Thermal Shim

Thermal shim is a thin, precisely sized piece of conductive metal, usually copper, inserted into a cooling interface to take up a height gap between a component and its heatsink. When a chip or die sits lower than the surface the heatsink actually contacts, a shim bridges the difference so that clamping pressure and a thin layer of paste can still form a solid, low-resistance joint, instead of relying on a thick and poorly conducting slab of compound to span the void. It is a simple part, but in rework it is often the difference between a chip that runs cool and a chip that cooks.

Shims occupy a small niche in the thermal toolkit, sitting between paste, which fills microscopic roughness, and pads, which fill larger gaps with a compliant but modestly conductive material. The shim is the option you reach for when the gap is too large for paste, too irregular for a stock pad, and too thermally demanding for anything but solid metal to bridge well.

Why a metal bridge beats a paste bridge

Thermal paste exists to fill microscopic surface roughness, not to carry heat across visible distance. Its bulk conductivity is orders of magnitude below copper's, so every extra fraction of a millimeter of paste thickness adds real thermal resistance to the path from die to sink. A copper shim inverts the problem: the metal carries heat across the span, while two thin paste layers, one from chip to shim and one from shim to heatsink, handle only the microscopic contact at each face. Done correctly, the total resistance of the shimmed stack approaches that of a direct mount. The aim is always the same, the shortest and most conductive path from silicon to airflow.

Where shims show up in miner repair

Shims earn their place wherever surfaces are uneven or component heights vary. On a hashboard, replacement chips may sit at a slightly different height than the originals, heatsinks removed and refitted during rework may no longer seat flush, and boards that have bowed under years of heat can leave individual ASIC packages hanging below the plane the heatsink expects. In each case, paste alone would have to span too large a gap, and a one-size thermal pad rarely matches the exact clearance. A shim cut or selected to the measured gap restores the intended mechanical stack. The same logic applies to power-stage components, where a MOSFET or regulator sitting proud or shy of its neighbors can starve for cooling while everything around it looks fine.

Cautions for the workbench

Shimming is a precision exercise, and the failure modes are mechanical, not just thermal. Too thick a shim over-pressures the joint when the heatsink is clamped down, and that pressure has to go somewhere: it can crack a bare die, fracture solder balls under a BGA package, or bow the board enough to disturb neighboring joints. Too thin a shim leaves an air gap that defeats the purpose entirely. Measure the clearance rather than guessing, keep both shim faces flat, clean, and free of burrs, and apply thermal paste thinly on both sides. Copper shims are also conductive in the electrical sense, so confirm the shim cannot shift into contact with exposed traces or component leads. Used with care during rework, a shim quietly restores proper contact on a chip that would otherwise drift into thermal throttling; used carelessly, it creates the very damage it was meant to prevent.

Shimming sits squarely in the craft end of miner maintenance, the kind of judgment call that separates a lasting repair from a comeback. If a board needs this level of attention and you would rather hand it to a bench that does it daily, start a repair with us, and see thermal resistance for the metric every one of these decisions is ultimately trying to minimize.