Choosing the Right Lead-free Solder for Hi-Rel Printed Wiring Assemblies
J.K. “Kirk” Bonner, L. del Castillo, A. Mehta
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Drive
Pasadena, CA 91109 USA
The use of lead in electronics has come under increasing scrutiny. Given the trends in both Japan and Europe, it is highly likely that the U.S. will be driven by commercial interests to phase out of lead in electronics usage. This paper presents data collected on a recent NASA project to focus on finding suitable alternatives to eutectic tin-lead solders and solder pastes. The first phase of this project dealt with determining the most feasible candidates to replace tin-lead and to determine suitable processing operations in assembling printed wiring boards.
Lead-containing solder has been used for past 60 years as the principal joining material for Level 2 packaging defined as attaching component level packages to a suitable substrate to produce printed wiring assemblies (PWAs). Generally the solder of choice has been either eutectic tin-lead solder containing 63 wt. percent tin (Sn) and 37 wt. percent lead (Pb) or near-eutectic tin-lead solder containing 60 wt. percent tin and 40 wt. percent lead. Eutectic tin-lead solder has a unique melting point of 183°C (361°F), whereas near-eutectic tin-lead solder melts within the range of 183°-189°C (361°-372°F).
Tin-lead solder forms stable solder joints capable of operating in a wide variety of service environments. It is easy to use, and because it has a relatively low melting point, rework and repair are also easy. In addition, the reliability of tin-lead solder joints is well understood. Using Coffin-Manson curves and Weibull distribution plots of thermally cycled solder joints, it is possible to estimate the amount of useful life remaining for tin-lead solder joints after exposure to a known number of thermal cycles.
Four (4) lead-free solder pastes were selected based on an extensive search of the literature. Two PWBs per solder type were assembled using the four different solder pastes resulting in total of eight assemblies. These are:
The next phase of the project will be to downselect to two (2) of these pastes, assembly four (4) PWAs per paste and assembly four using eutectic tin-lead (Sn63Pb37—183°C (TM)) as a control, and thermal cycle the boards.