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The effect of thermal aging on the microstructure evolution and solder joint reliability in hard disk drive (HDD) under mechanical shock was investigated. Significant coarsening of Ag3 Sn particles was found in SnAgCu solder, and AuSn4 intermetallic compound (IMC) changed from needle-type to layer-type during aging. For as-soldered SnAgCu solder joints after mechanical shock, the cracks were initiated in AuSn4 at the corner of the solder joints, and mainly propagated along the thin Ni3 Sn4 IMC layer. After aging at 150 degC for 21 days, the cracks were mainly propagated along the solder, Ni3 Sn4, Au-Sn-Ni-Cu, and Au-Cu-Sn. The significant coarsening of microstructure was found in SnPb solder joints, and only microcracks were found on the surfaces of as-soldered and aged solder joints after mechanical shock.