I have had the fortunate opportunity to be a part of two revolutionary trends in the electronics industry: surface mount technology (SMT) and Pb-free soldering. There are a number of similarities between the development of these two technologies. Thus, the experiences that were gained from the SMT learning curve can provide helpful guidance as our industry climbs the Pb-free learning curve, if for no other purpose, than to provide a sense of comfort that this “storm” can be weathered, as well.

Before comparing similarities between SMT and Pb-free soldering development efforts, it is important to recognize the following distinction: The change to SMT was not in response to looming legislative and regulatory deadlines. The argument can be made that shorter time-to-market requirements, which developed in response to the explosive growth of off-shore competition during the 1980s and 1990s, established surrogate deadlines that, even by today’s standards, may appear to be even harsher than the July 1, 2006 benchmark imposed by the Restrictions on Hazardous Materials (RoHS) directive. Nevertheless, strictly speaking, those reduced time-to-market goals were largely self-imposed and, as such, provided a sufficient flexibility that allowed manufacturers to overcome many of the hurdles associated with SMT implementation.

The implementation of SMT and Pb-free soldering have a number of similarities. First and foremost is simply establishing the need to embrace a new technology. Many manufacturers determined that, as a business decision, it was not necessary to hop onto the “SMT bandwagon” just because other corporations were doing so. Even today, there is still product being manufactured for which, it is most cost-effective from a processing as well as reliability (warranty) point-of-view to have hand-soldered, PTH interconnections. Similarly, the need to implement Pb-free technology must be a business decision based upon: production costs; long-term reliability (consumer electronics versus military or space systems); market share and market location (which includes the impact of regulatory requirements); part availability; and the manufacturing model (e.g., the availability of Sn-Pb or Pb-free processing lines in-house versus at the contract manufacturer) to name just a few factors.

Second, the electronics community had prior experience with SMT through hybrid microcircuit (HMC) technology. Of course, HMC technology represented a relatively small sector of the overall electronics industry as it also does so, today. Nevertheless, there was a knowledge base in-place from which, was developed materials (e.g., solder pastes), components (e.g., leadless passive chip devices), and processes (e.g., vapor phase reflow) that formed the groundwork for implementing SMT on organic laminate circuit boards. Such a knowledge base has also existed for Pb-free solders. Several studies were performed in the early 1990s in response to US legislation that proposed banning or heavily taxing the use of Pb-bearing materials in manufactured products. High-temperature, Sn-based solders – primarily the Sn-Ag, Sn-Ag-Bi, and Sn-Ag-Cu-Sb alloys as well as several other novel compositions – were investigated in consortia programs as well as through individual Pb-free solder programs in corporations, universities, and government laboratories. Those studies included materials properties testing as well as prototype circuit board projects (surface mount and through-hole) that examined specifically (1) the adaptability of these solders to large-volume assembly processes and (2) the ability of Pb-free interconnections to meet the reliability requirements of consumer, telecommunication, and military products. The important outcome of these prototype investigations was that they demonstrated a basic feasibility of using high-temperature, Sn-based solders in the second-level interconnections of printed wiring assemblies.

Third, recognizing that there was a limited knowledge base in place to establish SMT some twenty years ago, it was also clear that there was a considerable knowledge gap to be overcome for the successful implementation of this new technology. Reflow processes produced defects that required new inspection criteria because the solderability performance of component I/Os and conductor pads differed significantly from that of the corresponding structures on PTH interconnections. Also, it became readily apparent that the relatively new topic of thermal mechanical fatigue (TMF) would have to be quickly understood in order to address the growing concerns about SMT reliability. (Do you remember the early attempts to place leadless ceramic chip carriers, LCCCs, on FR-4 laminate?) By comparison, fatigue degradation was rarely considered with regards to the reliability of more robust PTH interconnections.

A similar knowledge gap exists with Pb-free soldering, today. The higher melting temperatures of candidate Pb-free alloys will require process modifications to accommodate specific package designs and materials. There is also an outstanding need for reliability data of Pb-free interconnections. Fortunately, the Pb-free technology database is slowly growing as test programs come to completion and the results are made available to the electronics community. As was the case with SMT, the business decision to design and/or assemble Pb-free electronics must necessarily take into account a shortfall of technical data and, thus, determine the costs versus benefits of generating the required information in order to support the design, manufacture, and reliability aspects of a particular product line.

Fourth, there is the shortfall of infrastructure. Twenty-some years ago, design organizations that were tasked with implementing SMT, were faced with an absence of surface mount components, laminates, design rules and guidelines, etc. Today, a similar situation exists with Pb-free soldering technology. There is lack of available component I/Os having Pb-free finishes as well as few plastic packages made from molding compounds that can survive the higher processing temperatures. Guidelines and specifications are only now being developed by the standards-writing organizations to address Pb-free interconnections. In the mean time, designers and process engineers will have to consider the development of “mixed” Pb-bearing and Pb-free assemblies in order to meet near-term product demand. Long-term reliability will be a key consideration with mixed assemblies. It is expected that the component market will provide the driving force for suppliers to increase the availability of Pb-free components. However, unlike the “SMT definition” of mixed technology, which refers to assemblies having both PTH and SMT solder joints, the production of interconnections that combine Pb-bearing and Pb-free alloys cannot continue indefinitely due to the July 1, 2006 RoHS deadline and those of other pending regulations worldwide.

In summary, the current effort to implement Pb-free soldering technology into electronics assemblies is, in many aspects, similar to the introduction of SMT twenty-some years ago. The fact that the industry was able to overcome the technical and business hurdles placed before it then, provides a much-needed precedent, which indicates that similar difficulties will be successfully resolved so that a Pb-free soldering technology can be ready to meet the July 1, 2006.

1. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Dept. of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.