Thermal conductivity of common alloys in electronics packaging

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