Center for Advanced Life Cycle Engineering

One of the remaining challenges associated with the use of lead-free materials in electronics is the potential failure risk associated with tin whiskers. Despite intense research in recent years, there are still no accelerated test methods to reliably predict whisker growth in field application conditions. Semiconductor component and electronic system manufacturers rely on a set of environmental exposure conditions defined by industry standards to qualify and monitor plating processes. While these test methods offer a common platform for package qualification, their value for quantitative whisker-induced failure risk assessment is quite limited. Additionally, these tests are timeconsuming and labor intensive, making it difficult to use for purposes such as process improvement, control, and monitoring. In this paper, a mechanical indentation method is evaluated for assessing the whisker growth propensity of tin and select tin alloys plated on copper, brass, and alloy 42 substrates. Indentation-induced growth results are compared with responses to standard test conditions including temperature cycling and elevated temperature and humidity.

The past decade has seen the development of commercial, third-generation, high reliability Pb-free solder alloys designed to meet the requirements of higher temperature use environments. Most of these offerings are based on the Sn-Ag-Cu (SAC) system, with major alloying additions of bismuth (Bi), antimony (Sb), or indium (In). These elements, individually or in combination, promote additional precipitation, solid solution, or dispersion strengthening that can resist microstructural degradation at elevated temperatures or during aggressive thermal cycling. Results from the literature show that an increase in thermal cycling dwell time can decrease the thermal cycling reliability of SAC solders. Because these high reliability solder alloys are designed for extended operation at higher temperatures, it is important to understand their behavior and characterize their reliability at extended thermal cycling dwell times. This paper presents the initial results from an experimental program designed to compare thermal cycling results for high reliability solder alloys using a typical dwell time of 10 minutes to an extended dwell of 60 minutes. The 10-minute data were generated in the initial phase of testing and preliminary results were published previously. The current data are based on a thermal cycling test condition of −55/125 °C (TC7 in IPC-9701B) and the test vehicles are a 192-pin chip array ball grid array (192CABGA) and an 84-pin thin core ball grid array (84CTBGA). The high reliability alloys were found to outperform the prevalent SAC305 (Sn3.0Ag0.5Cu) consistently. With the 192CABGA, SAC305 showed a moderate loss of reliability with increased dwell time, but some high reliability alloys had comparable performance with 60-minute and 10-minute dwell times, and others performed much better with the 60-minute dwell. With the 84CTBGA, the high reliability alloys have comparable performance with 60-minute and 10-minute dwell times, with only one alloy performing slightly better with the 60-minute dwell. The surprising finding was that SAC305, when tested with the 84CTBGA, exhibits no dwell time effect with the −55/125 °C test condition. All the alloys exhibited thermal fatigue failures in the bulk solder, but most of the high reliability alloys also exhibited interfacial or mixed mode failures, which complicates interpretation of the data.

The past decade has seen the development and introduction of commercial, third-generation, high-performance Pb-free solder alloys designed to meet the requirements of higher temperature use environments. Most of these offerings are based on the Sn-Ag-Cu (SAC) system, with major alloying additions of bismuth (Bi), antimony (Sb), or indium (In). These elements, individually or in combination, promote additional precipitate, solid solution, or dispersion strengthening that can enhance resistance to degradation at elevated temperature or during aggressive thermal cycling. Results from the literature show that an increase in thermal cycling dwell time can decrease the thermal cycling reliability of SAC solders. Because high-performance alloys are designed for extended operation at higher temperatures, it is important to understand their behavior and characterize their reliability at extended thermal cycling dwell times. This paper presents the initial results from an experimental program designed to compare thermal cycling results for high-performance solder alloys using an extended dwell of 60 minutes to a typical short dwell time of 10 minutes. The 10-minute dwell data were generated in the initial phase of testing and published previously. The data reported here are from a thermal cycling profile of -55/125 °C (TC7 in IPC9701B) and the test vehicle is a 192-pin chip array ball grid array (192CABGA). Contrary to the results for SAC solders, the 60 minute dwell time did not reduce the reliability consistently for all the high-performance alloys in the test matrix. Based on evaluation criteria of characteristic lifetime and 1% cumulative failure rate from a 2-parameter Weibull plot, the high-performance alloys had comparable reliability performance with 60-minute and 10-minute dwell times. Although all the alloys exhibited fatigue failures in the bulk solder, many of the alloys also exhibited interfacial and mixed mode failures, which complicates interpretation of the data. Multiple failure modes for these solder alloys also were reported for the 10-minute dwell testing.

The industry standard for accelerated temperature cycling, IPC-9701B, defines test conditions for characterizing solder interconnect fatigue. The document specifies a maximum cycling ramp rate of 20 °C/minute to avoid thermal shock conditions that can accelerate failure modes other than low cycle fatigue in the bulk solder. This investigation compares the performance and failure mode of two ball grid array (BGA) packages tested with cyclic ramp rates characteristic of thermal cycling and thermal shock. The accelerated temperature profile is from −40 °C to 125 °C for thermal cycling and thermal shock, with the cycling ramp rates approximately three times faster with thermal shock and with equal dwell times in cycling and shock. The test matrix includes BGA packages fabricated with eutectic SnPb and near-eutectic SAC305 solder alloys as the performance baselines, and three high-performance solder alloys based on the SAC system but modified with additions of bismuth (Bi) and antimony (Sb). The failure data are reported as characteristic lifetime η (the number of cycles to achieve 63.2% failure), slope β, and cumulative 1% failure from a two-parameter Weibull analysis. Destructive cross-sectional analysis was used to characterize the solder microstructures before and after testing and the interconnect failure mode. The results show that the thermal cycling ramp rate and the thermal shock ramp rate produced the same results quantitatively and the same failure modes. These findings apply to the two BGA components in combination with the five solder alloys used in the study. For these test conditions, components, and solder alloys, the same results were achieved in thermal shock compared to thermal cycling with a 30% savings in test duration.

Sintered silver materials (with and without epoxy matrices) are used in microelectronics, as high-temperature interconnect materials, and also as conductor trace materials in printed electronic circuitry. The sintering process results in an interconnected assemblage of discrete agglomerated particles. This results in intrinsic length-scale effects under the action of different stress gradients. In other words, the effective homogenized average continuum-scale material behavior changes with the local magnitude of the stress gradients. Consequently, regions of sharp, localized stress concentrations have to be modeled with different effective continuum material properties, compared with the properties that are relevant for regions that have a uniform stress field. In this study, the focus in on the effective creep behavior, in particular. This length-scale effect is empirically explored in this study using nanoindentation with indenters of different tip radii, causing different stress gradients. Properties estimated by each indenter are compared to demonstrate the dependence of the effective continuum properties on the local length scale effects (generated by the ratio of the tip radius to the characteristic discrete dimension of the sintered particles).