In case you missed it.

Material Reliability

"Characterization and Reliability of Increased Cu Levels in SAC Alloys"

Authors: Isabel de Sousa, Dr. Donald W. Henderson, Luc Patry and Robert Martel; [e-mail].

Abstract: Work was undertaken to characterize the effect of using SAC alloys with higher copper levels. SAC solder joints with 3.0 weight % silver content and copper composition levels up to 1.5 weight % were characterized as a function of reflow conditions. As the solder's initial copper content increased, important reductions to the copper pad structures' dissolution and consumption were observed. In the compositional range of 0.5 weight % to 1.2 weight % copper, for the initial copper content in SAC alloy solder balls, it was found that the copper content of BGA solder joints on copper pad structures reached identical copper concentrations within approximately two reflow cycles. Saturation values of approximately 1.2 weight % copper were found at a 250[degrees]C peak reflow temperature. Resultant solder joints were evaluated, using ball pull testing, metallurgical cross-sectioning, polarized light microscopy, SEM examination and solder hardness testing. Modules prepared with solder alloys with higher copper contents were also tested in thermal cycling and shock and vibration testing to resolve any potentially important differences in behavior; these results were compared to general application requirements.

It was found that increasing the initial levels of copper in solder had little impact on basic solder joint properties, but had the significant benefit of diminishing copper pad dissolution during reflow. Implementation of copper-rich SAC alloys will enable more flexibility for solder joint thermal treatment and dramatically improve joint reworkability in the context of thin copper pad structures. (IPC/JEDEC Global Conference on Lead Free Reliability and Reliability Testing for RoHS Lead-Free Electronics, April 2007)

Pb-Free Rework

"Reliability Challenges of Lead-Free (LF) Plated Through-Hole (PTH) Mini-pot Rework"

Authors: L. Ma, A. Donaldson, S. Walwadkar, I. Hsu; [e-mail].

Abstract: Excessive copper erosion was observed in Pb-free mini-port rework for PTH connectors that can potentially lead to quality and reliability issues. The root cause was a SnCu reaction, in which IMC was formed that flakes from the PCB because of molten solder flow/agitation.

This study aimed to identify key variables of Pb-free mini-pot rework and minimize copper erosion; develop an approach to determine minimum copper thickness that ensures performance reliability; determine allowable reduction of copper thickness for rework; capture and determine failure modes/mechanisms through modeling and empirical data collection.

Recommendations include revisiting current IPC specifications and considering an increase in minimum copper thickness limits to 0.001"; minimize variation of new incoming copper thickness specifications; educate manufacturers on the need to fully characterize internal wave soldering processes, inclusive of copper erosion; and enforce "good" PCB design rules; e.g., trace width/tear-drop transition region. (IPC/JEDEC Global Conference on Lead Free Reliability and Reliability Testing for RoHS Lead-Free Electronics, April 2007)

Surface Finishes

"Impact of Surface Finish Choice on Reliability Test Results: Data Analysis of HALT Testing on the Reliant TV-3"

Author: David Cavanaugh

Abstract: Highly accelerated life test conditions do not necessarily age test articles at each experimental level at the same rate. Other data analysis difficulties arise when the experimental design does not consist of a bi-level, fully balanced design. With the use of robust statistical tools and forensic engineering principles, these difficulties may be overcome.

This current study makes extensive use of scatter plots and regression analysis as the analytical framework to uncover the underlying relationships within the Reliant TV-3 statistical DoE and HALT experiment. Six input variables (paste type, wave solder flux, reflow profile peak temperature, wave solder temperature, reflow with nitrogen/air and PCB surface finish) were analyzed against six output factors: number of prior opens (as received), number of soft failures (intermittent during vibration); number of hard failures (persist after HALT); average time to hard failures (for each individual card under test), sum of prior and hard opens (both hard opens); sum of prior, hard and soft opens and the statistical subpopulation (four were recovered).

The three PCB surface finishes (ENIG, OSP and immersion silver) were found to be capable of performing better than the eutectic SnPb control, depending on chosen soldering processing parameters. (IPC/JEDEC Global Conference on Lead Free Reliability and Reliability Testing for RoHS Lead-Free Electronics, April 2007)

CIRCUITS ASSEMBLY provides abstracts of papers from recent industry conferences and company white papers. With the amount of information increasing, our goal is to provide an added opportunity for readers to keep abreast of technology and business trends.