The Junction-to-Coolant Stack — Where the Kelvins Go

The junction-to-coolant stack — where the kelvins go

A 1 kW package drives heat through die, TIM1, heat spreader, TIM2 and cold plate in series. Each interface contributes R_joint = BLT/(k·A) + (R″_c1 + R″_c2)/A — a bond-line term the datasheet advertises, plus two contact terms it rarely mentions. Raise the TIM's bulk conductivity tenfold and watch what the stack does.

TIM bulk k

Bond-line thickness

Area-specific contact resistance R″_c (each face)

Scenario:

Model: series resistances over a 16 cm² footprint at 1 kW, with fixed representative values for die conduction, spreader and cold-plate convection, and R = BLT/(k·A) + (R″c1 + R″c2)/A for each TIM — the sliders set the area-specific contact resistance R″c (mm²·K/W) applied at each face. Real stacks add spreading interactions, pressure- and temperature-dependent contacts, and pump-out/aging; per ASTM D5470 the measurable quantity is the total interface resistance versus thickness, from which apparent k and contact terms are separated by extrapolation. This simplified model shows where the kelvins go — not a qualification of any product.