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Friday, 29 February 2008 19:00
Technical Abstracts Hand Soldering

“A Study of Soldering Tip Life on Lead-Free Alloys”

Authors: Juthathip Fangkangwanwong, Jareerat Jintana, Jarinee Ketui and Teng Hoon Ng; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Abstract: Soldering tips operating with Pb-free alloys show less durability than those operating with conventional SnPb solder. This is exacerbated by the higher temperature, higher tin content (tin readily erodes iron plating) and more aggressive flux in Pb-free soldering. Since the implementation of the RoHS directive, many companies have adopted SAC 405. Hence, for this study, SAC 405 solder wire was chosen as the natural rework solder for the investigation on the durability performance of solder tips from various suppliers. One of the known concerns of SAC 405 is the higher copper dissolution rate, which sometimes causes increased defect rates, especially in multiple PTH reworks. Another concern is the relative higher price of the alloy because of the silver content. In this paper, an alternative Sn-Cu + Ni is proposed to determine the effect of alloy on solder tip life when compared with SAC 405 base alloy. (SMTA Pan Pacific Symposium, January 2008)

Jetting

“Enhanced Stability of Electrohydrodynamic Jets through Gas Ionization”

Authors: Sibel Korkut, Dudley A. Saville and Ilhan A. Aksay

Abstract: A new technique for printing extraordinarily thin lines quickly over wide areas could lead to larger, less expensive and more versatile electronic displays, as well as new medical devices, sensors and other technologies. Theoretical predictions of the nonaxisymmetric instability growth rate of an electrohydrodynamic jet based on the measured total current overestimate experimental values. The authors show that this apparent discrepancy is the result of gas ionization in the surrounding gas and its effect on the surface charge density of the jet. As a result of gas ionization, a sudden drop in the instability growth rate occurs below a critical electrode separation, yielding highly stable jets that can be used for nano- to microscale printing. (Physical Review Letters, Jan. 25, 2008)

Solder Reliability

“Methodology to Characterize Pad Cratering under BGA Pads in Printed Circuit Boards”

Authors: Mudasir Ahmad, David Senk, and Jennifer Burlingame; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Abstract: The conversion to Pb-free BGAs has raised several new assembly and reliability issues. One reliability concern becoming more prevalent is the increased propensity of pad cratering. In general, Pb-free solder joints are stiffer than SnPb solder joints, and some Pb-free compatible PCB dielectric materials are more brittle than conventional SnPb-compatible PCB materials. These two factors, coupled with the higher peak reflow temperatures for Pb-free assembly, could transfer more strain to the PCB dielectric structure, causing a cohesive failure underneath the BGA corner pads. The likelihood of pad cratering occurring in any given assembly depends on several factors, including BGA package size, construction and surface finish, PCB pad size, and material and surface finish. Standard assembly-level bend, shock and drop tests can be used to determine if the entire assembly can survive a given strain and strain-rate range without failures. However, with these standard assembly-level tests, it is difficult to determine if the failures occurred as a result of an unusually weak PCB dielectric/PCB pad size or a stiffer BGA package.

In this study, an easy-to-implement test method is presented, along with results comparing known good and known bad PCBs. Different dielectric materials and pad sizes were evaluated to develop a comparative metric that can be used to rank-order different material/pad size combinations. The results were generated over different temperatures, to study the effect of temperature on dielectric mechanical strength characteristics. Finite element analysis was performed to better understand the factors impacting variation in results. (SMTA Pan Pacific Symposium, January 2008)

 
“Analyzing and Predicting Electrochemical Migration Failures On Field Failure Returns”

Author: Renee J. Michalkiewicz; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Abstract: This paper outlines options available to analyze a specific lot of failed assemblies and steps that can be taken to prevent ECM failures on future lots. J-STD-001 is used as a guideline in preparing a customized test procedure. Exact procedures that may be used to assess working assemblies for ECM potential are outlined. Case studies are included. As a general procedure, monitoring points connected to the area of concern are isolated, often by removing components or cutting other traces, while wires are soldered. The assemblies are placed in a temperature/humidity chamber and a bias is applied across the suspect location. The resistance between these isolated points is monitored for sudden or slow drops indicative of leakage current development or dendritic growth. If ECM development is observed on these assemblies from the same lot, the entire lot should be considered at risk. (SMTA Pan Pacific Symposium, January 2008)

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.

 

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