On Wed, Jul 16, 2003 at 11:34:32AM -0800, Robert Smith enlightened us
thusly
> I routinly remove throughhole parts with out problems from my 4-layer
> boards with a basic solder/desolder rework station.
>
> Some thoughts --
>
> 1. Are you using a proper desoldering tool. They have a hollow tip
> to drop over the lead and a vacuum pump to suck out the solder.
I'm using a Weller 75W temprature controlled desoldering piece on a
general purpose Weller iron psu and supplying my own vacuum on the bench
with a vacuum pump on a footswitch. I am inclined to select the thickest
nozzle available for the particular hole size.
For the SMT stuff, I have a Metcal iron which is the Rolls Royce way to
get these bits off. It has shaped heads, no elements as such, but
creates heat in the tip using RF reacting with a special metalurgical
compound which absobrs rf up to a certain temprature, then reflects it.
A single metcal station will desolder chips up to qfp98; there are many
tips available which need two stations.
With that, life is simple; Wrap a single length of solder around the
chip legs, and drop on the appropiate tip. Desoldering is quick, and
near 100% recovery is guaranteed. I may lose an smt pad or two with no
tracks on it - that's all.
I need different things at different times. Nearly always I want the
board to be intact. I am dissappointed if I cannot get the chip as well.
I can want to desolder a chip that I can't replace. I don't splash money
very freely, but I'd spend a little here.
>
> 2. Do your connections to the inner layers have proper thermals
> etched in. If not, they will suck up the heat and make removal
> difficult.
Evidently not. I can desolder most boards, but the problems seem to come
with 4 layer boards with an inner earth layer. I meet no more that six
of any board in a year, unless it's a bummer. Most things I haven't seen
before, and may not again.
They are not my boards. I run a general repair house. Lying
around here today are drive boards for thermal blankets, fork lift
parts, an omron plc, a stepper drive, lift boards, and a German
solenoid which they want �1100 for with a small board in it.
>
> 3. Try more heat, my rework station uses about 85W elements. I
> usually set them to 750 deg F for desolder work.
I can only conclude we are mixing Centigrade and Farenheit here. 750
Farenheit is close to 400 degrees Centigrade. The power supply only goes
up to 450 Centigrade.
>
> Preheating the boards may help. Try heating them to about 200 deg F
> in an ordinary oven and keep them warm in towels on the way to the
> work bench.
This is a major pain when I am repairing, as you may imagine. I may be
reduced to it.
>
> Bob Smith
>
>
>
> > Any good tricks around for desoldering four layer throughole pcb
> > parts?
> >
> > Where there is a ground layer, the desolder weapon I am using at 370
> > degrees Centigrade doesn't heat the solder enough to melt it
> > through; what seems to happen is that the board soaks up the heat,
> > and the parts are only warmed.
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
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