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Microprocessorand ASIC designers must address the thermal and mechanical protection of IC diewhile considering system cost and reliability. Lids and heatsinks are commonsolutions for mechanical protection.
To ensurereliability, designers seek to minimize die junction temperature and oftenconsider high thermal conductivity to be the most important attribute of lidmaterial. Yet thermal performance and reliability hinges on other factors:match or mismatch of coefficient of thermal expansion (CTE) between the lid andassembly materials, lid stiffness/flatness, weight, dimensional tolerances, andpackage design.
THERMALMANAGEMENT
Thermalmanagement techniques provide adequate thermal dissipation without addingmechanical stress to the IC from thermal expansion differences between the IC,lid, substrate, interface materials, and other materials in the package.
The mostcommon lid materials for microprocessors and ASICs are copper (Cu), aluminum(Al), and aluminum silicon carbide (AlSiC). With a thermal conductivity valuearound 400 W/mK at room temperature, copper has the highest thermalconductivity of available materials. The thermal conductivity of AlSiC andwrought aluminum are 190 and 200 W/ mK, respectively.
Designersmust also consider thermal cycling issues associated with the CTE values of thedie and lid as well other combinations. CTE generally isn't an issue with a diesize less than 5 mm and heat flux less than 10 W/cm2. As die size and heat fluxincrease, CTE differences between lid, die, lid flatness, and weight have asignificant effect on thermal performance, and choosing a lid material with aCTE compatible with the die becomes important.
Compatiblelid material CTE values will reduce die assembly flexing and distortions duringthermal cycling. Comparing average CTE values of lid and common die materialsat 150'C, AlSiC most closely matches gallium-based IC materials. A solderconnection between lid and die yields maximum thermal dissipation in flip-chipapplications.
Theslightly higher CTE of AlSiC puts the die in slight compression during assemblyand thermal cycling. However, higher CTE materials may impart catastrophictensile forces on the IC with rising temperature. In any event, the closer CTEmatch of AlSiC will minimize package distortions during assembly and thermalcycling.
Withtwice the CTE of AlSiC, copper incurs greater system flexing, though it doeshave a higher thermal conductivity. Aluminum, with a 23-ppm/?C CTE, is unsuitablefor high-power large applications due to the CTE mismatch.
MATERIALDENSITY
Anotherconsideration is lid material density. Density (weight) is not a thermalproperty, but can influence die protection during assembly and service.Consider the weight per solder ball of the IC. During assembly, high lid weightcan deform solder balls during soldering (material creep). It also canpotentially cause shorts between the balls.
Inhigh-speed automated assembly, lid weight poses significant influence onpackage stress during acceleration/deceleration assembly shifts. Lid weightalso affects shock and vibration resistance and stress state due to packageorientation during service. These situations favor materials with lighterweight. Weight becomes more important for larger assemblies with lids largerthan 40 mm2.
LARGEASSEMBLIES
Assystems become larger, the combination of lid material, shape, stiffness,flatness, and dimensional tolerances becomes as important as CTE and thermalconductivity values. Stiffness and dimensional tolerances affect the lid's fitto the die.
Thecavity depth of the lid is important in minimizing the gap between the die andlid. This depth, somewhat dependent upon lid flexibility, must be large enoughto protect the die. For stiffer material, a shallower depth is acceptable, asstiffness will ensure no distortions of the lid during assembly, heatsinkattachment, and/or service.
Lid-materialstiffness increases with lid thickness, but this may not be acceptable due toweight constraints. With a less stiff lid, designers may need to impose tighterdimensions on cavity depth to maintain an acceptable bond line thickness.However, tighter dimensional tolerances increase the cost of manufacturing thelid.
MANUFACTURABILITY
Manufacturingprocesses and costs are additional considerations in choosing lid material.Each material has a preferential manufacturing process for lowest cost, butdesigners should consider full system costs, including the rate of quality.
Alow-cost manufacturing process, stamping lids from sheet stock material is theconventional method for manufacturing copper and aluminum lids, restrictingthem to primarily 2D shapes and with limited 3D features. Stamped aluminum lidstarget low-power applications only due to aluminum's high CTE and modestthermal conductivity. When die is small or power low, stamped copper lids canprovide a cost-effective solution.
AlSiCuses a slightly more expensive casting process, but provides greater geometricalshape capabilities. In addition to its CTE compatibility with IC materials,AlSiC also allows larger lids due to lighter weight and higher stiffness.
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