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24 The PCB Magazine • March 2014 to generate data that would allow investigators to generalize the effect of package compliance on the safe working strain of the assembly by correlation of test data from multiple packages to an existing simplified mechanical model. Assembly processing, test methods and results will be documented in addition to discussion on resultant data, failure analysis, distribution parameters. The effectiveness and predictive range possible from the simplified model will also be discussed. Introduction The transition to lead-free assembly is es- sentially complete for many product sectors. Low thermal mass products designed for sale directly into the consumer market such as handsets, smart phones, gaming systems, PCs and netbooks, are all routinely built with lead-free solder alloys and comply with widely enacted environmental legislation. The vast majority of these products are built with al - loys in the tin silver copper (SAC) but there have been some recent introductions of tin copper (SnCu) and tin bismuth (SnBi) alloy systems. There are a wide variety of choices available but the im- pact of SAC and SnCu alloys is that the minimum solder joint temperature for proper re-melting of solder spheres and powders is in the 230– 240°C range. SnBi systems reduce melting temperature significantly but are cur - rently most commonly used in conjunction with alloys of the other two systems to replace wave solder op- erations. While there have been some ver y extensive field failure issues related to material selection, in general the materials required for robust assemblies in these low warranty time, high replacement rate product categories are now readily available. Proper analysis, material qualification and product design will result in "wear out" failure mechanisms and in- warranty repair rates that are in line with those experienced with eutectic tin lead (SnPb) assembly processes. Two principal factors are now driving oth- er higher-reliability and higher thermal mass product sectors to Pb-free assembly processes. First, the resolution of the European Union discussions on the "lead in solder" exemption has produced reasonably clear timelines for the Enterprise Computing and Telecommunication (EC&T) sector. There is a wave of these serv- er room and backbone type products that will transition over the next few years. These prod- ucts are typically designed to the maximum area that can be processed through standard SMT, wave solder and test equipment. Current prod- ucts typically have 20–30 Cu layers and thick- nesses of 0.100–0.130 inches (2.5–3.5 mm) but certainly higher layer counts and thick- nesses of 0.25 inches (6 mm) are predicted for these products. These high layer counts are combined with board dimensions which can exceed 16 x 20 inches (40 x 50 cm). The high thermal mass associated with this type of assembly can drive a 5x to 10x increase in the heat energy requirements when compared to consumer prod- ucts. These increased energy requirements translate into longer thermal profiles and extended exposure times at high temperatures for all processing steps. In addition to thermal considerations these high complexity as- semblies have another com- mon feature: Multiple popu- lations of application specif- ic integrated circuits (ASICs) that usually exist as large BGA packages based on built up substrates with metal heat spreaders. The combination of thick laminate structure and large stiff package design almost certainly defines the maximum stress condition and therefore the lower boundary condition for mechanical in- tegrity under flexure in high complexity as- sembly. Second, EC&T and other sectors with The combination of thick laminate structure and large stiff package design almost certainly defines the maximum stress condition and therefore the lower boundary condition for mechanical integrity under flexure in high complexity assembly. " " SPHERICAL BEND TESTING continues