Reattaching Suface-Mounted Resistors

[billchase]

I have a motherboard that I unfortunately dislodged two surface mounted resistors.
The tabs on each end of the resistors are still soldered to the resistors.
Because of this there does not seem away to resolder them back into place.
What are my chances of "glueing" them back into place?  I only have $10 tied up in
the computer I screwed up, so any suggestions on repairing it will be welcome.

C Snyder
 

[Acill]

Well you cant glue them back on wwith normal glue because you will loose the conductive properties of the resistors. What you can try is going to Radio Shack or some place like it and getting some conductive epoxy. It may work, but you need to make sure only the end are connected and you are not shorting out the traces. With only $10 invested it may be just easier to get something new.

[Jeff]

Well, I'm not looking at it so...  Here are a couple of suggestions based on previous hacks I've done.  Locate a piece of copper and file yourself a small pile of fine copper powder.  Mix this with some standard 2 part epoxy.  This will work as conductive epoxy in a pinch if you get the mix right.   Short of that you can also buy conductive epoxy.

You might also be able to complete the circuit by following the traces to the next available points(another component or through hole) and scabbing a resistor between these two points with some TINY wire wrap wire.

You will probably have to apply the smallest amount with a pin to avoid shorting across the resistor with the epoxy. It's really easy to lift/destroy a smd trace or pad with too much heat.  So be really careful if your taking a standard small tipped iron to it.

Good Luck,
Jeff

[kgrach]

Replace them. once the tab comes off the are done.

but cheer up resistors are dirt cheap pennies really. any electronics place can set you up with the two you need for less than a can of soda.
look at the numbers on the surface mount resistors. 473 = 47 plus three zeros 47000 or 47k Ohms.

5R4= 5.4 Ohms  simple really

buy a few extra when you get them because if you are not careful you could leach the tin right off the end of the resistors with your Iron and be right back where you started from.

[kzin]

so you say, the pads from the PCB have come off, and are still attached to the resistors?

You can locktite them back down, tiny amounts of 406 wil do (under each pad). Are the tracks busted too, if they are right at the pads then you may be able to scrape back the insulation layer (usualy a brown or green layer) from the track next to the pads, then if you tin these bits you may be able to bridge the gap with solder (may have to add abit of solder). but this would have to be done quickly as the locktite and the track may come off again with too much heat.

[JetRacer]

Like someone else said; since I'm not looking at it... If you havn't held a solder iron in your hands before, then buy a new one. Otherwize:

You mean that some of the board (copper lead) was knocked off with them?

In that case I'd use a scalpel (or equivalent) to remove the plastic foil covering the lead on the board and take it from there (cut along lead, never across). You may or may not be able to replace the lead and wire it directly to the closest component. If it's digital you will probably mess up timings. If you're unlucky you can mess up timings by doing clumbsy soldering aswell.

If you really meant that the resistors where in pieces, then just replace them. If you are uncertain of what's what then take them with you to a place that sells components and ask for replacements.

In any case, don't use ordinary glue, it's futile. And conductive glue is probably too expensive anyway.

[Castellen]

Agreed, I would definately not use a resistor which has been dislodged in such a way, as the ceramic body of the resistor can develop hairline cracks making the resistor open circuit permanently or intermittently, causing future headaches.

Any place that does SMD work should have them, take along a chocolate bar or something as payment, the resistors cost less than 3¢ each.  Staff would not want to make an invoice for a few cents, they'd loose money, but they'd be overjoyed at the prospect of free chocolate!
Take along the original parts as there are various SMD resistor sizes, though if they're from an Amiga motherboard, they'll be 1206 size.  The smaller but more common 0805 size resistors will still fit OK.

If you have spare motherboards for parts, locate resistors with the same value and use them as replacements.
Look at the board schematic and see if they're really necessary.  Designers somethings do funny things like add pull up/down resistors on a TTL driven output and other such pointless things.

When you fit the new device, do not use any glue, as many types contain cyanoacrilate, and if you heat this (when soldering the ends) gives off extremely dangerous hydrogen cyanide fumes!

Use solder wick to remove all the old solder, hold the new SMD part in place with tweesers, and solder either end.  Simple.

[billchase]

Thank you to all that have replied.  The resistors themselves are not damaged
(BTW they are small, plastic, yellow, and  rectangular).  It appears that the contact
from the motherboard peeled off with the tabs.  I will try to post a couple of pictures
this evening to clarify my description.  Again thanks for all the suggestions.

C Snyder

[ShawnD]

BTW they are small, plastic, yellow, and rectangular

Those sound more like capacitors, either electrolytic or tantalum.  If they look like these then they are caps.

The board may work without them, but will not be very reliable.  They are mainly used to filter the power.  

If you try and repair it then make sure you get them the right way around.  Electrolytic and tanalum caps will explode violently if powered up backwards (or if the rated voltage is exceeded).

To fix it you will need a good soldering iron (a cheap 25w one will not work very well and will likely damage the board) and a steady hand.  A magnifier, tweezers, etc. also help.  

If the pads pads are missing try to follow the trace to the nearest point you can solder to or scrape off the soldermask (as mentioned above) and attach the cap with some wire.  Look for nearby caps that connect to the same power source.  Keep your wires as short as possible to minimize the effects on the circuit.

When you are done use some glue to hold everything down.

[billchase]

Yes, I believe you are correct.  They do appear to be capacitors.  I apologize for the mistake.
Here are links to some pics I took.  The lighting is not great.  I will try to make better ones, but
hopefully this will help.

C Snyder

Image Link1
Image Link2
Image Link3
Image Link4

[Castellen]

Yeah, they're tantalum capacitors.
Unfortunately you've managed to rip off the solder pads as well.  But those capacitors are usable again, they're not as brittle as the ceramic resistors and capacitors.

Do what ShawnD suggested, shouldn't be hard to get it going again, but will be fiddely to make it appear "nice".
As mentioned they are polarised, so make sure you get them round the right way.  There's no + indicator on the silkscreen (the white lettering) that I can see, but you should be safe putting them in the same way as the surrounding capacitors.

Gonna post some pictures so we can look at your handywork? :-)

[PiR]

Hi, sorry billchase to abuse your thread...

@Castellen

But these smaller, like black or gray, on these photos, are resistors, aren't they?

Or they produce so small capacitors novadays?

You know why I'm asking.  ;-)

[billchase]

Yes, I believe the little black and gray things are resistors.
I only assumed the yellow ones were resistors by association
with size and mounting.  BTW, the motherboard is still  functioning
without the two dislodged capacitors, but I want to "fix" it anyway.

C Snyder

[amigamad]

This site has a lot of info on surface mount soldering http://www.diyaudio.com/wiki/index.php?page=SurfaceMountDeviceRework

They are easy to ruin with a soldering iron if you apply to much heat.