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.

<Proceed to Page 2  -  Soldering, Adhesion Bonding, Diffusion Bonding, Mechanical Joining>