Reliability issues in direct chip attach assemblies using reflow or no-flow underfill

The encapsulation or underfilling of flip chips is critical for the widespread success of flip-chip-on-board type assemblies. The use of an underfill reduces the stresses on the solder joints that result from the coefficient of thermal expansion (CTE) mismatch between the different materials. The mismatch in the CTE between the chip, solder joint, and the substrate influences the reliability of the assembly. This mismatch results in cracks being initiated at the high stress points which eventually leads to failure of the flip chip assembly. In addition to increased performance and reliability that is achieved by the use of underfill, it is also necessary to have enhanced characteristics for faster, in-line processing of these materials. Reflow or no-flow encapsulants allow the underfill process to be a part of the surface mount process sequence, and do not require an off-line encapsulant dispense step. The use of these materials leads to other reliability concerns including delamination of the encapsulant material from the passivation interface, encapsulant fillet cracking, solder extrusions and bridging through fissures in the encapsulant, moisture ingress into substrates and issues due to mask mis-registration. This paper reports on experimental work that evaluated reflow or no-flow underfill materials. Some of the parameters varied for the evaluation include encapsulant dispense pattern, passivation type, die pitch, and bump layout.