The Effects of the Solder Composition on Interfacial Reactions in Flip-Chip Solder Joints
Date Issued
2010
Date
2010
Author(s)
Chang, Chih-Chiang
Abstract
This study investigated the intermixing of 95Pb5Sn solder bumps and 37Pb63Sn pre-solder in flip-chip solder joints. In this thesis, we use three experiments to investigate the composites solder. First, the reaction conditions included multiple reflows (up to 10) at 240℃, whereby previously solder-coated parts are joined by heating without using additional solder. Second, we studied interdiffusion and the interfacial reaction during high-temperature storage of 95Pb5Sn solder bumps and 37Pb63Sn pre-solder in flip-chip solder joints. The reaction conditions included aging at 100, 130, 150, and 175℃ for times as long as 4000 hr. Third, this study also investigated the effect of the thermal stress in flip-chip solder (37Pb63Sn) joints. The reaction conditions included multiple temperature cycling test (TCT) between -55 to 125℃.
In the reflow reaction, we found that the molten pre-solder had an irregular shape similar to a calyx (i.e., a cuplike structure) wrapped around a high-lead solder bump. The height to which the molten pre-solder ascended along the solid high-lead solder bump increased with the number of reflows. The molten pre-solder was able to reach the UBM/95Pb5Sn interface after three to five reflows. The molten pre-solder at the UBM/95Pb5Sn interface generated two important phenomena: (1) the molten solder dewetted (i.e., flowed away from the soldered surface) along the UBM/95Pb5Sn interface, particularly when the number of reflows was high, and (2) the molten pre-solder transported Cu atoms to the UBM/95Pb5Sn interface, which in turn caused the Ni-Sn compounds at the chip-side interface to change into (Cu0.6Ni0.4)6Sn5.
In the solid-state reaction, we found that Cu6Sn5 and Cu3Sn formed on the board side and (Ni,Cu)3Sn4 formed on the chip side after 100 h of aging. After 2000 h of aging at 175℃, the Ni under-bump metallization (UBM) was exhausted. This caused the (Ni,Cu)3Sn4 layer at the chip-side interface to be converted gradually into (Cu0.6Ni0.4)6Sn5. We also found that the consumption of the Ni UBM was faster than when eutectic SnPb solder was used for the entire joint. Nevertheless, the consumption of Cu on the substrate side was slower than when pure eutectic SnPb solder was used for the entire joint.
We also reported on the highly interesting microstructural evolution from a fine eutectic to a coarse laminar pattern caused by the cycling thermomechanical stress in eutectic PbSn flip-chip solder joints. The temperature was cycled between -55 and 125℃ for up to 2000 cycles. The shear strain on the corners joints, which experienced the maximum thermomechanical stress among all the solder joints in a flip-chip package, was estimated to be 0.13 for each cycle. A coarse laminar pattern, made up of the Pb-rich phase and the Sn-rich phase, gradually developed during the temperature cycling with phase boundaries that were approximately parallel to the direction of the shear stress. It is argued that the laminar pattern had developed in such a manner that the material with the lowest yield stress (the Pb-rich phase) could align itself with the direction of the shear stress. Because continuous soft layers were arranged in a direction that was approximately parallel to the direction of the shear stress, the stored deformation energy could be dissipated more efficiently.
The least section reports on the Ni/95Pb5Sn/Cu ternary diffusion couples were used to investigate the cross-interaction between Ni and Cu across a layer of 95Pb5Sn solder. High-lead solder layers with thickness of 100 or 400 μm was electroplated over Cu oils. A pure Ni layer (20 μm) was then deposited over the as-deposited high-lead solder surface. The diffusion couples were then aged at 150 to 250℃ for different periods of time. With this technique, the diffusion couples were assembled without experiencing any high temperature process, such as reflow, which would have accelerated the interaction and caused difficulties in analysis. This study revealed that the massive spalling also occurred during aging even though reflow was not used. The massive spalling began with the formation of micro-voids. When the micro-voids congregated into large enough voids, the intermetallics compounds (Cu3Sn) started to spall from the interface. This spalling phenomenon occurred sooner with increasing temperature and decreasing solder volume.
Subjects
flip chip
interfacial reaction
high-lead solder
composites solder
spalling
temperature cycling test
File(s)![Thumbnail Image]()
Loading...
Name
ntu-99-D96527006-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):858dffffe5d74eb880f74bb511f4b3bf