Thermal conductivity of common alloys in electronics packaging

Jim Wilson, Associate Technical Editor
 

Table 1. Thermal Conductivity Alloys with CTE Matched to Electronics Packaging

Table 1 lists several metal alloys used in electronics packaging and their approximate values of thermal conductivity and CTE. It is difficult to determine precise values of thermal conductivity because of differences in grain structure and alloy composition. Some of these alloys are proprietary to the manufacturer and are not widely available from multiple sources. Consequently, thermal engineers are often forced to rely on manufacturer-supplied values of thermal conductivity. Unfortunately, the manufacturers are not always diligent in specifying the test temperature and alloy composition. It should be acknowledged that this list is only a representative sample of the materials available and continuing research will likely expand the choices available. Some composites are available as laminates or with continuous fibers (e.g., graphite) and will have properties that are not isotropic. In addition, the composition of some of these composites may be adjusted to select a desirable CTE and given the wide range of potential combinations, the interested reader is encouraged to refer to [1]. Prior technical data columns in ElectronicsCooling include [4] which lists the thermal conductivity of leadframe materials and [5] and [6] which cover the coefficient of thermal expansion.

Metal alloys have a long history in electronics packaging, especially in packaging high value components. Requirements that lead to the choice of metal include package durability, the need for a hermetic environment in the package, complex geometry requiring intricate machining, electrical and RF grounding, and of course, thermal performance. Alloys are engineered to have desirable properties that differ from those of the parent materials. One mechanical property of interest is the coefficient of thermal expansion (CTE). Package reliability concerns require that differences in the CTE among the materials in the package be understood and managed. Semiconductors have relatively low expansion coefficients (Si ~ 2.8 ppm/°C and GaAs ~ 6 ppm/°C at room temperature). The standard choices for heat-spreading metals are aluminum and copper (CTE of ~ 23 ppm/°C and ~ 17 ppm/°C, respectively at room temperature). Making direct attachment of the semiconductor to the metal without a compliant attachment material often will result in excessive stress. The need for lower CTE metals has led to the development of metal alloys with CTE’s lower than copper or aluminum.

Conventional CTE constraining alloys used in heat spreading applications are the “refractory alloys” such as copper molybdenum (CuMo) or copper tungsten (CuW). Typically, the refractory metal constitutes about 75-85% of the compound, which results in the lower CTE, but the conductivity is also reduced compared to pure copper. These alloys are comparatively heavy and expensive and development continues on alternatives.

References

1. Zweben, C., “ ‘Revolutionary’ New Thermal Management Materials,”
   ElectronicsCooling, May 2005, Vol 11, No 2.
2. King, Joseph A., “Material Handbook for Hybrid Microelectronics,”
   Artec House, Norwood, MA, 1988
3. Pecht, M., el al, “Electronic Packaging Materials and Their
   Properties”, CRC Press, Boca Raton, FL, 1998
4. Lasance, C., “Technical Data, Thermal Conductivity”,
   ElectronicsCooling, May 1997, Vol 3, No 2.
5. Lasance, C., “Technical Data, Coefficient of Thermal Expansion”,
   ElectronicsCooling, Sept. 1997, Vol 3, No 3.
6. Lasance, C., “Technical Data, The Coefficient of Thermal Expansion”,
   ElectronicsCooling, Jan. 1998, Vol 4, No 1.