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Re: LF: My TS850

To: [email protected]
Subject: Re: LF: My TS850
From: "Dave Sergeant" <[email protected]>
Date: Sun, 7 May 2000 09:19:03 +0100
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
From Dave G3YMC
John Sexton wrote:
Hi Dave,
What about telling us how to do it - I refer to 64 pin Surface Mount
Devices?
I have to replace the occasional surface mount chip in the TVs and Videos I
repair, not that many, but growing.  Everyone has their own technique, some
of them unbelievable and some which require spending loads of money.

The method I use is straightforward.  Bear in mind though the golden rule -
your fault is nearly always somewhere other than in the big chip.  Dave
seems to have been lucky.

To remove chips I use a Weller Pyropen hot air gun.  These are 30-40 pounds,
there are cheaper ones but I have no idea if they work as well.  The chip
connections are heated up with the hot air, and after a minute of less it
comes loose and can be pushed away with a screwdriver.  Clean off any
surplus solder.  When practicing on scrap boards it is easy to get carried
away with the novelty and end up with a pile of chips beautifully removed in
a few minutes...

As Dave mentions, the secret in soldering chips in is to use liquid flux.
This is readily available in small bottles cheaply.  The second secret is
getting the chip in the right place to start with.  Some minutes carefully
positioning it with all the legs in the right place is necessary, then when
you are happy solder two opposite corner pins to secure.  Coat one side with
plenty of flux (using the brush in the bottle) and then solder.  I have done
this with a standard Antex bit, but it is easier with 1mm. Normal solder is
OK, don't waste your money on the special stuff.  Do not apply too much
solder, just enough on each pin.  Hold the board vertically with the pins
you are soldering facing down and run the iron along each pin such that the
excess solder runs downwards.  With care you do not get any shorts.  Repeat
on each of the other edges.  You can even apply loads of solder until all
the pins are shorted, apply more flux and run the excess off, though this
might be asking for trouble.

A tip on using manual solder suckers.  Put a rubber sleeve over the end of
the nozzle, the pump is then much more efficient and the nozzle lasts for
ever.  Replace the sleeve as required, suitable sleeves are readily
available in packs of 100.  I have had no success with solder braid myself
and don't use it.  Others seem to have great faith in this.

I shall be giving a talk at the Bracknell club this coming Wednesday on
consumer electronics repair, and hope to include a live demonstration of the
above techniques.  It depends on volunteers bringing some suitable hardware
with devices that can be removed (and that they don't want to work again...)
For more details e-mail me.

73s Dave
[email protected]
http://ourworld.compuserve.com/homepages/sergeantd





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