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...
