| Summary: | 碩士 === 國立中正大學 === 機械工程學系暨研究所 === 99 === The purpose of this study was to investigate the bonding strength of chip bonded onto flex substrate by thermosonic bonding after reliability test. A nickel layer was deposited to the flex substrate to improve the bondability. Reliability test including high temperature storage (HTS), temperature cycling test (TCT) and pressure cooker test (PCT) were carried out to serve as accelerated tests. Die shear test was applied to study the bonding strength between gold stud bump and flex substrate. The fracture morphology of die shear specimens and cross section of the bonding interface were analyzed by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) to reveal the microstructural characteristics and to evaluate the failure modes between gold stud bump and flex substrate. Auger electron spectrometer (AES) was used to detect the atomic interdiffusion at the bonding interface. The reliability of this new process was verified based on the experimental results.
From HTS test results we found three different fracture modes, they are bonding surface lift, bond lift and chip lift. Experimental data indicate that the percentage of chip lift fracture mode increase with increasing test duration. Due to mismatch of thermal expansion coefficient between silicon chip and gold bump, thermal stresses and delamination were generated at the interface. From AES analyses we found that the interdiffusion between Au and Ag atoms was enhanced with increasing storage time. The bonding strength did not increase because of the micro- delamination at the interface.
After TCT test, the bonding strength decreases with increasing test cycles. Similar to the results after HTS test, the percentage of fracture mode of chip lift significantly increased with increasing test cycles. In some specimens, gold bumps and chip separated after TCT test, yet there was only micro-delamination found at the bonding interface after HTS test. It is attributed to the more severe TCT test conditions than HTS test. Most of the joints on the edge of chip were broken at chip side, but the center ones were broken at bonding surface. This is due to larger deformation on the chip edge.
The bonding strength decreased sharply after PCT test. The fracture surface morphology could be a half-moon shape, or concentric-circles. Deposited layer was pulled out with prolonged test duration. Delamination could be observed at the periphery of interface between deposited layer and copper electrode after 48 hours PCT. Both periphery and center of the bond were separated from the deposited layer and copper electrode after 336 hours PCT. Since water vapor infiltrated into the interface toward periphery and center of the bond, the bonding strength decreased drastically after PCT test.
From the bonding strength results after reliability tests, the bonding scheme in this work only passed HTS while in TCT and PCT the specimens failed to comply the test standard.
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