Qualification of the Assembly Process of Flip-Chip BGA Packages ...

3.1.1.2 Underfill material [9]
Texas Tech University, Nivetha Shivan, May 2012
The space between the die and substrate is filled with a non-conductive adhesive, joining
the entire surface of the die to the substrate. Underfill materials are composed of organic
polymers and inorganic fillers. The organic polymer used for most underfills are
thermosets. A thermoset is a polymer material that irreversibly cures (Curing is a process
in which polymer material are hardened by cross-linking of the polymer chains. Curing is
done by adding chemicals, exposing to UV, electron beam or heat). Epoxy is the most
commonly used thermoset for underfills. The hardener used for the underfill is
anhydrides. The epoxy and anhydrides together make the thermoset polymer.
Relationship between Filler Content and CTE
Polymers usually have a high Coefficient of Thermal Expansion (fractional change in
size per degree change in temperature at a constant pressure). Underfill which contains
only the polymer will have a CTE that is three times greater than that of the solder joints
in a package.
This means that the solder joint will experience tension when the package is heated and
compression when it is cooled. This way destructive force is placed on the solder joints.
Hence it is important for the underfill to have CTE closer to that of the solder joints.
Hence fused silica is added to the polymer material. This is because fused silica has low
CTE. It is also compatible with the silicon die, has excellent dielectric properties and high
chemical and thermal stability.
As the filler content (by weight) is increased in the polymer material, the CTE of the
underfill reduces linearly as shown in Figure 3.5.
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