Dave Nelson and Hector Pallavicini
Qian Zhang, Paul Friesen, and Abhijit Dasgupta
CALCE Electronic Products and Systems Center
University of Maryland
College Park, MD 20742
Due to environmental and political concerns, and although significant uncertainties remain about the potential for lead contained in electronic products to have any significant impact on the environment, the use of lead in electronic products is an increasingly visible topic. Because of processing or reliability limitations, entire product categories are being exempted from legislation banning the use of lead. It appears that multi-track electronics manufacturing where some assemblies will use traditional eutectic solder while others will use various lead-free alloys is a certainty. A result of multi- track manufacturing will be products that require the use of both SnPb and Pb-free components. The intent of the study is to provide the avionics industry with reliability data that addresses Pb-free and mixed system assemblies.
A CCA test vehicle was designed to analyze several design and assembly process variables when subjected to environmental extremes. CCA variables included solder paste type, solder bump type, device type, and use of underfill materials. Solder paste included Sn63/Pb37 and (95.5Sn/3.8Ag/0.7Cu) alloys. Solder bump were either SnPb or Pb-free. Device types included area array components with pitches of 0.5, 0.75, 0.8 and 1.27 mm. Use of underfill material considered presence or absence of non-reworkable underfill. Test vehicle design permitted real time solder joint interconnection monitoring by measuring resistance through daisy-chained loops. Thermal cycling was used to stress the CCA in environmental testing. The thermal cycle profile consisted of temperature extremes from 55 to +125o Celsius with a 15- minute dwell at hot, a 10-minute dwell at cold, and a 5-10o Celsius per minute ramp rate.
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