Underfill Encapsulation

One of the additional steps used for flip chip mounting is the need for underfill. Its main purpose is to reduce the effect of the CTE mismatch between the silicon die and the organic substrate. Recently underfill is also seen as being a requirement in mobile electronics systems to provide a “shock” mounting for the die, especially in a 3D stack. Once the underfill is cured, the chip, underfill, and substrate deform together as a unit keeping the relative deformation between the chip and the substrate very small, so the shear strain in the solder joint is small. (See Flip Chip Reference 1.)


The most desired features of underfill are:

A recent advance in underfill encapsulation is the so-called “no flow” underfill. In this process, the underfill is applied to the substrate prior to the chips being placed. A typical process is a follows:

No-flow underfills can have a significant advantage over standard underfill processing by eliminating the lengthy capillary flow times needed with large devices. In addition, process speed is increased by having the no flow material completely cured during the reflow step, thus eliminating a timely post bake batch process. (See Flip Chip Reference 2.)