Low mechanical stress, high electrical and thermal conductance semiconductor die mount

This invention includes semiconductor devices including the heat sink with a slitted metal strip, such as copper, which is coiled or folded to produce an array of flexible flat fingers for mechanical, thermal and electrical contact with the silicon die, such as a power transistor. The use of a slitted metal strip instead of a bundle of wires makes fabrication of the flexible mount simpler and more economical. The flexible flat fingers are able to accommodate hot spots on the semiconductor device and change in thermal gradients as the device is operated. The slitted metal strip reduces mechanical stress on the die and the die attach solder to avoid solder fatigue which is a wear-out,mechanism that can cause failure of the semiconductor device; keep the magnitude of mechanical stresses from becoming large enough to fracture the silicon die; permit the use of higher melting temperature hard solders which are more resistant to solder fatigue; and permit peak operating temperatures of the semiconductor device to be higher than possible with devices that are mounted with soft die attach solder.

FIELD OF THE INVENTION 
This invention relates to semiconductor devices including die mounts, and 
more particularly a die mount constructed from a slitted metal strip such 
as copper. 
BACKGROUND OF THE INVENTION 
The function of an electronic package is to permit handling and 
installation of the electronic components without damage, and to protect 
the components from environmental factors such as moisture and corrosive 
agents. It must also provide a pathway to the environment for the 
electrical current controlled by the device and for heat generated by 
components within the package. The package of an electronic device or 
module is basically any part of the module except the silicon die. The 
package includes, for example, the die attach solder, heat spreader, any 
electrical insulator such as BeO, heat sink, bond wires, interconnections 
and connectors, and the housing. The cost of the packaging is usually 
greater than the cost of the silicon devices within the package and the 
quality of the package influences the performance and reliability of the 
device or module as well as the price. 
The extraction of heat generated by components within the package is 
necessary to prevent those components from reaching a temperature where 
their reliability is adversely affected. This is a particularly important 
problem for modules which contain power transistors which can dissipate 
tens of watts of energy during their normal operation and require low 
resistance electrical contacts to minimize parasitic power losses. The 
electrical resistance of the packaging must be much lower than the on 
resistance of the power transistor die so that the voltage drop of the 
packaged device is minimal and determined primarily by the die 
on-resistance. The thermal expansion coefficients of the various materials 
making up the package must be well matched to avoid excessive mechanical 
stresses which could rupture the electrical and thermal interfaces and 
cause failure. 
High power solid state devices, such as transistors, diodes, and silicon 
controlled rectifiers, commonly utilize the so-called vertical structure 
where the current passes vertically through the die from top side contact 
to the bottom side die mount (FIG. 1). The mechanical contact made by the 
die mount to the die must accommodate the mechanical stresses that develop 
when the die and the die mount are at a temperature different from the die 
attach solder solidification temperature and when the temperature of the 
die and die mount thermally cycle during normal operation. The stress 
relief function is a particularly demanding one in that the solder die 
attach layer plastically deforms to limit the stress transmitted to the 
die. This plastic deformation introduces structural defects, point defects 
and dislocations, which will ultimately lead to mechanical fatigue and 
failure of the die attach layer. Mechanical fatigue of the die attach 
solder can be reduced by using a harder, less pliable solder, but with the 
penalty of increasing the mechanical stresses applied to the die. In worse 
case situations, the stresses can become large enough to fracture the die, 
resulting in failure of the device and usually the circuit in which it is 
embedded. This is particularly true for large power transistor die. 
Industry rule of thumb is that the largest die that can be directly 
soldered to a solid copper heat sink that is about 0.63 to 0.76 cm (0.25 
to 0.3 inch) square. Many man years of research and development have been 
invested in improving the reliability of power transistor packages, the 
die attach solder in particular, to achieve a transistor operating life in 
excess of the useful life of the circuits in which they are used. However, 
the package will ultimately fail because the principal failure mode, 
solder fatigue, is built into the structure of the package. 
In a study of solder fatigue by Vaynman and McKeown, "Energy-Based 
Methodology for the Fatigue Life Prediction of Solder Materials," IEEE 
Transactions On Components, Hybrids, and Manufacturing Technology, Vol. 
16, No. 3, pp. 317, 1993, the number of shear stress-strain cycles a 
solder joint can experience before failing is correlated with the damage 
to the solder caused by the deformation. Shear stress-strain cycles are a 
natural consequence of the temperature cycling a component experiences in 
normal operation. The damage function is defined as the product of the 
shear stress and the shear strain, i.e., the work performed on the solder 
in the plastic deformation cycle. With repeated cycling, the damage 
accumulates and the joint fails. Reducing shear strain reduces solder 
damage, and extends solder joint life. The shear strain is reduced by 
reducing the shear stress. 
To improve die attach reliability and increase operating life, it is 
desirable to decouple the electrical and thermal contact function from the 
stress accommodation function. This would make a wider parameter space 
available for packaging power electronic systems. One could envision 
several ways to achieve this decoupling--use a liquid metal contact 
between die and heat sink, with the risk that the die could be dislodged 
by shock or vibration, or by making a dry pressure contact between die and 
heat sink, but dry contacts have higher electrical and thermal resistance. 
The use of copper wire bundles in place of solid copper heat sinks to 
relieve stress on power transistor die was first reported by H. H. 
Glascock and H. F. Webster, "Structured Copper: A Pliable High Conductance 
Material for Bonding to Silicon Power Devices", IEEE Transactions on 
Components, Hybrids, and Manufacturing Technology, Vol. CHMT-6, No. 4, pp. 
460-466, 1983. J. F. Burgess, R. O. Carlson, H. H. Glascock, II, C. A. 
Neugebauer, and H. F. Webster, "Solder Fatigue in Power Packages", IEEE 
Transactions on Components, Hybrids, and Manufacturing Technology, Vol. 
CHMT-7, No. 4, pp. 405, 1984, also discloses packaging arrangements of 
interest. Glascock and Webster were concerned with packaging the very 
large (several square inches in area) power devices used in electric power 
transmission equipment. They prepared their wire bundles by winding skeins 
of copper wire, inserting the skein in a copper tube, and drawing the tube 
to compress the wires tightly together. Their test results clearly showed 
the effectiveness of this concept in minimizing the stresses on very large 
die and significantly extending, their reliable operating life. However, 
the complexity and time consuming nature of the process used to prepare 
the copper wire bundles evidently intimidated others in the field because 
there have been no other reports in the literature exploring this novel 
concept in the eleven years since the publication of their results. 
The present invention overcomes many of the disadvantages of the prior art. 
SUMMARY OF THE INVENTION 
This invention includes semiconductor devices including a die mount made 
with a slitted metal strip, such as copper, silver, platinum, nickel and 
mixtures thereof, which is coiled or folded to produce an array of 
flexible flat fingers for mechanical, thermal and electrical contact with 
a silicon die, such as a power transistor. The use of a slitted metal 
strip instead of a bundle of wires makes fabrication of the flexible mount 
simpler. The flexible flat fingers are able to accommodate hot spots on 
the semiconductor device and change in thermal gradients as the device is 
operated. The slitted metal strip reduces mechanical stress on the die and 
the die attach solder to avoid solder fatigue which is a wear-out 
mechanism that can cause failure of the semiconductor device; keep the 
magnitude of mechanical stresses from becoming large enough to fracture 
the silicon die; permit the use of higher melting temperature hard solders 
which are more resistant to solder fatigue; and permit peak operating 
temperatures of the semiconductor device to be higher than possible with 
devices that are mounted with soft die attach solder. 
These and other objects, features and advantages of the present invention 
will become apparent from the following brief description of the drawings, 
detailed description and appended claims and drawings.

DETAILED DESCRIPTION 
The present invention includes a slitted metal strip, such as copper 
silver, platinum, nickel and mixtures thereof, which can be coiled or 
folded to produce the array of flexible arms for mechanical, thermal and 
electrical contact to a semiconductor device. Using a copper strip instead 
of a bundle of copper wires makes fabricating the flexible die mount 
simpler than previous approaches. 
FIG. 1 illustrates a semiconductor device according to the present 
invention which may be a vertical power transistor, Where the current 
passes vertically through the die mount (current flows in the direction of 
the arrow). Such a device may include a silicon power transistor die 10, 
and a first wire 12 making connection through a solder layer 13 with a 
gate contact on the die. A die attach solder layer 14 connects the die to 
a die mount 16 which provides a drain contact. A top side die mount 18 is 
soldered 19 to a second wire 20 and soldered 21 to a source contact on the 
die 10. The fingers 25 of the second die mount are positioned to contact 
the die. 
We start with a strip of copper 22 whose width is equal to the thickness of 
the desired die mount. The thickness of the copper strip is kept thin to 
keep the mechanical compliance high. Strip thicknesses down to 0.002 inch 
(0.051 mm) are routinely available from the supplier industry and 
thicknesses down to 0.0005 inch (0.013 mm) are available from specialty 
suppliers. The strip may have a variety thicknesses, for example ranging 
from 0.01" to 0.002" to 0.0005". In general, the thinner the strip the 
lower the mechanical die attach solder stress. To obtain high mechanical 
compliance perpendicular to, the plane of the strip, slits 24 are made 
part way through the strip at regular intervals, every 0.5 mm for example 
(FIGS. 2A and 2B). The unslitted portion of the copper strip holds the 
slit portions together. Slitting can be accomplished with a roller die. 
The slits may be perpendicular to the longitudinal axis of the strip, or 
they may be angled with respect to the longitudinal axis to increase their 
length and further lower the die attach solder stress on the device. The 
strip is oxidized to create a solder resist layer on the surfaces of the 
strip, including the slits. This oxidation could be done most economically 
in an additional process immediately following the slitting operation. 
Other solder resist coatings, such as a plated chromium coating or a 
varnish such as that used to insulate magnet wire, could be used in place 
of the oxide solder resist layer. The resist coating must be stable at the 
soldering temperature and not stick to the adjacent copper strips. The 
strip is then folded or coiled to the desired size. The term "coil" means 
a series of connected concentric rings formed by gathering or winding. The 
term "folded" means to bend over or double up so that one part lies on 
another part. The end of the coiled or folded strip is attached to the 
preceding ring or layer by tacking or glue to keep the coil or folded 
strip tightly held together. Coiling is particularly attractive in that 
once coiled, the sample can be easily handled and no further cutting is 
required because the thickness of the coil is set by the thickness of the 
copper strip and the number of turns. If the copper was not oxidized 
immediately after slitting, it would be done at this point. After 
oxidation, the solder surfaces (die mounting face and package mounting 
face) have to be cleaned of oxide, by sanding for example, in order to 
solder at those locations. A nickel plated layer can be applied to the 
cleaned surface to prevent formation of brittle Cu-Sn intermetallic 
compounds with the solder. 
Coiling allows one to make die mounts of the desired size one at a time. 
One could also make a coil the size of the silicon wafer which could be 
soldered to the wafer before it is cut into individual die. The wafer size 
coil would be separated into individual die mounts bonded to the die 
during the wafer sawing operation. This would simplify the attachment of 
the die mount to the die since all die would be bonded to the flexible die 
mount simultaneously. If the top side of a die is metallized, for example 
in a vertical transistor or diode, a top side contact can be formed with 
the slitted die mount according to the present invention. Such top side 
contact (or die mount) 18 will lower total package resistance and could 
provide an additional thermal path way to remove heat dissipated in the 
die. This contributes to cooler operation and high reliability. 
The coiling process produces round die mounts that may not be acceptable 
for mounting square or rectangular die. The unused copper area adds cost 
to the die mount and there may not be space available in the package or 
module for a round mount. To avoid this problem, after coiling, the round 
die mount may be pressed into a square or rectangular shape to more 
closely conform to the die dimensions as illustrated in FIG. 3. Uniaxial 
pressure may be applied to collapse the hole in the center of the coil 
left by the coiling mandrel. 
For applications Where minimum electrical and thermal resistance is not 
required, one could use unslitted copper strip and then cut multiple slits 
the surface of the completed coil using an abrasive cutoff wheel or 
electro-discharge machining (EDM). This would give a die mount with lower 
copper "fill factor" but retains the high mechanical compliance. This 
approach might be suitable for mounting semiconductor devices like large 
microprocessors, micromechanical sensors, and safety-related electronics 
chips (air bags, ABS, traction control) where low mechanical stress is 
required for high reliability even though the electrical and thermal loads 
are not large. 
The approach to making a flexible copper die mount using coiled or folded 
copper strip is simpler than the fabrication process conceived for copper 
wires and has comparable electrical, thermal, and mechanical performance 
to the die mount made from copper wires. The strip-based approach will 
allow the easy fabrication of die mounts to any specified size up to and 
including full wafer size. 
The coiled die mount according to the present invention is made by the 
following steps. A strip of copper is slitted along at least one of the 
edges and the strip is oxidized. The strip is then wound into a coil. The 
end of the coil is attached to the previous layer with adhering solder or 
welded to hold the coil together. Solder faces of the strip are sanded to 
remove the oxide. Optionally, the coil can be pressed into a rectangular 
shape. A Ni plating can be deposited on the ends of the fingers. The 
fingers 25 of the die mount are positioned to engage the solder layer 
between the die and the die mount. 
The folded, slitted strip die mount according to the present invention is 
prepared by the following steps. Again, a strip of copper is slitted along 
at least one of its edged (FIGS. 2A and 2B) and the strip is oxidized. The 
strip is repeated fold over on itself to form a rectangular shape as 
illustrated in FIG. 4. The folded strip is compressed in a fixture which 
holds its folded structure. Solder faces of the strip are sanded to remove 
the oxide. A binding layer is applied to the non-slitted edge of the 
folded strip stack to hold it together. The binding layer may be formed by 
electroplated copper, flame sprayed copper or other methods. The fingers 
25 of the die mount are positioned to engage the solder layer between the 
die and die mount.