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'[EE] Fixing solder bridges where irons just don't '
2006\03\19@032343 by Peter Todd

picon face
I've got a display module that I designed and built that consists of two
double-sided PCB's sandwitched together. One PCB has 35 5x7 LED modules
on it, the other PCB has the driver circuitry. The former PCB is
entierly through hole while the latter is almost entierly fine pitch
surface mount, soldered using a toaster oven. The connection between the
two boards is made by in-line strip connectors, soldered to both boards.
72 individual pins in total go between the boards.

In short, the gap between the boards is about 0.100" and getting them
apart is going to be a real pain. I did carefully check that none of the
protruding pins from the through-hole components are shorting anything,
that is not an issue. I really need things to be compact in this
application.

The whole assembly works perfectly except... Somewhere on the LED board
there are two shorts, between two pairs of column lines to the LEDs. I'm
pretty certain that they are caused by tiny solder bridges, the layout
of the boards was very tight in some areas.

Any clever tricks you guys can think of to fix this, short of taking
everything apart? I'm half inclined to try simply running everything
through the reflow oven again, but the LED modules would probably melt.

And if I am going to take things apart, how the heck do you reliably
desolder pins from through-hole plated circuit boards? I've always found
the solder sticks too well for solder-wick to remove it. Using those
solder suckers is a nice theory, but the joints cool before I can
position them and release. Unfortunately if I manage to damage any of
the pins, I've really got to then take off the led modules to solder in
new pins as the joints are obscured...

Somehow, after looking at how compact Cray's were built, I think they
had better pre-assembly testing than I did!

--
spam_OUTpeteTakeThisOuTspampetertodd.ca http://www.petertodd.ca

2006\03\19@045347 by Ruben Jönsson

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face
I would try to cut the stip connectors with a fine side cutter, pin by pin to
separate the two boards. Then desoldering each pin, one by one at the same time
as pulling them out with a plier. If the strip connectors are held together
with some plastic, cut it of first so the pins are totaly separated.

/ Ruben

{Quote hidden}

> -

2006\03\19@051154 by Richard Prosser

picon face
If you know where the shorts are you may be able to blow them by
passing a high current through the short.
Keep the voltage low & you minimise the chance of damaging anything
else but it is a risky process. I've had mixed results but have fixed
a problem caused by a short inside a void in a multilayer pcb using
the prcoess.

Alternatively
After using the soldersucker to remove as much solder as possible you
can generally free the pins by pushing them sideways with a small
solderining iron tip. Once all are done then you can pull all pins
through.

Or the opposite method
Run as much solder as you can onto the pins and keep the whole lot
molten by running the iron backwards and forwards to create a pool
along the pin line. Once everything is hot enough you shpuld be able
to pull out the pins. Clean up with a soldersucker and/or solderwick.

Your milage may vary.

Richard P

On 19/03/06, Ruben Jönsson <rubenspamKILLspampp.sbbs.se> wrote:
{Quote hidden}

> > --

2006\03\19@090133 by w d myrick

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Peter,   If you must take it apart take a Dremel and use an abrasive cutting
tool and cut all the
pins.  Now you can take the pins out of each board.

Derwrad


{Original Message removed}

2006\03\19@141003 by Matt Pobursky

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On Sun, 19 Mar 2006 03:39:54 -0500, Peter Todd wrote:
> And if I am going to take things apart, how the heck do you reliably
> desolder pins from through-hole plated circuit boards? I've always
> found the solder sticks too well for solder-wick to remove it. Using
> those solder suckers is a nice theory, but the joints cool before I
> can position them and release. Unfortunately if I manage to damage
> any of the pins, I've really got to then take off the led modules to
> solder in new pins as the joints are obscured...

I know how *I* would do it. This is one of those "right tool for the
right job" issues. I fought this same problem many times over the years
and finally asked the advice of some of my contract assembly guys -- I
figured they must have to deal with this all the time.

Their advice? "The only way to easily and safely desolder through hole
pins is with the right tool". Several of them recommended one of these:

http://www.hakkousa.com/products.asp?PID=808&Page=1

They make really short work of through hole pins and component leads. I
can typically desolder a DB-9 connector in less than a minute. I've
never pulled a PCB pad or damaged a board in the process either.

Since I've had the tool I have found I use it a lot more than I thought
I would. It does a great job on connector and header pins and it has
been worth it's weight in gold removing faulty electrolytic caps from
PC motherboards. If you've ever tried to desolder these, they are
almost impossible because of very tiny pads and solder connections to
internal ground planes.

It may seem like a "pricey" tool, but I almost never pay anywhere near
retail price for tools, thanks to Google and internet shopping. Ebay is
good too. I found a brand new Hakko 808 desoldering tool for $89 on
Ebay. I also picked up a brand new 850D hot air rework station for less
than half retail price on Ebay as well.

If you can't afford to buy the tool (or a similar one) then maybe you
know someone in the contract manufacturing business? They will almost
certainly have the proper desoldering tool and might desolder your
board as a favor to you.

Doing it any other way will end up being very time consuming and most
likely result in damage to your PCB assemblies.

Matt Pobursky
Maximum Performance Systems

2006\03\19@143027 by Denny Esterline

picon face
I recently did some desoldering work with an normal, everyday heat gun. It actualy worked suprisingly well. It was on some SOIC parts with a thermal pad in the center and I heated the backside of the board for about a minute - tapped it on the table and hey presto, ALL the parts fell off. This was a simple board so I didn't mind putting a few discretes back.

With a little thought you might be able to shield you displays and whatnot from the hot air and just get the pins hot.

What about that chipquick desoldering solder? It's markedted for SMD rework, but it might help here too: http://chipquik.com/index.htm

Good luck,
-Denny

{Original Message removed}

2006\03\22@013354 by Peter Todd

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On Sun, Mar 19, 2006 at 01:10:02PM -0600, Matt Pobursky wrote:
{Quote hidden}

Well I added Hakko 808 to my watchlist... Thanks for the tip, it looks
like a wonderfull tool, if I can get one cheaply.

> If you can't afford to buy the tool (or a similar one) then maybe you
> know someone in the contract manufacturing business? They will almost
> certainly have the proper desoldering tool and might desolder your
> board as a favor to you.

No luck right now, but I may manage to get a job in the electronics
assembly business this summer...

> Doing it any other way will end up being very time consuming and most
> likely result in damage to your PCB assemblies.

Looks like it alright. There's no way I'm taking a dremel to anything so
tiny and fragile.


I tried to use a solder sucker on the pins as another poster suggested,
and then carefully push them free. No luck, the solder sucker didn't
even get any solder out, I suspect the way the pins are shaped on the
back side seals off the air, I did push everything tightly together
while I was assembling it to maintain a consistant board-to-board
distance. Oh well. I may try the high current trick, but only once I'm
really desperate, now that I've got the displays up and running I'm
finding that the shorted lines aren't too noticable unless you look for
them.

Fortunately I've got spares, but not many spares. I better not screw up
again!

--
EraseMEpetespam_OUTspamTakeThisOuTpetertodd.ca http://www.petertodd.ca

2006\03\22@085707 by Robert Young

picon face
>
>
> I tried to use a solder sucker on the pins as another poster
> suggested, and then carefully push them free. No luck, the
> solder sucker didn't even get any solder out, I suspect the
> way the pins are shaped on the back side seals off the air, I
> did push everything tightly together while I was assembling
> it to maintain a consistant board-to-board distance. Oh well.
> I may try the high current trick, but only once I'm really
> desperate, now that I've got the displays up and running I'm
> finding that the shorted lines aren't too noticable unless
> you look for them.
>
> Fortunately I've got spares, but not many spares. I better
> not screw up again!
>

This may not be the time to practice, but I have found that when using a
Soldapult (the blue ones with the spring loaded plunger) I get better
results if I modify the solder sucker's tip.

Cut a small notch in the tip such that it will fit squarely over the pin
to be desoldered while the soldering iron is still in contact with the
pin, solder and pad.  

Leave the iron on the solder joint while "popping" the solder sucker.

Another trick that seems to help, especially with pins that have
connections to internal copper planes, is to "float" them with a little
fresh solder and flux before trying to use the solder sucker.  You have
to work fast and don't let the joint cool down.

It is definitely possible to damage a part this way due to overheating
so practice on old boards first.  

Working under a deadline is no time to learn new techniques, I'm sure
you would agree. :-(

Rob

2006\03\23@041627 by Peter Todd

picon face
On Wed, Mar 22, 2006 at 07:55:38AM -0600, Robert Young wrote:
> This may not be the time to practice, but I have found that when using a
> Soldapult (the blue ones with the spring loaded plunger) I get better
> results if I modify the solder sucker's tip.
>
> Cut a small notch in the tip such that it will fit squarely over the pin
> to be desoldered while the soldering iron is still in contact with the
> pin, solder and pad.  
>
> Leave the iron on the solder joint while "popping" the solder sucker.

Thanks, that sounds like a great idea. The school's Soldapult looks
like that's been done already, though I *highly* doubt that was
intentional... :)

> Another trick that seems to help, especially with pins that have
> connections to internal copper planes, is to "float" them with a little
> fresh solder and flux before trying to use the solder sucker.  You have
> to work fast and don't let the joint cool down.

Can't say I have the money to be using internal copper planes... But I
did find that when fixing solder bridges on TQFP and PLCC ICs that same
principle works very well. Put some fresh solder where the bridge is,
and clean with solder braid. Lets the solder braid actually contact the
(now much bigger) short rather than just sit there ineffectually.

> It is definitely possible to damage a part this way due to overheating
> so practice on old boards first.  
>
> Working under a deadline is no time to learn new techniques, I'm sure
> you would agree. :-(

I'll say, this project's been using more new techniques than I'd care,
SMT, stacked circuit board assemblies, multiplexed led displays and 3d
programming.

Fortunately I have lots of experience with the technique of sleep
depravation. :)

--
petespamspam_OUTpetertodd.ca http://www.petertodd.ca

2006\03\23@043342 by Alan B. Pearce

face picon face
> This may not be the time to practice, but I have found that when using a
> Soldapult (the blue ones with the spring loaded plunger) I get better
> results if I modify the solder sucker's tip.
>
> Cut a small notch in the tip such that it will fit squarely over the pin
> to be desoldered while the soldering iron is still in contact with the
> pin, solder and pad.
>
> Leave the iron on the solder joint while "popping" the solder sucker.

The other trick I use is to have the iron on the pin on the component side
of the PCB, with the solder sucker on the other side.

But yes, the solder sucker I have at the moment has a nick out of the side
of the nozzle for just this purpose.

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