Gregory Henshall

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.

Design, materials, and assembly process of high‐density packages with a low‐temperature lead‐free solder (SnBiAg)

Purpose – The purpose of this paper is to discuss the design, materials, and assembly process aspects of a study, conducted by The High Density Packaging Users Group Consortium, into process development and solder joint reliability of high‐density packages on printed circuit boards using a low‐melting temperature lead‐free solder (Sn‐57 wt%Bi‐1 wt%Ag).Design/methodology/approach – The components studied include several SMT package types and various lead configurations. The assembly process addresses the low‐temperature lead‐free assembly process, inspection and analysis of these boards and packages.Findings – It was found that, the assembly process of the SnBiAg lead‐free test boards is very robust and the assembly yield is almost 100 percent.Originality/value – The paper is of value by presenting a description of the rationale and material set used for an experiment to test SMT assembly and reliability characteristics using the 57Bi‐42Sn‐1Ag alloy, which has a melting point of 139°C.

Reliability test and failure analysis of high‐density packages assembled with a low‐temperature lead‐free solder (SnBiAg)

Purpose – The High Density Packaging Users Group Consortium has conducted a study of process development and solder‐joint reliability of high‐density packages on printed circuit boards (PCB) using a low‐melting temperature lead‐free solder. The purpose of this paper is to investigate the reliability tests (e.g. temperature cycling and shock and vibration) and failure analysis (FA) of high‐density packages on PCB with the low‐melting temperature lead‐free solder (Sn‐57 wt%Bi‐1 wt%Ag).Design/methodology/approach – The design for reliability, materials, and assembly process aspects of the project have been discussed in “Design, materials, and assembly process of high‐density packages with a low‐temperature lead‐free solder (SnBiAg)” also published in this journal issue. In this study, reliability tests (e.g. temperature cycling and shock and vibration) and FA of high‐density packages on PCB with the low‐melting temperature lead‐free solder (Sn‐57 wt%Bi‐1 wt%Ag) are investigated.Findings – Lead‐free solder‐jo...

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.

Acceptance testing of BGA ball alloys

The industry has seen the development of a wide range of new Pb-free BGA ball alloys. A significant element of uncertainty regarding these new alloys is the lack of defined data requirements for alloy acceptance. This paper describes recent efforts at Hewlett-Packard to develop Pb-free solder alloy testing requirements. To facilitate the standardization of alloy testing, the required tests are divided into three major areas. • Material properties • Solder joint reliability • Impact to manufacturing processes This paper presents the approach to assess the risk of using new Pb-free BGA ball alloys on printed circuit assemblies.