Richard D. Parker

Tin Whisker Test Development—Temperature and Humidity Effects Part I: Experimental Design, Observations, and Data Collection

The effects of temperature and humidity on tin whisker growth were investigated through a collaborative project sponsored by the International Electronics Manufacturing Initiative (iNEMI) and its member companies. A broad range of testing conditions was adopted to test a variety of components with matte tin (Sn) plating and copper (Cu)-based leadframes. The primary goal of the study was to collect data that could be used to develop mathematical models (acceleration functions) that describe the dependence of tin whisker growth and corrosion on temperature and humidity. This paper describes the background, experimental design, data collection and reports results. Part II of the study (J. W. Osenbach et a. ?Tin whisker test development-Temperature and humidity effects part II: Acceleration model development,? Electronics Packaging Manufacturing, Vol. 33, no. 1, pp., Jan. 2010) discusses in the data analyses and acceleration model development. Storage testing was performed over a wide range of temperature and humidity conditions from 30?C to 100?C and from 10% to 90% relative humidity (RH). Commercially produced components with both 3 ?m and 10 ?m thicknesses from three sources were evaluated. For components with the 10 ?m-plating, the plating was evaluated in both the as-plated and reflowed (260?C) conditions. These variations resulted in a large experimental matrix that included 13 different Sn platings, aged at ten different temperature and humidity combinations. Further, the aging test was done at five different laboratories with inspections performed at eight different laboratories. The data collected include 1) corrosion incubation time, 2) tin whisker incubation time, and 3) dependence of the maximum whisker length on storage time at each temperature/humidity condition. Data suggest that corrosion is not a unique driving force for whisker initiation and growth. Whisker formation differs in corroded and non-corroded regions. Due to the scope of this work, it is broken down into two papers. The data and experimental observations are discussed in this paper. The mathematical model development, discussion of results and conclusions are included in Part II of this study.

Tin Whisker Test Development—Temperature and Humidity Effects Part II: Acceleration Model Development

The incubation time for both whisker growth and corrosion in thin Sn platings (3-10 ¿m thick) on Cu-based alloys have been found to be well represented by an exponential function of humidity and an Arrhenius function of temperature for both as-deposited and reflowed tin platings. Furthermore, whisker growth was found to follow the same functionality in both corroded and non-corroded regions of the plating. The effective activation energies and humidity coefficients were found to depend upon plating thickness, exposure to reflow, and presence of corrosion. The effective activation energies ranged from 0.23 eV to 0.41 eV and the humidity coefficients ranged from -0.012% to -0.031%. Corrosion enhanced whisker growth occurred by lowering the effective activation energy for whisker growth. A theory based on excess, non-creep relaxed, oxidation induced strain was developed to explain the corrosion induced energy barrier lowering. The data showed that 60°C/87%RH appears to be the optimal high temperature/high humidity test condition at this time for Sn over Cu substrates. Within the limits of the whisker and corrosion (incubation) acceleration functions developed in this study, it is concluded that the JEDEC tests can be used to indicate behavior at other temperature/humidity points that could be relevant storage or service conditions.

The effect of nickel microalloying on thermal fatigue reliability and microstructure of SAC105 and SAC205 solders

This study explores the effect of a nickel (Ni) microalloy addition on the thermal fatigue performance and microstructure of two low Ag content, Pb-free solder alloys, Sn-1.0Ag-0.5Cu (SAC105) and Sn-2.0Ag-0.5Cu (SAC205). The alloy performance was evaluated using two different area array component test vehicles, an 84-pin chip scale package (CSP) and a 192-pin fine pitch ball grid array (BGA). The baseline alloy microstructures were characterized using polarized light microscopy and scanning electron microscopy with backscattered electron imaging for phase identification. Thermal fatigue performance was assessed with accelerated thermal cycling (ATC) using four temperature cycling profiles with distinct temperature ranges (ΔT) and temperature extremes. Additionally, each temperature profile used a standard 10 minute dwell time or an extended 60 minute dwell time. A microalloy addition of 0.05% Ni was found to alter the base microstructures of the SAC 105 and SAC205 alloys. Generally, the Ni addition improved the thermal fatigue life but the improvement was not consistent in both alloys, both components, and across all thermal cycling profiles. The most consistent response was with the 84CTBGA component, which showed improved reliability with the Ni addition in all of the thermal cycles.

Thermal Cycling Reliability of Alternative Low-Silver Tin-Based Solders

Sn-3.0Ag-0.5Cu (SAC305) alloy is the most widely used solder in electronic assemblies. However, issues associated with cost and drop/shock durability have resulted in a continued search for alternative solder alloys. One approach to improve the drop/shock reliability has been to reduce the silver content in Sn-AgCu alloys. Another approach is doping Sn-Ag-Cu solder with additional elements. Moreover, conflicting results have been reported in literature on the effects of aging on Sn-Ag-Cu alloys. In 2008, International Electronics Manufacturing Initiative (iNEMI) started the “Characterization of Pb-Free Alloy Alternatives” project to provide a comprehensive study of fifteen tin-based solder interconnect compositions benchmarked against the eutectic tin-lead solder. For this study, temperature cycle durability was the primary focus and solders were selected to study the effect of varying silver content, microalloy additions, and aging. This paper reports the preliminary findings from one of the test conditions conducted under the iNEMI project. The cycles to failure for a temperature cycling test condition from -15oC to 125oC, with dwell times of 60 minutes at both extremes are presented. The test assembly consisted of sixteen 192 I/O BGAs and sixteen 84 I/O BGAs soldered on to LG451HR laminate. Preliminary findings revealed that the reduction of silver resulted in a reduction in cycles to failure. In all cases, the fifteen tin-based solders were more durable than the eutectic SnPb solder. Aging did not affect the cycles to failure in SAC105 solder; however, the cycles to failure decreased with aging in SAC305 solder. In addition, aging resulted in a wider distribution of cycles to failure in 192 I/O BGAs.

iNEMI Pb-free alloy characterization project report: Thermal fatigue results for low and no-Ag alloys

Significant innovations in Pb-free solder alloy formulations are being driven by volume manufacturing and field experiences. As a result, the industry has seen an increase in the number of Pb-free solder alloy choices beyond the common near-eutectic Sn-Ag-Cu (SAC) alloys first established as replacements for Sn-37Pb. The increasing number of Pb-free alloys provides opportunities to address shortcomings of near-eutectic SAC, such as poor mechanical shock performance, but also introduces a variety of technical and logistical risks. Since 2008, the Pb-Free Alloy Characterization Program sponsored by the International Electronics Manufacturing Initiative (iNEMI) has been working to fill the gap in knowledge associated with thermal fatigue resistance of these new solder alloys. Results from the extensive experimental program are now becoming available and are being published through a series of publications (see References). This paper provides a summary of the overall iNEMIs program goals, the experimental structure, and the results and analysis of thermal cycling for low silver alloys, containing 1 wt.% or less Ag. Results indicated that there is a correlation between the characteristic life of short dwell thermal cycles and Ag content. Increase in the Ag content increased the characteristic life. Another important finding is that all low-and no-Ag alloys performed better than Sn-37Pb under the test conditions. Finally, as the stress levels increase during thermal cycling, the performance differences between the Pb-free alloys diminish, and their performance appears to be approaching that of Sn-37Pb.