Semiconductor-to-metal attachment with silver-filled TPI bondlines

Abstract

In the attachment of semiconductor chips and submounts to metal heat sinks, bondlines utilizing silver-filled thermoplastic polyimide adhesive (TPI) are very durable across a wide range of environmental conditions (thermal, physical, chemical, radiation). TPI bondlines can withstand an extreme CTE-mismatch between the laminated materials, and have excellent bonding with adhesive-layer thicknesses down to only a few microns. To provide electrical conductivity and enhance thermal conductivity, the TPI bondline can be compounded with a high concentration of silver particles, and retain durability and adhesion. There are two general constructions of silver-filled TPI bondlines:Bondfoil – thin layers of silver-filled B-staged TPI coatings on either side of a metal carrier/substrate. The thickness of the (cured) TPI coatings would be 2–10 μm/side. TPI-priming (B- or C-staged; partially or fully cured) of a semiconductor surface may be required.TPI coating only – thin layers of silver-filled TPI polymer (1–3 μm/surface; B- or C-staged) on the interface surfaces to be bonded. A minimal amount of A-staged TPI (liquid: polymer in solvent) may be added to the bondline construction to optimize surface wetting during lamination.The ultimate in robustness and thinness, silver-filled TPI bondlines can provide:Continuous operation at 350°C, as well as temporary exposure to 450°C. Thermogravimetric analysis (TGA) of the TPI polymer shows that degradation does not start until well above 500°C. [See opposite.]Thermal shock durability -- the CTE-mismatched TPI bondline between silicon and aluminum can survive repeated thermal shocks with a ΔT of 300–400°C.Thermal impedance as low as 0.06 °C-cm2/W (0.01°C-in2/W) when using a silver-filled TPI bondfoil, and about 0.01 °C-cm2/W (0.002°C-in2/W) when using just silver-filled TPI coatings on the interface surfaces (no metal foil). In both constructions, the thermal impedance includes all interface resistances.Shear strength of 10 MPa to an aluminum surface and 1–2 MPa to silicon. These features make TPI bondlines ideal for demanding, CTE-mismatched semiconductor packages. As opposed to cross-linked thermoset bondlines -- which are brittle, especially when highly filled -- thermoplastic polyimide bondlines remain ductile and resist cracking, even when highly stressed.