Lead-free solder joints tend to be more susceptible to brittle fracture, and thus susceptible to drop-damage. Drop testing of handheld ultrasound devices revealed broken solder joints on a large inductor component. Analysis of the cracks showed a dual intermetallic compound (IMC) layer of Ni 3 Sn 4 (closest to the nickel) and (Ni,Cu) 6 Sn 5 , with the crack occurring in between the two layers. The inductor had a tinned nickel lead finish; the solder was SAC305 (a common lead-free solder comprising Sn, Ag, and Cu); and the printed circuit board (PCB) had a standard copper finish. The failure occurred very soon after manufacture and had not been enhanced by temperature cycling or aging, but it was not a time-zero failure: mechanical shocks from drops were required to propagate the crack through the joint fully. Strain measurements did not find any large strains after reflow and assembly, and no other components on the board showed cracking. There was no cracking observed at the PCB (Cu) side of the solder joint. The solution ultimately was to redesign the board, replacing the large single component with several smaller ones.

The application of an individual failure analysis technique rarely provides the failure mechanism. More typically, the results of numerous techniques need to be combined and considered to locate and verify the correct failure mechanism. This paper describes a particular case in which different microscopy techniques (photon emission, laser signal injection, and current imaging) gave clues to the problem, which then needed to be combined with manual probing and a thorough understanding of the circuit to locate the defect. By combining probing of that circuit block with the mapping and emission results, the authors were able to understand the photon emission spots and the laser signal injection microscopy (LSIM) signatures to be effects of the defect. It also helped them narrow down the search for the defect so that LSIM on a small part of the circuit could lead to the actual defect.