Reliability study of board-level lead-free interconnections under sequential thermal cycling and drop impact

The sequential thermal cycling (TC) and drop impact test are more reasonable to evaluate the reliability of lead-free solder interconnections compared with separate TC test or drop impact test. In this paper, sequential TC (−40 °C/125 °C, 13 min of soak time, 12 min of dwell time, totally 50 min of cycle time) and drop impact test (a sine impact pulse with a peak acceleration of 1500g and a duration time of 0.5 ms) were conducted to study the failure mechanism of solder interconnections under sequential TC and drop impact test. The TC load has larger effect on the Cu/solder interface at the PCB side than that of Ni (P)/solder interface at the component side. For the thermally cycled samples, the failure location of solder interconnections under drop impact has changed from initiation and propagation along the thin reaction layer (mode 1) between intermetallic compound (IMC) layer and Ni (P) pad at the component side to initiation at the bulk solder and propagation along the Cu₃Sn IMC layer (mode 2) or entirely through the bulk solder (mode 3) at the PCB side. The failure mechanism has also changed from the entirely brittle crack to the mixture of fatigue crack and brittle crack.