packaging is a complex process of joining many different parts and
pieces together. To achieve this, semiconductor packaging employs
a multitude of
joining processes. The ability to choose which
joining process is appropriate at every step of packaging is critical in
producing highly reliable packages for semiconductor devices. Below are
brief descriptions of common joining processes used not only in semiconductor
assembly, but in other industries as well. These are welding,
brazing, soldering, adhesive bonding, diffusion bonding, and mechanical
is a joining process for metals wherein the work pieces to be joined as
well as the welding or filler material used experience some melting.
A common method for welding, known as arc welding, consists of
generating an electric arc between an electrode (which contains the
welding or filler material) and the work pieces to be welded together.
The arc generates enough heat to melt the electrode and the areas of the
work pieces were the welding is performed.
electrode passes over a region while the arc is present, molten metal
from the electrode and molten base metal from the work pieces all get
mixed together, solidifying to form a strong joint upon cooldown.
The electrode contains some flux material, the purpose of which is to
stabilize the arc formed by generating gases (carbon dioxide, carbon
monoxide, water vapor) that shield the arc from the surrounding
temperature of the welding process alters the microstructure of the
welded areas of the work pieces, i.e., these areas (known as the
undergo grain coarsening at the very least. This results in a
reduction of the tensile strength and toughness of the metal.
Residual stresses that develop as the metal cools down also reduce the
strength of the welded joints. Thus, the welding process must be
optimized (by optimizing heat generation, metal compositions, and
cooling rates) in order to minimize microstructural changes and residual
stresses in the welded joints. Post-welding treatments are also often
performed to relieve residual stresses and make the microstructure of
the welds more uniform.
semiconductor industry, one common application of welding is in the
sealing of metal cans.
is a process for joining two work pieces together with a filler metal
that is sandwiched between them, wherein only this filler metal
undergoes melting, i.e., the work pieces do not experience any melting. The
temperature at which brazing is done must therefore be high enough to
melt the filler material, but not the work pieces. Materials used
as fillers for brazing are those that melt above 450 deg C. Flux
is also used during brazing for the purpose of eliminating oxide films
from the surfaces of the work pieces and
oxidation. This would ensure a good metallurgical bond between the
work pieces and the filler once the brazing process is completed.
the brazing material fills up the spaces between the surfaces being
joined, and even manages to get into tight spaces by capillary action. A
strong joint is obtained after the brazing material has cooled down.
semiconductor industry, a common application of brazing is in the
attachment of leads to certain ceramic packages (such as 'side-brazed
packages'). To allow brazing of metal features to a ceramic
package, the mating surfaces of the package must first be coated with a
thin film of refractory metal (e.g., molybdenum, tantalum, etc.).
This is usually done by applying the metal to the ceramic package in
powder form and then heating it. The resulting metal film is then
electroplated with copper. The feature for attachment to the package may
then be brazed to this copper plated areas.
is a joining process that's similar to
except that it is performed at much lower temperatures than brazing.
Thus, soldering is also a
process of joining two work pieces together with a filler metal, such
that only this filler metal undergoes melting, i.e., the work pieces do
not experience any melting. Brazing materials are those that melt
above 450 deg C, so soldering materials would be those that melt at less
than 450 deg C.
soldering materials include tin-lead, tin-zinc, lead-silver, and
cadmium-silver alloys. Like welding and brazing, soldering also
employs flux materials to clean the surface to be soldered and improve
metallurgical bonding. Residues from fluxes, however, must be
removed after soldering to reduce the risk of the occurrence of
manufacturing, one common application of soldering as a joining method
is in eutectic die attach.
This is a process for mounting the die on the package cavity using a
metal preform that forms a eutectic alloy with the die.
is a process for joining parts using bonding chemicals or materials
known as adhesives. It is employed to join polymers and
polymer-matrix composites, as well as polymers to metals, metals to
metals, and ceramics to metals. Adhesive-bonded joints can
withstand shear, tensile, and compressive stresses, but they do not
exhibit good resistance against peeling. To overcome the weakness
of adhesive bonding against peeling, joints mated together by adhesives
must have a good design, i.e., the adhesion area is maximized and
mechanical interlocking is employed.
bond strength is achieved if chemical bonds are formed between the
adhesive and the base material, or adherent. However, compared to
welding, brazing, and soldering, it is not easy to achieve primary
bonding (ionic, covalent, and metallic) in adhesive bonds because of the
relatively larger interfacial gaps between the adhesive and the
adherent. Secondary bonds, which do not involve electron transfer or
electron sharing but instead rely on coulombic forces of attraction, is
therefore more likely to form in adhesive bonding.
adhesive bonding strength, surfaces to be joined by adhesives must be
cleaned thoroughly. This, in essence, minimizes the interfacial
gap between the adhesive and the adherent. Making the surfaces
'rough' also improves adhesive bond strength because of the mechanical
interlocking that the 'roughness' provides.
bonding is also widely used in semiconductor manufacturing, a common
application of which is
adhesive die attach.
or diffusion welding,
is a solid-state joining process wherein the joined parts undergo no
more than a few percent macroscopic deformation. It can be
accomplished at temperatures higher than half the absolute melting point
of the base material. Diffusion bonding generally occurs in two
stages: 1) deformation processes that result in the surfaces to be
joined coming into intimate contact; and 2) formation of bonds by
diffusion-controlled mechanisms such as grain boundary diffusion and
power law creep. Diffusion bonding is capable of joining a vast
array of combinations and sizes of metal and ceramic parts.
is a process for joining parts through mechanical methods, which often
involve threaded holes. Joining parts using screws or nuts and bolts are
common examples of mechanical joining. The threaded holes employed
for mechanical joining are vulnerable to fractures. In ductile
materials, the fracture can come from fatigue, while in brittle
materials, the fracture can simply result from mechanical overloading.
Thus, mechanical joints must be designed with fatigue failure and
brittle fracture in mind. Another issue to be considered when
designing mechanical joints is galvanic corrosion, which is a type of
affects different types of metals that are in contact with each other.
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