Electrolytic Caps desoldering motherboard

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edfcmc
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Electrolytic Caps desoldering motherboard

Post by edfcmc » Tue Aug 12, 2003 9:50 am

Ok folks, I'v read numerous posts on the internet for electrlytic capacitor removal from a computer motherboard. It appears that the crux of the problem, is that new computer motherboards are "multilayered" and that poses a problems with basic desoldering techniques. I am very familiar with basic soldering/desoldering techniques on a regular circuit board, (parts go on top, traces on the bottom--nothing in the middle). What do I need to know to replace electroytic capacitors on a multilayerd board? Some people on the net have done it with solder wick, #60 drill bits.. What is the best way?

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haklesup
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Re: Electrolytic Caps desoldering motherboard

Post by haklesup » Tue Aug 12, 2003 1:38 pm

There is no trick to desoldering a multi layer board, you just need to be careful not to damage the trace or pad when working on it.<p>Don't heat the connection too long or too hot. This will weaken the substrate. The hot tip of a soldering iron should never be used as a chisel to bend the leads up.<p>Don't wiggle, or yank the lead out of the hole. Trim it so that it pulls out smoothly. A lead with a snag on the end can damage the hole on the way out.<p>With that said, you may want to clean off as much solder as possible with solderwick or a solder sucker. Trim the leads if they look bent and then just heat and pull gently. If solder is left in the hole you can clean it out or just wait until you solder in the new device.<p>You have the option of cutting the cap off the top of the board and removing the leads one by one.<p>Finally, a good coating of flux and maybe a little fresh solder will help the old solder to flow more freely.<p>BTW, how do you know you need to replace the caps. Did you get one of the MBs with the defective chinese caps.

edfcmc
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Re: Electrolytic Caps desoldering motherboard

Post by edfcmc » Tue Aug 12, 2003 3:03 pm

Yeah. I have a motherboard with the cheap taiwanese caps. The caps are rather large 4700uf 10v electrolytics (at least six out of eight ) where some of them have bulged and leaked and bubbled electrolyte on the power supply which is positioned on top of the caps. I will post pics later on. Oddly enough, the computer still works fine, but I have taken the computer off-line as I dont want to cause further damage when the rest of the caps blow (there are numerous horror stories on the net concerning these faulty taiwanese caps). I want to repair the motherboard (i know I can buy a replacement for rather cheap) because I hate to throw-away a perfectly working motherboard where I possibly could repair the thing for about ten bucks and some time. Anyway, thanks for the fast reply. I just havent kept up with the ever changing repair techniques for the proceses out there and wanted to double check before I bust out the solder wick, flux, and soldering iron.

bodgy
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Re: Electrolytic Caps desoldering motherboard

Post by bodgy » Tue Aug 12, 2003 5:10 pm

Remember that power supply caps on most if not all modern motherboards are rated at 105deg not 85deg and have a low ESR. Most important!<p>I have never had a major problem desoldering these caps by hand, IE without a hot air desoldering tool. Just use a small bit and the correct size desoldering braid, if you can get electronic liquid flux or paste(not plumbers flux), wipe it over the solder pads to be unsoldered and use plenty of solder on the iron bit it should come out easily. You may find you need to gently ease the caps as you apply the iron. But try to have removed most of the solder before you do this, otherwise the solder will wick up the plated pin hole cool and you'll then have difficulty cleaning this out without a hot air sucker.<p>Colin.
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Chris Smith
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Re: Electrolytic Caps desoldering motherboard

Post by Chris Smith » Tue Aug 12, 2003 6:27 pm

If the leads are exposed, you can snip them short, and piggy back onto the short leads. Only If you have room, or you are in doubt?

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haklesup
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Re: Electrolytic Caps desoldering motherboard

Post by haklesup » Tue Aug 12, 2003 8:17 pm

Just a little background info:<p>Those caps (and almost all caps on a motherboard) are used primarily for filtering transient noise out of the power supply voltage. These transients are primarily caused by rapid shifts in the demand for supply current as the CPU changes from task to task although some could originate in the power supply itself.<p>As long as the caps do not fail to the point of shorting, everything might work fine. The description you give indicates that the electrolyte has leaked out and the ESR (the equivelent resistance that an AC signal sees)is probably quite high now. Most likly the failure mode is "open" not shorted or leakey.<p>In any case an open cap (or one with very high ESR) will be a very poor high pass filter. Transients that would otherwise pass right through to ground end up causing voltage changes in the supply line locally on the board.<p>This is bad for a number of reasons. A voltage overshoot can cause junction breakdown resulting in a hard failure or shifts in logic levels that cause soft failures. A negative bounce can cause similar logic problems or "Transient Latch-Up". So long as the failure was non-destructive to the device (which it usually is) you experience this as an unexpected lockup or spontanious reboot.<p>I work with reliability and FA engineers at AMD and Intel and we have discussed failures where particular types of noise on supply pins can cause latch up of internal macro cells resulting in destructive and non destructive failures. <p>Much engineering effort at all chip companies goes into preventing these types of failures and some designs do a better job than others. As devices run faster, the internal feature size shrinks rapidly. Expect these types of sensitivities to increase with super fast CPUs and chipsets.

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