Consider Your Materials Carefully In Microprocessor And ASIC Design

Microprocessor and ASIC designers must address the thermal and mechanical protection of IC die while considering system cost and reliability. Lids and heatsinks are common solutions for mechanical protection.

To ensure reliability, designers seek to minimize die junction temperature and often consider high thermal conductivity to be the most important attribute of lid material. Yet thermal performance and reliability hinges on other factors: match or mismatch of coefficient of thermal expansion (CTE) between the lid and assembly materials, lid stiffness/flatness, weight, dimensional tolerances, and package design.

THERMAL MANAGEMENT Thermal management techniques provide adequate thermal dissipation without adding mechanical stress to the IC from thermal expansion differences between the IC, lid, substrate, interface materials, and other materials in the package.

The most common lid materials for microprocessors and ASICs are copper (Cu), aluminum (Al), and aluminum silicon carbide (AlSiC). With a thermal conductivity value around 400 W/mK at room temperature, copper has the highest thermal conductivity of available materials. The thermal conductivity of AlSiC and wrought aluminum are 190 and 200 W/ mK, respectively (Fig. 1).

Designers must also consider thermal cycling issues associated with the CTE values of the die and lid as well other combinations. CTE generally isn’t an issue with a die size less than 5 mm and heat flux less than 10 W/cm2. As die size and heat flux increase, CTE differences between lid, die, lid flatness, and weight have a significant effect on thermal performance, and choosing a lid material with a CTE compatible with the die becomes important.

Compatible lid material CTE values will reduce die assembly flexing and distortions during thermal cycling. Comparing average CTE values of lid and common die materials at 150°C, AlSiC most closely matches gallium-based IC materials (Fig. 2). A solder connection between lid and die yields maximum thermal dissipation in flip-chip applications.

The slightly higher CTE of AlSiC puts the die in slight compression during assembly and thermal cycling. However, higher CTE materials may impart catastrophic tensile forces on the IC with rising temperature. In any event, the closer CTE match of AlSiC will minimize package distortions during assembly and thermal cycling.

With twice the CTE of AlSiC, copper incurs greater system flexing, though it does have a higher thermal conductivity. Aluminum, with a 23-ppm/°C CTE, is unsuitable for high-power large applications due to the CTE mismatch.

MATERIAL DENSITY Another consideration is lid material density (Fig. 3). Density (weight) is not a thermal property, but can influence die protection during assembly and service. Consider the weight per solder ball of the IC. During assembly, high lid weight can deform solder balls during soldering (material creep). It also can potentially cause shorts between the balls.

In high-speed automated assembly, lid weight poses significant influence on package stress during acceleration/deceleration assembly shifts. Lid weight also affects shock and vibration resistance and stress state due to package orientation during service. These situations favor materials with lighter weight. Weight becomes more important for larger assemblies with lids larger than 40 mm2.

LARGE ASSEMBLIES As systems become larger, the combination of lid material, shape, stiffness, flatness, and dimensional tolerances becomes as important as CTE and thermal conductivity values. Stiffness and dimensional tolerances affect the lid’s fit to the die.

The cavity depth of the lid is important in minimizing the gap between the die and lid. This depth, somewhat dependent upon lid flexibility, must be large enough to protect the die. For stiffer material, a shallower depth is acceptable, as stiffness will ensure no distortions of the lid during assembly, heatsink attachment, and/or service.

Lid-material stiffness increases with lid thickness, but this may not be acceptable due to weight constraints (Fig. 4). With a less stiff lid, designers may need to impose tighter dimensions on cavity depth to maintain an acceptable bond line thickness. However, tighter dimensional tolerances increase the cost of manufacturing the lid.

MANUFACTURABILITY Manufacturing processes and costs are additional considerations in choosing lid material. Each material has a preferential manufacturing process for lowest cost, but designers should consider full system costs, including the rate of quality.

A low-cost manufacturing process, stamping lids from sheet stock material is the conventional method for manufacturing copper and aluminum lids, restricting them to primarily 2D shapes and with limited 3D features. Stamped aluminum lids target low-power applications only due to aluminum’s high CTE and modest thermal conductivity. When die is small or power low, stamped copper lids can provide a cost-effective solution.

AlSiC uses a slightly more expensive casting process, but provides greater geometrical shape capabilities. In addition to its CTE compatibility with IC materials, AlSiC also allows larger lids due to lighter weight and higher stiffness.