Evaluating Polymer, Free Assembly,

Soldering is one of the oldest and most successful manufacturing technologies in use today-boasting a 5,000 year history and an immense database.’ Ancient artisans perfected the use of precious metal hard solders, but Roman engineers optimized the use of soft solders for practical industrial use. Tin-lead (SnPb) solders were used to join lead sheets that lined the magnificent aqueducts.2 Low-temperature SnPb solders were the ideal “adhesives” then, just as they are today (setting aside their toxic characteristics). Tin and lead form alloys with unique properties that may never be duplicated with another system. While others decry, defend and debate the continued use of solders that contain lead, our goal should be to look at alternative technologies and to examine successful strategies for identifying and evaluating them. Science and logic are appropriate tools for studying and predicting the ultimate fate of “electronic lead,” but they will not suffice for predicting the outcome of lead reduction legislation. While the electronics industry remains reactive in dealing with regulatory efforts, we, as technologists, must be proactive in discovering, confirming and implementing new technologies for joining electronic elements. Government, industrial and university laboratories around the world are studying new alloys and polymerbased joining materials. Several groups are analyzing non-lead metal alloy data, and the evaluation of some candidates has already begun. In the US., the Center for Solder Science and Technology at Sandia National Laboratories (Albuquerque, NM) is playing a major Those working in metal alloy research, including Alpha Metals, view the task as very difficult due to the large number of criteria that must be simultaneously met and the limited number of metals that are economically and environmentally acceptable. Non-lead solders are in use today but only for very limited applications. No commercial product even comes close to being a general purpose drop-in replacement for SnPb solders. Many candidates could require new equipment and circuit finishes. The answers will not come quickly. The other approach to lead-free electronic joining is the search for suitable, non-solder alternative technologies. Under its Environmentally Conscious Manufacturing Program, the National Center for Manufacturing Services (NCMS) will play a coordinating role in this arena within North America.’ Nordic countries, however, began to evaluate interconnect alternatives many years ago through the efforts of IVF in Sweden and Elektronik-Centralen (now called DELTA) in Denmark.4.5 Both are government-supported laboratories with significant industrial sponsorship. Through NASA, the US. did examine solder replacements in the 1970s and with good results.6.7 But this was an era when fine-pitch, low-temperature processing and no-lead were considered by many to be relatively unimportant.* Today’s leading candidate is a metal-filled, thermoset isotropic conductive adhesive. The properties, processes and electrical conductivity mechanism are different for polymer systems since they are metal/polymer composites. Although we may agree that “polymer solders” should be required to meet most of today’s metal solder performance criteria, new tests must be added, and some old methods will need to be modified. Solder junctions have intrinsic electrical conductivity, and solder testing focuses on mechanical performance. Commercial polymers are electrical insulators, and conducting fillers must provide the electrical pathway. We will first examine the differences between conductive adhesives and metal solders and then explore test criteria and methods.