HARMONIC VIBRATION DURABILITY OF LOW-TEMPERATURE SOLDER INTERCONNECTS IN PRINTED BOARD ASSEMBLIES

Abstract

The harmonic vibration durability of several low-temperature solder (LTS) interconnects in printed board assemblies (PBA) is examined and compared against benchmark results from SAC305 interconnects. Test vehicles consist of 24 surface-mounted daisy-chained components assembled on an FR4 substrate, including four replicates each of CTBGA192, CVBGA228, CABGA360, and QFN68 components. Each test vehicle is assembled using one of four distinct solder alloys, consisting of either SAC305, LTS1, LTS2, or LTS3. All the BGA interconnects are hybrid bi-material joints with SAC305 ball and respective LTS solder paste. All specimens were isothermally pre-aged at a selected condition and then instrumented with strain gauges, accelerometers and resistance measurement wires to quantify the response and electrical continuity.Vibration durability testing of all test specimens was conducted under clamped-free-clamped-free (CFCF) boundary conditions, using harmonic excitation at the test vehicles’ first resonant frequency. The first resonant frequency was identified for each test specimen using a sine-sweep test and the load levels for the durability tests were established by identifying the overstress limit (through stepped-stress testing) for one SAC305 board and one representative LTS board. During durability trials, components were considered to have failed when the resistance of the corresponding daisy-chain net exhibited at least a 20% increase, over five consecutive scans. The flexural strain amplitudes in the PBA at the footprint of each component (estimated with a calibrated dynamic finite element model of the PBA) and the cycles-to-failure (CTF) of each component were used to construct durability plots for each tested solder alloy and for all four tested component types. A multiscale finite element modeling approach was developed to estimate the volume-averaged Von Mises’ strain within the critical region of the critical solder joints of the CABGA192 and QFN68 components. This was accomplished by starting with the calibrated PBA model mentioned above, which is a simplified linear model that was calibrated against the measured dynamic behavior (natural frequency and response amplitude) of the PBAs under test. The calibrated global model provided nodal displacement boundary conditions to detailed local nonlinear half-models of the modeled components, enabling solder strain estimation in the critical solder joint, for fatigue characterization. All solders were modeled with SAC305 stress-strain curves since LTS stress strain curves are not widely available. The solder strain at the failure site was used to develop strain-life (SN) fatigue models for all four tested solders. SN models showed that all LTS joints consistently outperformed the SAC305 joints across all component types, with LTS3 generally exhibiting the best vibration durability. The alloy with the highest vibration durability was dependent on component type and response level, with LTS3 and LTS1 generally showing the highest durability. Destructive failure analysis was conducted for all component styles. For the BGA component styles, cracking generally occurred in the neck region of the solder ball, with no copper trace failures being observed. In the QFN68 components, fatigue crack locations were found in the bulk of the solder (biased towards the component side), and in some cases, also in copper traces. Thus, the SN models presented here are considered to be lower-bound representations for design purposes. Analysis of the strain and acceleration test data revealed the onset of superharmonic response due to non-linear dynamic behavior at large base excitation levels. In some cases, this nonlinearity allowed transfer of energy to the third resonant mode although the excitation was only in the first resonant frequency. There was significant piece-to-piece variability of this nonlinear response. Nonlinear Finite element analysis confirmed that this variability is attributed to resonance dependent amplification, which occurs when the third superharmonic frequency coincides with the third resonant frequency of the test vehicle, resulting in a substantial increase in the third mode response. In summary, this work has provided strain-based vibration durability assessments for selected high-Bi LTS interconnects, by relating the local PBA flexural strain amplitudes and solder strain amplitudes to CTF across multiple component geometries. The combined experimental and finite element methodology used in this study establishes a framework for vibration fatigue characterization and provides a method for developing high-cycle fatigue (HCF) models for solders, under harmonic loading. The study also elucidates the sensitivity of the interconnect strain fields, and resulting HCF response, on: (i) hybrid bi-material construction of BGA joints; and (ii) geometric differences between BGA and QFN interconnects.