Underfill filler Characteristics

Filler Size

Successful underfilling requires that the filler particles are smaller than the gap size. The maximum particle size is critical to ensure underfill of small gaps typical of flip chip assemblies. Maximum filler diameter should be 1/2-to-1/3 the gap size. Filler particles may also become trapped between bumps. Flux often surrounds bumps and can significantly reduce the channel size. Very small filler particles (< 5 μm) may reduce flow rate.The supplier should report the maximum particle size of the filler to determine compatibility with users minimumgap size. Particle size of the filler can be determined by using progressively smaller mesh screens or by means of aparticle size analyzer.

Filler Material Type

There are a number of different types of fillers that can be used in underfill encapsulants. Silica is most prevalent, however in high power dissipation applications a more thermally conductive filler may be used. Alumina is an example of a more thermally conductive filler. The underfill supplier should report the filler type to users.

Percent by Weight

Typically, the filler comprises a large portion (60% - 75% by weight) of the formulation. Underfill suppliers prepare formulations in terms of weight percent versus volume percent. The weight percentage may be significantly different than the volume percentage.

 Density

Typically, density of the underfill is reported by suppliers. It is usually in the 1.5 g/cc to 2.0 g/cc range. The underfill density is influenced greatly by the filler density. Typically, ASTM-D-792 or similar method is used to determine density. In low-viscosity underfill systems, filler settling is a concern. Underfills with higher density fillers may be more predisposed to filler settling or stratification, particularly during high-temperature cures.

Thermal gravimetric analysis (TGA) residue of a “wet” specimen may be used to determine relative filler content. The TGA may also be used to determine if filler settling may have occurred by curing a highly vertical but thin specimen and analyzing the top and bottom of the specimen for relative filler content.