Volumetric Reduction
A reduction in the physical space occupied by a thermosetting resin occurs during its chemical transition from a liquid to a dense polymer network. In microelectronic manufacturing, epoxy volumetric shrinkage represents this change and is typically expressed as a percentage of the starting volume. The contraction occurs as monomers rearrange into tightly packed crosslinked chains.
This change in dimensions can pull components out of alignment, which is critical for optical or capacitive sensors that require sub-micron precision.
Physical Mechanism
Chemical reactions convert weak van der Waals forces between molecules into much tighter covalent bonds as the cure progresses. This process causes epoxy volumetric shrinkage, which is directly proportional to the density of the reactive groups in the resin. Hardening before completion of this process can trap high internal stresses.
Calculating the exact contraction rate helps in designing the molds and dispensing systems.
Package Deformation
Unbalanced strain across a substrate causes the assembly to warp or twist. High values of epoxy volumetric shrinkage are particularly damaging to thin-die packages, because the contracting resin can pull on the silicon and cause micro-cracking. It can also cause the adhesive to pull away from the metallic lead frame, compromising the seal.
Minimization Approach
Inorganic fillers like silica or glass beads are mixed into the resin formulation to replace a portion of the reactive polymer. This reduces epoxy volumetric shrinkage by introducing stable particles that do not contract when heated. This method allows microchip assembly processes to remain precise and repeatable.
