Reliability of lead-free interconnections under consecutive thermal and mechanical loadings

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 Cu 3 Sn 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).

Thermal Cycle and Vibration/Drop Reliability of Area Array Package Assemblies

“…Prior work by Mattila et al [49] on compounded loads for UCSP devices suggests that the location of failure can be one of the following: (a) the two interfaces (component-solder or solder-PWB), (b) bulk solder matrix, and (c) void-assisted crack propagation of pad side Cu 3 Sn layer. The failure type was shown as a function of the load application.…”

“…The primary objective of this part of the experimental research, carried out in collaboration with the Rochester Institute of Technology (RIT), is to characterize isothermal aging followed by drop integrity of surface mount mixed (Sn 37 Pb and lead-free) assembly solder joints [11,[42][43][44][45][46][47][48][49]. The investigation involves both forward and backward compatibility.…”

Board‐Level Reliability of Lead‐Free Solder under Mechanical Shock and Vibration Loads

“…The nominal interconnection composition is approximately Sn0.5Ag0.5Cu when the solder paste and the component bump alloy have been mixed at the reflow (perfect mixing assumed). For more details, refer to [32].…”

“…If we consider the entire life span of portable products, the accidental shock loads caused by dropping take place between the thermal cycles caused by internal heat production. Dense continuous networks of grain boundaries may have been produced in the solder interconnections by recrystallization and the above-mentioned two failure modes are mixed when the products are dropped [31,32]. Thus, even though the thermomechanical loads are not always considered as the primary cause of failures in portable electronic products, the changes of microstructures initiated by the cyclic thermomechanical loads make the reliability of these devices dependent on their loading histories.…”

Lead-free soldering: Materials science and solder joint reliability

“…For example, intermetallic compound formation at the solder/pad interface is not the only infl uential factor for solder joint reliability. Resistances of solders to thermal fatigue, 2 recrystallization, 3 and deformation at high strain rate 4 are even more critical. The industry is hoping that materials scientists would do more to address reliability issues of solder joints.…”