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.

Several failures in Chip-On-Lead (COL) package from the customer were returned for Failure Analysis (FA). Containment activities were able to find similar failures. The connectivity of the silicon die to the leads uses gold wire. The die is in live bug position with respect to the package and is being held in place using non-conductive die attach epoxy. The identification of the Failure Mechanism (FMECH) utilized analysis flow involving non-destructive and destructive FA techniques. A hairline crack was found on the die between the two (2) corner pins. Based on lot history reviews, hairline die crack had a very low detectability at electrical test. Further collaboration with the process owners showed the need to identify the crack initiation, propagation and the factors that could result to this FMECH. Analysis of fracture or fractography was utilized in identifying the crack initiation point and propagation. Due to low detectability, identifying the factors resulting to die crack would require several evaluations and process mappings. Finite element analysis (FEA) was utilized to create models and simulation to identify factors that would result to highly stressed area identified through fractography. These additional data for the hairline crack were vital on the identification of root cause and formulation of corrective/preventive actions.

Failure to apply the proper systematic analysis procedure can result in loss of valuable evidence required to understand the root cause of package failures. For example, in the case of marginal current leakage fail, decapsulation from package front-side may result in loss of the electrical failure signal so that root cause of the leakage failure cannot be understood. In such case, a systematic backside fault isolation method can improve the success rate of isolating the defect. These electrical failures are often due to zero solder bond line thickness (BLT), or filler particle compression on the die, which are key assembly defects encountered in clip style surface mount packages (SMX). In this paper, the first case study is to determine the failure mechanism of an electrical short. A silicon micro-crack propagating through the junction at the dimple clip center, which is due to the ultra-thin solder BLT close to zero micron is found to be the root cause of failure. The second case presents the failure mechanism for a low leakage fail. The pointed tip of a silica filler particle compressed on the die surface leads to excessive leakage.

A land-grid array connector, electrically connecting an array of plated contact pads on a ceramic substrate chip carrier to plated contact pads on a printed circuit board (PCB), failed in a year after assembly due to time-delayed fracture of multiple C-shaped spring connectors. The land-grid-array connectors analyzed had arrays of connectors consisting of gold on nickel plated Be-Cu C-shaped springs in compression that made electrical connections between the pads on the ceramic substrates and the PCBs. Metallography, fractography and surface analyses revealed the root cause of the C-spring connector fracture to be plating solutions trapped in deep grain boundary grooves etched into the C-spring connectors during the pre-plating cleaning operation. The stress necessary for the stress corrosion cracking mechanism was provided by the C-spring connectors, in the land-grid array, being compressed between the ceramic substrate and the printed circuit board.

This paper demonstrates the application of fractography on ductile fracture in gold wire bonding and brittle fracture in micro lateral crack of silicon chip. Different separation mode of a lifted wire ball was mapped through the study of various sizes of dimples, ductile zone and non-welded area. Multiple Focus Ion Beam (FIB) cuts were required at lateral crack area in order to expose the horizontal fracture features to determine the crack propagation direction.

We have been paying attention to the development of the nondestructive physical analysis (NDPA) technology. We think that NDPA is a technology which doesn't depend on the worker's capability or experience. There are many NDPA techniques, and analysis using X-ray imaging is one of the principal techniques. Due to the progress of the image analysis using computers in recent years, X-ray imaging have been evolving from two dimensional images to three dimensional imaging. We have been applying X-ray CT imaging to actual failure analysis and reliability evaluation since 2008. At ISTFA 2009, we reported on the effectiveness of X-ray Computed Tomography (CT) images in the failure analysis. [1] We confirmed that the X-ray CT image had various applications, for example, screening for counterfeit parts, the detection of the defect of the multi-layers printed wiring boards (multi-layers PWB), the structure confirmation of caulking contacts, and the detection of cracks or voids of the solder joint. This paper discusses the effectiveness of X-ray CT imaging in failure analysis and discusses the effectiveness of applying X-ray CT imaging to the propagation of cracks occurring at solder joints during temperature cycling test.