02 Nov 2013, 22:29
The problem with graphite is, it's the same stuff used in carbon resistors, and is far less conductive than silver (in fact, at room temperature, nothing is more conductive than silver, even copper). I would never use a carbon-containing material to repair a wire bond...higher resistance means less current will get through the connection, so, you might get your segment back, but it will be slightly dimmer because it's not getting enough power. Do it right and use the silver epoxy; the best stuff is made by MG Chemicals. It has ultra-fine silver particles produced by precipitation out of solution; gives the most uniform distribution of silver grains and the lowest-possible-resistance connection. A single successful repair pays for the kit, and there is enough to do more than a hundred (14 grams). There is nothing useful to be gained by cutting corners here. BTW, I wouldn't recommend trying to do wire-bond repair at only 10X magnification. I have a B&L StereoZoom microscope here, and my experience suggests that at least 40X, and preferably 60X, is needed to get a clean, reliable repair. Bonding pads are really tiny, on the order of .006 inch (0.15mm) square (some are even smaller), so there's no room at all for sloppiness (you might end up complicating your problems, perhaps beyond the point of solution). Finally, you need an appropriate method of applying these vanishingly-tiny specks of adhesive. Ordinary pins and needles are about as useful as railroad spikes in this situation, and I have found only one good alternative...what are called "biological probe needles". These things are solid tungsten, diamond-ground to a taper and a point with a radius no greater than .0001 inch (.0025mm, or 2.5 microns); at 60x, a pin looks as dull and rounded as a baseball bat...the probe needle still comes to a virtually dimensionless point (you actually have to be careful handling these; touch them to your skin under any pressure, and they sink, virtually without resistance or pain, into deeper tissues...they are much sharper than acupuncture needles!). This is a case a bit like brain surgery; you need the right tools for the job, and improvisations usually end up badly.