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.