Minimizing Thermal Resistance with Direct Attach Heat Spreaders

As commercial and military electronics applications continue to “push the envelope” with higher powers and smaller packaging requirements, it is becoming more critical to minimize the thermal resistance from the heat load to the heat sink. Ideal packaging materials must have high thermal conductivity and coefficient of thermal expansion (CTE) values that are compatible with the integrated circuit device while remaining lightweight and affordable. Given these constraints, removing high heat loads and/or high heat flux from these electronics presents some interesting challenges for design engineers.

Figure 1. Heat Pipe Operating Principles
One approach is to spread the heat to a larger area for dissipation through high thermal conductivity bulk materials or custom designed heat spreaders. Bulk metallic components such as copper or aluminum have high CTE values compared to most electronic components. Because of this CTE mismatch, heat spreading materials require the use of stress reducing components such as intermediate substrates, thermal pads, or grease for attachment to the semiconductor device. These materials compensate for the expansion differences between the semiconductor electronics and the heat spreader. However, they often provide significant thermal resistance due to their poor thermal conductivity. Thermal gap pads have thermal conductivities ranging from 0.5 to 3 W/m-K. This is significantly less than the metallic spreaders which range from 180 W/m-K in aluminum to over 400 W/m- K in copper spreaders.

Packaging materials with device and substrate compatible CTE values minimize the thermally induced stresses during power cycling. Thermal stresses often result in the delamination of substrates disrupting the thermal dissipation path and causing premature electronics failure. Good CTE compatibility mitigates this issue by eliminating the need for intermediate substrates and allowing direct attachment using high conductivity solder.

Figure 2. Thermal Profile of an AlSiC Plate and AlSiC Hi-K Plate
An optimum low CTE heat spreader combines low cost, lightweight materials, with the high heat transfer rate of passive, two phase heat transfer technology (heat pipes). The two most common systems, with a proven record of high performance and reliability in industrial applications are embedded heat pipe plates and vapor chambers. Solutions for improved CTE for both systems have been developed. An aluminum silicon carbide (AlSiC) embedded heat pipe plate and an aluminum nitride /direct bond copper vapor chamber allow for direct component attachment as well as distinctive thermal advantages. The two device structures, applications and implementation are different and will be examined here.