Material Joining Processes

Semiconductor 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 joining.

Welding 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. 

As the 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 atmosphere.

The high temperature of the welding process alters the microstructure of the welded areas of the work pieces, i.e., these areas (known as the 'heat-affected zone' or HAZ) 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.

In the semiconductor industry, one common application of welding is in the sealing of metal cans.

Brazing 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 preventing oxidation.  This would ensure a good metallurgical bond between the work pieces and the filler once the brazing process is completed.

Once melted, 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.  

In the 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.

Soldering is a joining process that's similar to brazing, 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. 

Common 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 corrosion.

In semiconductor 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.   

Adhesive Bonding 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.

High adhesive 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.

To maximize 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.

Adhesive bonding is also widely used in semiconductor manufacturing, a common application of which is adhesive die attach.

Diffusion Bonding, 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.

Mechanical Joining 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 corrosion that affects different types of metals that are in contact with each other.