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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.

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Abstract-In this paper, we investigate the mechanical and electrical properties of an anisotropic conductive film (ACF) on the basis of high-density vertical fibers for a wafer-level packaging (WLP) application. As part of the WaferBoard, a reconfigurable circuit platform for rapid system prototyping, ACF is used as an intermediate film providing compliant and vertical electrical connection between chip contacts and a top surface of an active wafer-size large-area IC. The chosen ACF is first tested by an indentation technique. The results show that the elastic-plastic deformation mode as well as the Young's modulus and the hardness depend on the indentation depth. Second, the efficiency of the electrical contact is tested using a uniaxial compression on a stack comprising a dummy ball grid array (BGA) board, an ACF, and a thin Al film. For three bump diameters, as the compression increases, the resistance values decrease before reaching low and stable values. Despite the BGA solder bumps exhibit plastic deformation after compression, no damage is found on the ACF film. These results show that vertical fiber ACFs can be used for nonpermanent bonding in a WLP application.