Robust Optimization of a Lead Free SMT Process

This paper will focus on Dr. Taguchi’s Robust Engineering methodology, measurement methods and experimental results for the optimization of a lead free SMT process for use in an Automotive Electronics application. The key strategy is to find process parameters that make the process insensitive to noise factors. Traditional optimization approaches focus on maximizing the response variable while the Robust approach focuses on consistent results regardless of variation in noise factors. The Robust method was utilized in the development of a lead free process for manufacturing an Automotive SMT product. Major factors that can create variation in a lead free process were identified, including lead free solder paste brand, paste print speed, oven reflow temperatures and times, and reflow environment. Several noise factors were studied including volume of solder paste, location of components on the board, and lead frame plating materials, namely tin and palladium/nickel/gold. A series of measurements were made on the lead free product that assessed the strength and reliability of lead free solder joints, measurements such as visual scoring, cross-section, surface insulation resistance, and pull strength. Using the Robust experimental design, these measurements were optimized to create high quality and reliable lead free SMT solder joints that were the most insensitive to the noise. In essence, quality was increased by using variable measurements rather than by counting attributes (good/bad). Overall, a gain of 2.1 dB was realized. In Robust terms, this equates to reducing variation in the lead free process by ~22%. This study also revealed which of the processing factors were most significant in controlling the lead free process. The results of this study and the use of Robust Engineering methodology provide a means for developing a full range of lead free technology, components and products used on Automotive Electronics. Introduction One of the challenging new fields in the electronics industry is the implementation of lead free into electronic products. This change is being driven primarily via legislation, especially in Europe, specifically through the WEEE and RoHS Directive in Europe, in its current revision banning the use of lead in electronics by 2006. 2 An essential element to developing these lead free products is the implementation of the lead free manufacturing process. The lead free manufacturing process poses new challenges not encountered in the traditional tin/lead world of electronics. One key difference in the lead free manufacturing footprint from traditional processes is the reduced wetting potential of the lead free solders that will be used. The physics of wetting dictate this reduced wetting since the lead free alloys have higher surface tensions than tin/lead systems, therefore making it more difficult to solder parts. In addition to the wetting challenges, lead free alloys require higher melting temperatures than their tin/lead counterparts, increasing the stresses on the components to be soldered. An important element to lead free manufacturing is limiting the maximum process temperatures and thermal energies to which components are exposed. For example, in the recently released IPC/JEDEC J-STD-020B, a component classification industry standard, lead free packaged parts are only required to be classified up to peak oven temperatures of 245°C or 250°C depending upon package size. Once a process is put into place that can produce soldered parts within such temperature requirements, only then can a sufficient component supply base be made available that can match product requirements. For the automotive electronics industry, the reliability of the manufactured electronic product is important due to the stressful environments seen by these products. In this high volume business, first time quality (FTQ) must be high, and the need for an effectively optimized manufacturing process is an absolute requirement. This is especially true in the emerging field of lead free. This paper discusses the optimization of a surface mount only (SMT) process for a lead free automotive electronics assembly process. SMT was chosen due to the drop-in replacement potential for this soldering technique, as opposed to wave soldering or selective soldering, which are much more immature fields in terms of lead free development. This immaturity is partially the result of the huge capital investments that are necessary for wave soldering or selective soldering since dedicated soldering pots are required because Pb poisoning is problematic. The Robust Engineering methodology was used to optimize this lead free SMT manufacturing process. This methodology is a powerful tool for selecting the best Presented at IPC SMEMA Council APEX 2003 www.GoAPEX.org