>Date: Mon, 7 Oct 2002 09:26:44 -0400 >From: John McGilvary <[EMAIL PROTECTED]>
>I happen to have an old 7200 (and even a 6100) from which I can scarp >one of these from. >I was an Electronics Tech (waaaaay back) in the Navy and am comfortable >with a soldering iron, so I may just locate this chip and change it. >Cannot be any worse off. If I get nervous looking at it I may contact >you about it, as $20 is not too much to pay to avoid scrapping this >machine. You might want to pick up one of the simple Chip Quik kits from Digi-Key. Do a search on Chip Quik with the second word apparently misspelled. That's their trademark name. I think they have a little starter kit for $15 or $20. The thing you're really after here is the Chip Quik alloy (and maybe the liquid flux if you don't have any on hand). This stuff looks like a thick brittle solder. It is an alloy which you melt into existing solder which lowers the melting point of the solder. As you know, it can be difficult to get all the pins to come loose at the same time when you're trying to desolder a SM component. That's not a problem on a dead component where you can clip the pins, but if you want to reuse the component, it's an issue. The way I would do this (obviously there are a multitude of possible methods, but here's what I would do as an outline, feel free to improvise using your experience) is apply a little liquid solder flux to the pins of the CUDA chip, then melt some of the alloy onto the pins using a 15 watt pencil. Melt enough to form a good bead on each side of the chip. The alloy melts very quickly, so I use the 15 watt pencil to just get the alloy down in the right place. A higher power pencil is likely to melt too much of the alloy too quickly. If you have temp. controlled equipment this isn't so much an issue. I do everything with cheap Radio Shack pencils, so I use a few different wattages for different tasks. Anyway, once the beads are down, I then use a 30 watt pencil to heat the beads, alternating from one side of the chip to the other. Starting on one side, I'll go down the pins and touch the 30 watt pencil to each pin to make sure that the existing solder is melted and has the chance to mix wiht the Chip Quik. The whole bead will be liquid while you're doing this, as the bead will carry the heat to all the pins. Often, when I reach the last pin, the chip just comes loose. It may be necessary to touch the first bead again to reheat it slightly and sometimes it's even necessary to alternate sides a couple of times, until both of the beads are hot enough for the chip to come loose. If you have a bit of sheet metal or a kitchen pan you don't mind using, it's handy to have such a surface to set the loose chip on to cool. Then you can use the 15 watt pencil to melt the remaining alloy on the board and bead it up and sort of roll it to the end of the pads. It helps to incline the board while doing this--gravity helps. Get the ball/bead to a clear area of the board and let it cool, then lift it up and set it aside. You can reuse it and the alloy is expensive. If you hold the chip vertically (the little red handled heat sink/clip from RS is handy for holding the small chip), and apply the 15 watt pencil at the top pin and work your way down you can get all the residual alloy to run to the bottom pin and come off on the pencil in a ball. Then dump it off the pencil onto a metal surface and collect the ball after it has cooled. Then apply a little more liquid flux to the pads, and use some solder wick (desolder braid) and a pencil to clean the residual solder/alloy off the pads. Finish by spraying or wiping with some flux remover. You may also need to clean the "bottom" pins of the chip where the alloy was collected with some desolder braid. This method has worked really well for me. Digi-Key also has a largish package of just the Chip Quik alloy for about $80, but unless you're doing a lot of desoldering, it's obviously not worth it. For larger chips, it's sometimes necessary to use a heat gun set to 500 - 600 F to heat all the pins simultaneously. If I need the heat gun, I cover the surrounding components in modeling clay so that they don't fly away. I've managed to remove and reattach chips up to 208 pins using this method and those tools. The Bandit and Hammerhead are 208 pin chips. Oh, and I really like the Chemtronics brand Chem-Wik desolder braid much more than the easier to find Easy Braid. It may be my imagination, but I think it works better. Digi-Key has the Chemtronics product. But it really may be my imagination. I don't know why the brand of desolder braid would matter, but the Chemtronics stuff just seems to wick the solder up better. Jeff Walther -- PCI-PowerMacs is sponsored by <http://lowendmac.com/> and... 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