Surface mount soldering/desoldering

[rgb-man]

I have a board that has a faulty surface mount chip - the package is
just over 1 inch square and has 176 pins.

I have another faulty board (with missing parts) but which has the
same part that I could, in theory, remove and transplant to the board
with the faulty IC.

Trouble is that I've never desoldered or soldered a surface mounted
device in my life, least of all one with 176 pins! I have though
soldered and desoldered plenty of chips with the usual plated through
holes.

Should I give up before I even start?

If I decide to give it a go, will I need to buy much specialised
equipment?

Are there maybe any web sites which people here would recommend that
are particularly useful?


Thanks

 

[Mark Fergerson]

rgb-man wrote:

I have a board that has a faulty surface mount chip - the package is
just over 1 inch square and has 176 pins.

I have another faulty board (with missing parts) but which has the
same part that I could, in theory, remove and transplant to the board
with the faulty IC.

Trouble is that I've never desoldered or soldered a surface mounted
device in my life, least of all one with 176 pins! I have though
soldered and desoldered plenty of chips with the usual plated through
holes.

Should I give up before I even start?

If I decide to give it a go, will I need to buy much specialised
equipment?

Are there maybe any web sites which people here would recommend that
are particularly useful?

What kind of pins? Flat and J-leads are (relatively)
easy; first suck up most of the solder with pre-fluxed
copper braid (buy it as "desoldering braid") then GENTLY pop
the pins loose from the pads with a BRAND-NEW Exacto knife.
On the bad chip, you can just cut the pins loose from the
package (with good quality fine-tip cutters) and desolder
them individually to minimize the amount of heat you apply
so as to avoid lifting the pads from the board. That may
risk lifting the pads anyway though because cutting them
produces a mechanical shock that has nowhere else to go.

Then, straighten the leads on the good chip, pre-tin the
target board pads with solder paste applied with a toothpick
or very fine wire solder, tack the chip down at opposite
corners (make sure you get it aligned just right or you'll
have to start over), then solder all the pins on each side
en masse by gently wiping the iron slowly along them (if you
tinned the pads right you won't get bridges). With flat pins
you can do them individually if you're feeling froggy; press
the pin down with the Exacto while heating with the iron.
I've never had solder stick to an Exacto blade.

You may need a lighted magnifiying lamp to see what
you're doing. Finally, check for solder bridges between pads
(use the braid again to clear them). You'll want a very
fine-pointed iron (preferably one that's temperature
controlled), though I've done it successfully with a clumsy
Weller gun in my younger days.

So, you're looking at acquiring (in approximate
descending order of cost); a magnifying lamp, a good
sm-capable iron, maybe a pair of fine-tip cutters, an Exacto
knife set, some fine wire or paste solder, and desoldering
braid. Much of it you'll find indispensable for lots of jobs
once you have it.

A set of "third hands" or small bench vise will help hold
the board steady, too.

Get some practice with the other sm chips (if any) on the
donor board first to build your confidence. Oh, and lay off
the caffeine that day (ask me how I know).

Mark L. Fergerson

 

[michael turner]

On Sun, 15 Feb 2004 14:18:44 +0000, rgb-man wrote:

I have a board that has a faulty surface mount chip - the package is just
over 1 inch square and has 176 pins.

I have another faulty board (with missing parts) but which has the same
part that I could, in theory, remove and transplant to the board with the
faulty IC.

Trouble is that I've never desoldered or soldered a surface mounted device
in my life, least of all one with 176 pins! I have though soldered and
desoldered plenty of chips with the usual plated through holes.

Should I give up before I even start?

If I decide to give it a go, will I need to buy much specialised
equipment?

Are there maybe any web sites which people here would recommend that are
particularly useful?

Put it in a pre-heated oven at gas mark 6, 2 hours should do it.

--
Michael Turner
Email (ROT13)
zvxr.gheare1963@ivetva.arg

 

[Garrett Mace]

"rgb-man" <spamfree949@btinternet.com> wrote in message
news:402f7fc1.13607436@News.cis.dfn.de...

I have a board that has a faulty surface mount chip - the package is
just over 1 inch square and has 176 pins.

I have another faulty board (with missing parts) but which has the
same part that I could, in theory, remove and transplant to the board
with the faulty IC.

Trouble is that I've never desoldered or soldered a surface mounted
device in my life, least of all one with 176 pins! I have though
soldered and desoldered plenty of chips with the usual plated through
holes.

Should I give up before I even start?

If I decide to give it a go, will I need to buy much specialised
equipment?

Are there maybe any web sites which people here would recommend that
are particularly useful?


Thanks

I have done this successfully with a $20 heat gun. It is best if you can
come up with some kind of mount to hold it over the chip so you have two
hands free to remove it.

Heat guns, on the low setting, put out about 650 degrees F. That temperature
will damage the chip internals, and if you have a moisture sensitive chip
you might generate steam bubbles inside and damage the die with internal
stresses. So the plan is to heat up all the pins as quickly as possible, tap
with a dental pick until you see the entire chip wiggle, and then quickly
flip it over with the dental pick and turn off the heat gun. This gets the
chip off the board before the inside of the chip has the chance to heat up
to dangerous levels. The ideal distance seems to be about 1.5 inches above
the device. This is really no different from the way a professional hot air
surface mount rework station operates, except they have metal airflow
concentrators that direct most of the hot air directly to the outer edges.
You could find a piece of ceramic tile that fits the top of the device, and
use that to help keep air flow off the top of the chip. Perhaps even take
some clay (real, not plastic) and form little pyramids of different sizes
for different chips. My heat gun happens to blow most of the air around the
edges of the barrel, so it works fine for me the way it is. I have used this
technique on devices up to PQFP-208.

For soldering, you want to keep in mind from the very beginning to use as
little solder as you can get away with. Go ahead and buy the thinnest solder
you can find, or a syringe of solder paste if you can get it. Use the barest
amount of solder and a fine-tipped iron to tack down the chip in alignment
on the board, then go around with a small screwdriveror razor blade and
ensure every pin is in contact with the pad below it. Run a flux pen along
the length of the pads. Then take a length of fine solder (.016" or smaller)
and lay it across the pins, and use a LARGER soldering iron, about 40W, with
a bigger tip to smoothly drag along the pins and solder. If you do it right,
you should have nearly every pin soldered. If you used too much solder, it
will wick up in between the pins and lodge itself right up against the chip,
where it is difficult to remove even with solder wick. That's why you need
the thin solder, because if you at any point have too much in one area it
will wick up. You need the large soldering iron because it stores more
energy and won't cool down before you get to the end of the row. ONLY use it
for dragging though, and don't keep it in contact with any single pin for
more than a second. Use the fine tipped iron for touchups.

Alternately you could just run a thin bead of solder paste along the pads,
position the chip, and heat up again with the heat gun. Solder paste is
expensive so I don't do this with large chips that are easily done with the
above method.

 

[Glenn Ashmore]

Mark Fergerson wrote:

What kind of pins? Flat and J-leads are (relatively) easy; first suck
up most of the solder with pre-fluxed copper braid (buy it as
"desoldering braid") then GENTLY pop the pins loose from the pads with a
BRAND-NEW Exacto knife. On the bad chip, you can just cut the pins loose
from the package (with good quality fine-tip cutters) and desolder them
individually to minimize the amount of heat you apply so as to avoid
lifting the pads from the board. That may risk lifting the pads anyway
though because cutting them produces a mechanical shock that has nowhere
else to go.

I have had good luck slavaging big gull wing SMDs by slipping a piece of
wire wrap wire behind the pins and temprarily tacking it down somewhere
off to the side. Then after soaking up as much solder as possible with
braid I touch the end pin with the iron and give the wire a light tug to
lift the pin off the pad and proceed down the row.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

 

[Jamie]

you need a hot pencil with vacuums and a cross heating tip would help.


rgb-man wrote:

I have a board that has a faulty surface mount chip - the package is
just over 1 inch square and has 176 pins.

I have another faulty board (with missing parts) but which has the
same part that I could, in theory, remove and transplant to the board
with the faulty IC.

Trouble is that I've never desoldered or soldered a surface mounted
device in my life, least of all one with 176 pins! I have though
soldered and desoldered plenty of chips with the usual plated through
holes.

Should I give up before I even start?

If I decide to give it a go, will I need to buy much specialised
equipment?

Are there maybe any web sites which people here would recommend that
are particularly useful?


Thanks

 

[Jamie]

hey, thats a hell of an idea!
i think i will try that next time!
i do have some very small bare wire here that would work perfectly
for that.


Glenn Ashmore wrote:

Mark Fergerson wrote:

What kind of pins? Flat and J-leads are (relatively) easy; first
suck up most of the solder with pre-fluxed copper braid (buy it as
"desoldering braid") then GENTLY pop the pins loose from the pads with
a BRAND-NEW Exacto knife. On the bad chip, you can just cut the pins
loose from the package (with good quality fine-tip cutters) and
desolder them individually to minimize the amount of heat you apply so
as to avoid lifting the pads from the board. That may risk lifting the
pads anyway though because cutting them produces a mechanical shock
that has nowhere else to go.


I have had good luck slavaging big gull wing SMDs by slipping a piece of
wire wrap wire behind the pins and temprarily tacking it down somewhere
off to the side. Then after soaking up as much solder as possible with
braid I touch the end pin with the iron and give the wire a light tug to
lift the pin off the pad and proceed down the row.

 

[rgb-man]

On Sun, 15 Feb 2004 10:16:54 -0800, Jamie
<jamie_5_not_valid_after_5_Please@charter.net> wrote:

Thanks very much for all the replies - now I've got to summon up the
courage to have a go.

 

[Mark Fergerson]

Glenn Ashmore wrote:

Mark Fergerson wrote:

What kind of pins? Flat and J-leads are (relatively) easy;

<snip>

I have had good luck slavaging big gull wing SMDs by slipping a piece of
wire wrap wire behind the pins and temprarily tacking it down somewhere
off to the side. Then after soaking up as much solder as possible with
braid I touch the end pin with the iron and give the wire a light tug to
lift the pin off the pad and proceed down the row.

Yet another use for wire wrap wire! I agree with Jamie;
I'll have to give this a try.

<humility rant>

I was so sure I knew all the worthwhile tricks (assuming
no access to "pro" tools) that I didn't bother looking for
the kind of relevant website rgb-man asked for. Most of the
non-pro stuff I know I figured out on my own (after ruining
a sufficient amount of stuff to know what not to do) and
learned from other techs, but obviously none of us can know
everything. Might be a good idea to put all this up
somewhere so all us hotshots can fill the gaps in our
knowledge, or at least know what's appropriate to a given
task without having to guess, or risk ruining something
irreplaceable. I have no objection to leapfrogging the
learning curve as long as I know why what I skipped isn't
worth slogging through.

Sometimes "hard-won experience" isn't worth the pain
involved in winning it.

Mark L. Fergerson

 

[David]

You might fold a piece of Aluminum and tape/glue it to the top of the chip.
Aluminum reflects of IR radiation quite well and would therefore reduce or
eliminate heating of the chips internals while leaving the pins exposed.
Just FYI.

"Garrett Mace" <g.ryan@macetech.com> wrote in message
news:0aNXb.5505$Dg1.648@twister.rdc-kc.rr.com...

"rgb-man" <spamfree949@btinternet.com> wrote in message
news:402f7fc1.13607436@News.cis.dfn.de...
I have a board that has a faulty surface mount chip - the package is
just over 1 inch square and has 176 pins.

I have another faulty board (with missing parts) but which has the
same part that I could, in theory, remove and transplant to the board
with the faulty IC.

Trouble is that I've never desoldered or soldered a surface mounted
device in my life, least of all one with 176 pins! I have though
soldered and desoldered plenty of chips with the usual plated through
holes.

Should I give up before I even start?

If I decide to give it a go, will I need to buy much specialised
equipment?

Are there maybe any web sites which people here would recommend that
are particularly useful?


Thanks



I have done this successfully with a $20 heat gun. It is best if you can
come up with some kind of mount to hold it over the chip so you have two
hands free to remove it.

Heat guns, on the low setting, put out about 650 degrees F. That
temperature
will damage the chip internals, and if you have a moisture sensitive chip
you might generate steam bubbles inside and damage the die with internal
stresses. So the plan is to heat up all the pins as quickly as possible,
tap
with a dental pick until you see the entire chip wiggle, and then quickly
flip it over with the dental pick and turn off the heat gun. This gets the
chip off the board before the inside of the chip has the chance to heat up
to dangerous levels. The ideal distance seems to be about 1.5 inches above
the device. This is really no different from the way a professional hot
air
surface mount rework station operates, except they have metal airflow
concentrators that direct most of the hot air directly to the outer edges.
You could find a piece of ceramic tile that fits the top of the device,
and
use that to help keep air flow off the top of the chip. Perhaps even take
some clay (real, not plastic) and form little pyramids of different sizes
for different chips. My heat gun happens to blow most of the air around
the
edges of the barrel, so it works fine for me the way it is. I have used
this
technique on devices up to PQFP-208.

For soldering, you want to keep in mind from the very beginning to use as
little solder as you can get away with. Go ahead and buy the thinnest
solder
you can find, or a syringe of solder paste if you can get it. Use the
barest
amount of solder and a fine-tipped iron to tack down the chip in alignment
on the board, then go around with a small screwdriveror razor blade and
ensure every pin is in contact with the pad below it. Run a flux pen along
the length of the pads. Then take a length of fine solder (.016" or
smaller)
and lay it across the pins, and use a LARGER soldering iron, about 40W,
with
a bigger tip to smoothly drag along the pins and solder. If you do it
right,
you should have nearly every pin soldered. If you used too much solder, it
will wick up in between the pins and lodge itself right up against the
chip,
where it is difficult to remove even with solder wick. That's why you need
the thin solder, because if you at any point have too much in one area it
will wick up. You need the large soldering iron because it stores more
energy and won't cool down before you get to the end of the row. ONLY use
it
for dragging though, and don't keep it in contact with any single pin for
more than a second. Use the fine tipped iron for touchups.

Alternately you could just run a thin bead of solder paste along the pads,
position the chip, and heat up again with the heat gun. Solder paste is
expensive so I don't do this with large chips that are easily done with
the
above method.