Definition
A 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 contacts, a shim bridges the difference so that pressure and a thin layer of paste can still form a solid, low-resistance joint instead of relying on a thick, poorly conducting slab of compound.
When it is used
Shims earn their place where surfaces are uneven or components vary in height, situations where paste alone would have to span too large a gap and a one-size pad would not match the exact clearance. Copper's high conductivity makes it a far better gap-filler than a thick paste bridge: the shim carries heat across the span while two thin paste layers, chip-to-shim and shim-to-heatsink, handle the microscopic contact at each face. The aim is always the same, the shortest, most conductive path from die to sink.
Cautions for repair work
Shimming is a precision exercise. Too thick a shim over-pressures the joint and risks cracking a die or bowing a board; too thin leaves an air gap that defeats the purpose. The shim's thickness must match the measured clearance, and its faces must be flat and clean. Used correctly during hashboard rework, a shim restores proper contact on a chip that would otherwise overheat; used carelessly, it creates the very damage it was meant to prevent.
For the compound that pairs with it and the broader concept it improves, see Thermal Paste and Thermal Resistance.
In Simple Terms
A thermal shim is a thin, precisely sized piece of conductive metal, usually copper, inserted into a cooling interface to take up a height gap…