Method for a microwave heatable IHS (31-Mar-2004)

Method for a microwave heatable IHS

Disclosed is a method for a microwave heatable integrated heat spreader (IHS). Benefits include improved functionality, improved performance, and improved reliability.

Background

         Low units-per-hour (UPH) throughput and high process cost are problems associated with the reflowing/curing of thermal interface material (TIM) and sealant cure. Conventionally, they are two separate process steps using indium as the TIM material, and siloxane-based material as the sealant. However, attempts to combine the two steps using a convective oven have not been successful be of the high heating rate (~40°C/min) and long cure time for the sealant (~60 minutes, see Figure 1).

         While as convection heating is surface heating, microwave heating is volumetric. As a result, microwave heating can typically achieve faster heating. For example, a ~120°C/min ramping rate is experimentally verified. Being an alternative to convective heating, microwave heating has been applied into electronic packaging area for the applications such as underfill/encapsulant cure, adhesive cure, thick/thin film cure, and structural bonding of electronic assemblies.

         A variable frequency microwave (VFM) sweeps through >4000 frequencies each 0.1 second, preventing a charge from building on the electronic device within a microwave field. This feature prevents potential damage to the device and enables VFM to be used in electronics.

         The heating of a material by VFM is mainly caused by the dipole polarization of the material. VFM has two unique features that are absent in convective heating - fast heating and selective heating. For materials that have high dipole polarization, the heating by VFM is very fast and effective (>80% conversion from electromagnetic energy to thermal energy). Selective heating is due to the differences in the material response to microwave energy. Carbon is a very good microwave absorber. Carbon-containing materials can be heated up quickly by variable frequency microwave (VFM). However, metals are reflective to microwaves and cannot be effectively heated.

         Carbon/carbon and carbon/metal composites are lightweight thermal conductive materials. For example, the density and thermal conductivity of a C/C composite is 1.9 g/cm³ and 450 W/moK. C/Al composite is 2.1 g/cm³ and 200 W/moK. Copper is 8.9 g/cm³ and 390 W/moK, which is typically used in IHSs.

         Metal foils can be cohered to composite materials with a hot-press process. A Au layer can be cohered to a heat spreader as the wetting surface for TIM solder reflowing. Heat can be conducted to TIM quickly using microwaves, resulti...