In this study, the interfacial fracture toughness between passivation layer and underfill was investigated at elevated temperatures by performing conventional four-point bending test and finite element simulation. Since the board level reflow temperature for Pb-free flip chip packaging is reaching as high as 260 °C, the concern about delamination at the interfaces between underfill and its surrounding components becomes a more serious reliability issue. To estimate the interfacial strength, interfacial fracture toughness at a specific mixed mode loading condition is calculated. As temperature is increased, the interracial toughness was also increased up to the glass transition temperature. Beyond the glass transition temperature, however, interfacial fracture toughness is decreased.

The impact of phase change (from solid to liquid) on the reliability of Pb-free flip-chip solders during board-level interconnect reflow is investigated. Most of the current candidates for Pb-free solder are tin-based with similar melting temperatures near 230 C. Thus, Pb-free flip-chip solders melt again during the subsequent board-level interconnect reflow cycle. Solder volume expands more than 4% during the phase change from solid to liquid. The volumetric expansion of solder in a volume constrained by chip, substrate, and underfill creates serious reliability issues. The issues include underfill fracture and delamination from chip or substrate. Besides decreasing flip-chip interconnect reliability in fatigue, bridging through underfill cracks or delamination between neighboring flip-chip interconnects by the interjected solder leads to failures. In this paper, the volume expansion ratio of tin is experimentally measured, and a Pb-free flip-chip chip-scale package (FC-CSP) is used to observe delamination and solder bridging after solder reflow. It is demonstrated that the presence of molten solder and the interfacial failure of underfill can occur during solder reflow. Accordingly, Pb-free flip-chip packages have an additional reliability issue that has not been a concern for Pb solder packages. To quantify the effect of phase change, a flip-chip chip-scale plastic ball grid array package is modeled for nonlinear finite-element analysis. A unit-cell model is used to quantify the elongation strain of underfill and stresses at the interfaces between underfill and chip or underfill and substrate generated by volume expansion of solder. In addition, the strain energy release rate of interfacial crack between chip and underfill is also calculated.Index Terms-Contact analysis, flip-chip, interfacial failure, Pb-free solder, phase change, reflow process, strain energy release rate, underfill fracture, volume expansion.

In this paper, the underfill failure such as void and delamination is shown in the Pb-free flipchip package through solder ball reflow process. Since most of Pb-free solder candidates have similar melting point, Pb-free solder bump can melt again during board level Pb-free BGA reflow cycle. From the phase change of Pb-free solder bump, the volume expansion puts pressure on the surrounding materials, i.e. underfill, chip and substrate. This unique phenomenon is tested by board level solder ball reflow and the changes after reflow are presented by using X-ray and cross-section pictures. In addition, the volume expansion rate of pure tin is measured to confirm the effect of phase change of Pb-free solder.

No abstract

The change in solder/underfill adhesion and its effect on fatigue life were investigated for Pb-free solder joints for which, during the reflow process, the solder has melted and resolidified inside the underfill cavities. The change in interfacial adhesion was simulated and its strength compared using button shear test. Surprisingly, the difference was found to be only about 11%. Suspecting the validity of the result, the study was extended to further investigate the adhesion effect on fatigue life under thermal cycles. The effect was assessed analytically using FEA model. Energy-based Darveaux’s fatigue life model [1] is used to calculate solder fatigue life under two extreme conditions: perfect adhesion (without delamination or void between underfill and solder) and non adhesion. The failure parameter, accumulated plastic work per cycle for non adhesion was significantly less than that for perfect adhesion case suggesting adverse effect of strong adhesion to the enhancement of structural integrity. In this simulation, the room temperature was taken as the stress free state.