Interactions between Variable Frequency Microwave Underfill Processing and High Performance Packaging Materials

Abstract

Variable frequency microwave (VFM) has been recently proposed as an alternative underfill curing method that provides flip chip package warpage improvement as well as potential underfill cure time reductions. The current paper outlines how such advantages in VFM processing of underfill can be compromised when applied to high performance organic packages. VFM recipes for three underfill materials were developed by performing several VFM curing runs followed by curing rate measurements using the differential scanning calorimetry method. The VFM curing rate was seen to strongly dependent upon the underfill chemistry. By testing flip chip parts that comprised large and high-end substrates, we showed that the underfill material has negligible impact on VFM warpage with the major cause attributed to the coefficient of thermal expansion mismatch between the die and the substrate. Comparison between the convection and the VFM methods indicated two warpage tendencies that depended upon the VFM curing temperature. First, when both curing methods used comparably high temperatures, warpage increases up to about + 20% were found with VFM. This unexpected result was explained by the high-density Cu loading of the substrate which systematically carried heat generated by VFM energy from the die/underfill system to the substrate. Since this high-end substrate consists of sequential dielectric/Cu layers with asymmetric distribution of Cu, additional stresses due to local CTE mismatches between the Cu and the dielectric layers were induced within the substrate processed with VFM. Second, warpage reductions down to about-22% were obtained at the VFM curing temperature of 110°C with a curing time similar to that of convection cure. This suggests that the negative effect of the local CTE mismatches were no longer at play at the lower VFM temperatures and that the significantly lower final cure temperatures produced lower total shrinkage of the die and the substrate. Finally, due to lower elastic moduli, the cured VFM parts showed better mechanical reliability with no fails up to 1500 cycles.

Void-Free Underfill Process With Variable Frequency Microwave for Higher Throughput in Large Flip Chip Package Application

“…Indeed, the severity and the failures seen after 1500 cycles were comparable for the three cure profiles, therefore for both cure methods. A more clear difference was seen in the work of Diop et al where, for the type of package and UF material tested, the VFM cured packages demonstrated a better mechanical reliability than the convection cured package due to a lower post cure storage modulus [16]. Nevertheless, the fact that the optimized VFM profiles proposed here did not impair reliability is of particular interest in that these profiles were carried out in the absence of a prior package bake-out.…”

“…VFM heats absorbing polymers faster than convection resulting in faster processing times. For instance, at the same convection cure temperatures, VFM is usually seen to reach a full cure of the UF with only 10% of the recommended convection time [16]. Also, VFM demonstrates a full cure of the UF at lower temperatures within times similar to that of convection cure [16].…”

“…Essentially, the VFM heating is selective since only the UF and the doped Si die are heated by the microwave energy whereas the organic substrate and the cavity air are not directly heated [17]. However, heat generated in the UF-die system can be transferred by conduction to the substrate by means of the dense Cu loading used in high-end packages [16]. VFM heats absorbing polymers faster than convection resulting in faster processing times.…”

“…For instance, at the same convection cure temperatures, VFM is usually seen to reach a full cure of the UF with only 10% of the recommended convection time [16]. Also, VFM demonstrates a full cure of the UF at lower temperatures within times similar to that of convection cure [16]. These advantages are explained by the ability of the microwave energy to allow higher energetic molecular motions at the UF dipolar groups (O − H, N − H, C = O, .…”

“…This was reached by curing the UF with the variable frequency microwave (VFM) oven. In flip chip application, the VFM-UF cure is essentially dedicated to package warpage reduction [16]- [18]. The UF void-free study presented here was done with a reduced assembly cycle time since the bake-out step was eliminated.…”