|Some alternative, lead-free materials developed to comply with RoHS requirements can produce tin whiskers, which can cause shorts and device failures.|
At the MD&M West Conference on Thursday, February 14, Simin Bagheri, customer engagement lead at Celestica Inc. (Toronto, Ontario, Canada), will speak on “Materials Selection Based on the New RoHS Requirements: Selecting Alternative Lead-Free Alloys for Circuit Board Assembly.” In the following conversation with MPMN, she details some of the issues associated with the new generation of lead-free, RoHS-compatible materials and provides insights in how to address them.
MPMN: What do you hope to accomplish by presenting at the MD&M West conference?
Bagheri: At MD&M West, I plan to talk about RoHS and its implications for printed circuit board (PCB) assemblies. I also intend to speak about the work that industry has performed to date on developing RoHS-compliant materials and to address some of their limitations. In addition, I will present some of the research that we at Celestica have conducted into evaluating next-generation, alternative materials with improved properties compared with existing RoHS-compliant materials.
MPMN: Please summarize the new RoHS requirements and how they will impact medical device design, development, and manufacturing.
Bagheri: Originally, the RoHS directive excluded medical devices because of concerns that it could have serious device failure consequences. However, in May 2011, the medical device exemption was terminated. Medical device manufacturers must comply with the RoHS requirements by July 2014 to remove from their products six hazardous substances covered by the directive.
The impact of this shift on the medical device industry will be huge. First of all, the time allowed for companies to fully comply with the new directive is limited to slightly more than a year. The reason for this is that medical devices are high-reliability products that have to last for years to come. Thus, manufacturers have to be ready, and time is not on their side.
MPMN: Now that six hazardous materials are to be stricken from the list, what are some of the main alternatives to lead-based materials? What types of RoHS-compatible materials are science and industry developing for use in medical devices?
Bagheri: For more than 50 years, lead-containing alloys have been used in circuit-board assembly applications. Now, the use of lead will no longer be permitted. We mostly refer to RoHS-compatible materials as lead-free because the main element to be removed is lead. While five other constituents must also be removed, these constituent materials may not directly apply to the PCB assembly area. Thus, the materials to be used in the PCB sector are characterized as lead-free alloys.
Forgoing the use of lead and implementing new, alternative alloys has been difficult, however, because the new materials exhibit a range of limitations. We have learned through years of testing and field failure examination of nonexempt products that the limitations associated with alternative, lead-free soldering materials may lead to reliability issues for high-reliability devices.
MPMN: What types of limitations?
Bagheri: The first of the industry-selected alternative alloys with properties most similar to those of tin lead in terms of mechanical properties and thermal requirements are alloys that exhibit an ~30°C higher processing temperature. However, this temperature puts a great deal of strain on other circuit-board assembly materials such as components and laminates. It also causes new failures and failure modes that didn’t exist with lead-based materials. The new alloys also cause solder joints to form that are stiffer than lead-based joints, thus reducing the overall mechanical reliability of PCBs.
In addition, these new alloys have high tin content, which may lead to the growth of tin whiskers. The problem with tin whiskers—small electrically conductive filaments that are observed on nearly all tin alloys—is that if formed, they can serve as bridges between two conductors, resulting in short circuits and subsequent failures. For example, tin-whisker growth has been associated with the cases of unintended acceleration that affected a major car company’s vehicles. And the FDA reported recalls of some heart pacemakers because of the possibility of tin whisker growth. Now, the medical device industry is dealing with the possibility that tin whiskers may grow when new RoHS-compliant materials are used to assemble electronic circuitry.
MPMN: What efforts are being undertaken to overcome these limitations in the PCB materials area?
Bagheri: Among other things, Celestica has shown that tin whiskers are not merely a supply-chain issue but also an important overall manufacturing concern that can be affected by component plating quality, ionic cleanliness levels, and electronic circuit board processing materials and their interactions. In our collaboration with aerospace and defense industry customers, we have gained much experience in identifying the causes of tin whiskers, and we have also developed some mitigation methods to prevent their growth.
MPMN: Could you go into the question of failure analysis for the new materials?
Bagheri: In dealing with the limitations associated with materials that have already been selected for use by the medical device industry, my presentation at MD&M West will discuss a failure mechanism for laminate materials. Because the new lead-free alloys require higher process temperatures, laminate suppliers had to improve the properties of their products to withstand the higher processing temperature requirements. But in doing so, they added fillers to the laminate matrix that can introduce strains inside the PCBs, making them more prone to a dominant failure mechanism in lead-free assemblies known as ‘pad cratering.’
MPMN: In your presentation at the MD&M West conference, what technological solutions do you intend to discuss for addressing the limitations associated with current-generation RoHS-compatible materials?
Bagheri: Among other topics, I will discuss the use of alternative, lower-melting-point alloys for which Celestica has filed two patent applications. The ingredient in these alloys that we think may mitigate the growth of tin whiskers is bismuth. In addition, because these materials have lower melting temperatures than other materials currently selected by the industry to replace lead-based alloys, they may be employed to prevent such failures as pad cratering.
I expect that most of those who attend my presentation will be from major medical device OEMs. Because some medical device manufacturers may previously have thought that RoHS deals only with the supply chain, they may not have done enough research in the short period of time before the RoHS deadline to be fully ready for the transition. Thus, they may be interested in learning about the limitations associated with some of the new alloys and knowing that they should start engaging with a strategic partner to quickly get up to speed in developing new processes.
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