Underfill Encapsulation
Flip Chip Assembly Contents
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:
- Low Viscosity (fast flow)
- Low curing temperature/fast curing time, which can reduce cost and be less harmful to other components
- Low TCE, which can reduce thermal expansion mismatch between the chip, solder bumps and the substrate
- High modulus, which leads to good mechanical properties
- High glass transition temperature, which enables endurance of higher temperature environments
- Low moisture absorption, which can extend shelf life
- Good adhesion, which can improve product life time
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:
- A controlled volume of no flow underfill material is dispensed over the bond pads on the substrate
- Solder paste is printed onto the board for SMT component assembly
- SMT components are placed
- Bare chips are then aligned using a vision system to orient the chips, relative to bond site fiducials on the substrate
- The chips are placed on the substrate, compressing the no flow liquid underfill to form a compression bond to the substrate
- The solder interconnects are reflowed simultaneously while fluxing and polymerization of the no flow underfill takes place
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.)