Printed Circuit Design & Fab - January 2008 - (Page 22)

PTH RELIABILITY The Survival and Long-Term Reliability of LEAD-FREE PCBs A DfA program that designs and specifies PCBs compatible with lead-free soldering is often at odds with a cost driven manufacturing metric. by WERNER ENGELMAIER Significant attention and a great deal of effort have been expended on understanding the reliability of lead-free solder joints. However, difficulties posed by lead-free soldering as it relates to the reliability of PCB interconnect structures is perhaps more significant. Problems with PCB viability after the assembly process comes primarily from one source – the temperatures required to attach components during the lead-free assembly soldering process. These reliability issues required the development of a White Paper1 from which the materials in this article were taken. With SnPb soldering, PCB reliability was typically not an issue, but in our brave new lead-free world, this is no longer the case. PCBs now need to be designed and specified to be fully compatible with lead-free soldering. Unless these changes are included in the specification of a PCB, this will not happen, because PCB fabricators will build based on lowest possible cost. To carry out the changes needed to assure lead-free assembly success often comes with an increase in manufacturing cost. PCB Base Materials To assure the survival of PCBs during the high soldering temperatures and their subsequent reliability afterward, improvements in PCB resin properties are necessary. The integrity of the PCB interconnect structures – plated through-hole copper barrels and barrel/inner-layer interconnects – requires improved glass transition temperatures (Tg) and lower coefficients of thermal expansion (CTE). Improvements in Tg and decomposition temperature (Td) are also necessary for the thermal stability of the PCB resin. PCB base and prepreg materials are frequently specified according to industry document IPC-41012. It should be JANUARY 2008 FIGURE 1. Micro-photograph of PTV with fractured copper barrel near the center of the PCB (some resin recession also shown.) 22 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - January 2008 Contents Our Line Market Watch Around the World Happenings ROI PTH Reliability: Designing to Improve PTH Reliability EMC for the Real World PTH Reliability: The Survival and Long-Term Reliability of Lead-Free PCBs iNEMI's Emerging Technologies: Research Priorities for the Electronics Industry Printed Circuit Design & Fab Annual Buyers Guide Special Suppliers Section Guide to Products and Services Off the Shelf: Designcon Preview Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - January 2008

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