Program in Lead-Free Solder Alloy Development

We are currently involved in a new research effort on development of lead-free solder alloys for use in harsh environment, vehicle, and aircraft applications.

There is currently widespread, almost exclusive use of tin/lead (Sn/Pb) solder in the manufacture and assembly of electronic circuit boards. While Sn/Pb solder is reliable and cost-effective, potential legislation in America, Japan, and Europe to ban or restrict the use of lead in solder alloys is of vital concern to the electronics industry.  Lead is considered to be a health hazard because of the large history of human lead toxicity problems and its documented impact on human populations.  Given the competitiveness of the industry and because all equipment and manufacturing requirements have been based on eutectic Sn/Pb solder, the costs of conversion to a Pb-free solder and its associated reliability impact are not insignificant.  

The goal of our work in this area is to develop a fundamental understanding of alternate solder alloys that will meet the high volume, high yield, and high reliability assembly process for automotive electronics.  

Major Scientific Issues.  Soldering is a complex technology. Production of durable, reliable, safe, and affordable electronic Pb-free solders requires scientific evaluation of materials properties, manufacturing processes and equipment, toxicological effects, cost, and reliability.  Our starting point has been a thorough analysis of current effort in the field, beginning with the Lead-Free Solder Project conducted by the National Center for Manufacturing Sciences (NCMS).  This work has established a starting Pb-free solder alloy database.  NCMS has concluded that there is currently no universal drop-in replacement for Sn/Pb solder and that additional research in critical areas is still needed before implementing wholesale changes to present industry practice.  Further, their assessment showed that modifications in board design and process parameters are required for establishing a manufacturability of Pb-free solders that is equal to or better than what currently exists for Sn/Pb solders.  

In the NCMS study, most lead-free solders tested exhibited adequate surface mount and bottom-side through-hole joint fillets but poor topside through hole fillets.  No Pb-free solder wet as well as eutectic Sn/Pb on most board finishes.  Careful temperature profiling was required to avoid reflow of the topside surface mount components during wave soldering, indicative of the narrower process window for lead-free solders than from eutectic Sn/Pb.  Cross-sections of through-hole joints showed that the solder fillets had separated from the through-hole in many of the high-Sn alloys after cooling from the reflow temperature.  The specific mechanisms that make one alloy more susceptible to fillet lifting than another are unknown.  Further, it is unknown whether Pb-free replacements can tolerate exposure to the higher ambient temperatures in vehicle engine compartments.

Project Organization.  Solder joints in automotive applications typically fail by fatigue and creep under thermal cycling.  A basic understanding of these phenomena in new solder systems must be developed.  Older models for predicting solder joint failure based on eutectic Sn/Pb solder may no longer apply and new ones must be developed.  Basic material properties as a function of temperature will have to be measured to allow finite element modeling of electronic assemblies.  

Unique Solder Materials Facilities
We utilize a broad array of materials, mechanical, and thermal tests to characterize the solder alloys.  A wetting balance allows for measurement of the adhesive forces during wetting.  Video recording and heating capabilities on SEM/EDX enables in-situ, real-time observation of solder wetting characteristics.  A variable-pressure, bell-jar wetting system allows studies of process gas variations on solder wetting.  A high-pressure, UHV-compatible wetting chamber attached to one of our surface analysis systems allows us to study the surface physics during solder wetting and spreading.  A torsion oscillating viscosity system enables measurement of solder viscosity. The oscillating sessile drop method measures solder surface tension.

A unique set of experiments has examined the behavior of the alloys during the wetting process by dynamic surface chemistry studies of the advancing liquid alloy front.  Previous work using surface techniques such as Auger (AES) and X-ray photoelectron (XPS) spectroscopies has shown that low-level alloy impurities can segregate to the liquid surface and cause dramatic changes in wettability.  One of our aims is to determine the critical parameters that affect wetting and provide a comprehensive, atomic-scale picture of solder wetting phenomena.  

The photograph below shows the high-pressure and temperature cell attached to one of our UHV multi-technique surface systems, where in-situ alloy wetting under a variety of conditions can be studied.