Thermal Interface Material is material between die and heatsink enabling efficient heat transfer while accommodating mechanical mismatch — critical for temperature management. Thermal Conductivity k: air ~0.026; silicon ~150; copper ~400 W/mK. TIM goal: maximize k. Thermal Resistance R_th = L/(k·A); minimize thickness, maximize area, maximize k. Types phase-change (PCM): solid, melts slightly when compressed. Thermal grease: oil + particles. Phase-Change Materials excellent wetting; no pump-out. Preferred for reliability. Thermal Grease convenient; risk oil pump-out over thermal cycling. Fillers ceramic particles (alumina, BN, AlN) ~70 wt% increase k. Bondline Thickness typical 0.1-0.5 mm; thinner reduces resistance but harder to apply uniformly. Pressure Dependence resistance decreases with applied pressure (5-50 psi). Compliance needed. Surface Finish rough surfaces increase contact resistance (Ra ~0.4-0.8 μm ideal). Compliance TIM deforms accommodating CTE mismatch. Prevents die cracking. Pump-Out grease exudation over time; minimize via viscosity, particle structure. Long-Term Stability thermal properties degrade: oxidation, phase separation, hardening. Testing thermal cycling, pressure cycling, humidity validate reliability. Cost high-k TIM expensive; choose lowest-cost meeting requirements. TIM optimization crucial for thermal management of high-power packages.