Reliability of lead-free interconnections under consecutive thermal and mechanical loadings |
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Authors: | T T Mattila J K Kivilahti |
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Affiliation: | (1) Laboratory of Electronics Production Technology, Helsinki University of Technology, 02150 Espoo, Finland |
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Abstract: | To simulate more realistically the effects of strains and stresses on the reliability of portable electronic products, lead-free
test assemblies were thermally cycled (−45°C/+125°C, 15-min. dwell time, 750 cycles) or isothermally annealed (125°C, 500
h) before the standard drop test. The average number of drops to failure increased when the thermal cycling was performed
before the drop test (1,500 G deceleration, 0.5 ms half-sine pulse). However, the difference was not statistically significant
due to the large dispersion in the number of drops to failure of the assemblies drop tested after the thermal cycling. On
the other hand, the average number of drops to failure decreased significantly when the isothermal annealing was carried out
before the drop test. The failure analysis revealed four different failure modes: (1) cracking of the reaction layers on either
side of the interconnections, (2) cracking of the bulk solder, (3) mixed mode of component-side intermetallic and bulk solder
cracking, and (4) voidassisted cracking of the component-side Cu3Sn layer. The assemblies that were not thermally cycled or annealed exhibited only type (1) failure mode. The interconnections
that were thermally cycled before the drop test failed by mode (2) or mode (3). The drop test reliability of the thermally
cycled interconnections was found to depend on the extent of recrystallization generated during the thermal cycling. This
also explains the observed wide dispersion in the number of drops to failure. On the other hand, the test boards that were
isothermally annealed before the drop testing failed by mode (4). |
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Keywords: | Reliability lead-free thermal cycling isothermal annealing drop test JESD22-B111 chip-scale packed (CSP) |
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