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The Thermal Cycling Ramifications of Lead-Free Solder on the Electronic Assembly Repair Process

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To be published: IEEE Transactions on Components, Packaging and Manufacturing Technology, 2011 CALCE, University of Maryland 1 February 25, 2011 The Thermal Cycling Ramifications of Lead-Free Solder on the Electronic Assembly Repair Process Andrew Chaloupka, Peter Sandborn, and Anthony Konoza CALCE Electronics Products and Systems Center Department of Mechanical Engineering University of Maryland College Park, MD 20742 USA Abstract - The conversion from tin-lead to lead-free electronics has created concern amongst engineers about the reliability of electronic assemblies and the ramifications that reliability changes may have on the life cycle cost and availability of critical systems that use lead-free electronics. In order to analyze the impact of the tin-lead to lead-free electronics conversion in terms of life cycle cost and availability, a simulation of fielded electronic systems to and through a board-level repair facility was created. Systems manufactured with tin-lead parts or lead-free parts that are fielded, fail and have to be repaired are modeled. The model includes the effects of a finite repair process capacity, repair prioritization, multiple possible failure mechanisms, no-fault-founds, and un-repairable units. The model is used to quantify and demonstrate the system- and enterprise- level risks posed by the conversion from tin-lead to lead-free electronics. Example analyses were performed on electronic assemblies that use SAC (tin, silver and copper) and tin-lead solder using a repair process modeled after a NSWC Crane Aviation Repair Process (8000 assemblies with 30 year support lives were modeled). The components considered consisted of Ball Grid Array (BGA), Column Grid Array (CGA) and Leadless Chip Carrier (LCC) packaged parts that experienced three different thermal cycling profiles. The case studies revealed that when exposed to usage profiles characteristic of consumer electronics, low maximum and mean thermal cycling temperatures with long dwell times, SAC exhibited significantly reduced repair costs compared to tin-lead. For usage profiles characteristic of aerospace and high-performance applications, high maximum and mean thermal cycling temperatures with short dwell times, SAC exhibited significantly increased repair costs when compared to tin-lead. Index Terms - Lead-free electronics, Repair Simulation, RoHS, AHP, cost, availability I. INTRODUCTION The impact of transitioning from tin-lead to lead-free solder parts is affecting the electronics industry and most severely the aerospace and defense industries that produce products that require high levels of reliability. Products produced for applications known as AHP (Aerospace and High Performance) [1] are characterized by severe or harsh operating environments, long service times, and high consequences of failure [2]. Due to the high consequences of failure, AHP systems are currently excluded from the Restrictions on Hazardous Substances (RoHS) directive [3,4]. The current directive excludes equipment solely for the purpose of national security and military purposes that are not included in the consumer categories described in the RoHS Directive. Although excluded from requirements to use lead-free parts, most defense and aerospace manufacturers must utilize the same supply chain as commercial electronics manufacturers for parts and boards. While the supply chains can still produce legacy products that use tin-lead solder, they have relatively little motivation to continue toTo be published: IEEE Transactions on Components, Packaging and Manufacturing Technology, 2011 CALCE, University of Maryland 2 February 25, 2011 do so because the defense and aerospace industry represent less than 5% of the total market share [5]. As a result, commercial manufacturers are focused on providing lead-free parts for the commercial electronics industry. The limited availability of lead-based items has become a major driver in the design and sustainment of defense and aerospace systems as the number of tin-lead electronic suppliers has decreased. This challenge will require the defense and aerospace industry to convert to lead-free long before the RoHS directive requires it to (if ever), i.e., their current exclusion from RoHS is effectively a moot point. Irregardless of the reasons for conversion from tin-lead to lead-free electronics, the conversion is a reality and the ramifications of the conversion need to be understood. Many AHP lead-free products will be serving in platforms where long-term (greater than 15 years) reliability is a critical requirement. For these long field life systems, the impact of reliability may be most prevalent at the system-level and enterprise-level when the sustainment (support) of products must be considered. Enterprise-level impact, refers to the impact on support logistics (sparing and repair flow: repair time, repair cost, backlog) over the support life cycle of a larger population of systems. The impact of the conversion to lead-free must be quantified in order to provide performance expectations and provide risk mitigation if and when needed to program-level management. The next section of this paper discusses the motivation behind the development of the repair-process model described in this paper. The development is followed in Section III by a description of the model, and a detailed thermal cycling case study performed in Section IV. II. MOTIVATION Engineers communicate to program-level management every day that the “sky is falling” due to some previously unforeseen technical issue (lead-free, tin whiskers, counterfeit parts, obsolete parts, etc.), but management is rarely moved to action without a quantitative demonstration of the system-level and/or enterprise-level risks posed by the issue. The potential for reduced and less predictable reliability of lead-free electronics increases the probability that a serious technical issue will arise. While engineers have the resources to model and quantify system reliability, they often lack the ability to articulate the


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