the
holes are, um, less than generous -- the component pins tend to be a
close fit and there's not much room around the pin even when you get all
the solder out.
I normally use a 100W Weller soldering gun for things like those
trannies (I have a 50W Weller temperature-controlled iron for anything
else). The trick is to get the solder hot enough to flow, and to do so
quickly. If you can't do that, you're unlike to succeed, and the longer
you leave the heat on the pad, the more likely you are to damage
something. Then use a large pump to suck it off. Sometimes adding
fresh solder and trying a second time helps -- the first attempt removes
most of the oxidised stuff and the flux from the fresh solder helps
remove the last of it. Then push the pin from side to side to break the
last whisker of solder that bridges from the pad across the gap to the
pin -- if you can! from a small screwdriver. If that doesn't work, I
put the iron on the
lead with sideways pressure. Just as the lead pops, I remove the
iron.
The idea is to heat it just enough to make the solder start to soften
but not so much as to melt it all and attach the lead someplace else.
If this fails, I refill the hole with solder and try to adjust the lead
again, while hot, to center it in the hole.
As a last resort, I go to a friend house that manufactures burn-in boards.
He has a solder pot. It is full and I just set the board on the brim
of solder. I then just pull the part out.
Do remember through all this, do not put pressure on the traces
with the iron. Use a clean iron well tinned iron. You can't clear
a partially cleared hole, you need to refill it and try again.
Use bottled flux if you can get some, it helps to keep the solder
surfaces fresh.
Also, although many like solder wick, I'd done more damage with
it then I'd like. I now only use the pump( not the small one with the
yellow lever on the side. It is junk.The larger the better ).
Dwight
I'd suggest getting a bigger iron if you can, and don't use a narrow tip
for large components. I wouldn't waste any time on solder braid, either
-- fine for small stuff but probably not for this.
--
Pete Peter Turnbull
Network Manager
University of York