Printed Circuit Design & Fab - January 2008 - (Page 18)

PTH RELIABILITY FIGURE 4. Cross-section inspection is necessary to determine if there are cracks in the via barrel. FIGURE 3. Jumper wire is used on a PCB as a result of a cracked or broken via barrel. Aspect ratio, which refers to the comparison of board thickness to the smallest via created on that board, must also be carefully reviewed, as it plays an important role in PTH reliability. The greater the aspect ratio, the more difficult it is to fabricate the board and to assure that it’s plated through holes will maintain reliability throughout the product life. A 10:1 or 12:1 aspect ratio is standard, but there are now 30:1 ratio PCBs constructed for specialized applications. This 30:1 aspect ratio board requires more precise pad-to-hole ratio calculations for PTH reliability. It’s important to maintain robust drill sizes and annular rings for high aspect-ratio boards to avoid half-moons and breakouts. The different board fabrication cycles need to be followed and precisely implemented. This includes a number of precise process sequences that must be at specific temperatures for defined periods of time. There is always the possibility that a quick turn order will have a few corners cut. The OEM must be sure that the PCB fabricator doesn’t shorten the bake cycles in order to meet tight shipment schedules. This can severely compromise long-term product reliability. Such shortcuts could result in numerous problems, from insufficient material cure and moisture absorption to incomplete stabilization of the processing materials. In addition, if preventative maintenance is not regularly conducted on the various chemistries used plate and process the PCBs, they it may become contaminated or may stop performing as required. This contamination, as in the case of a contaminated copper electroplating bath, can cause vias to be more susceptible to cracking during the reflow cycles in the assembly stage. A cross-sectional inspection and analysis, as well as other relevant testing, are often required to pinpoint hairline cracks in the via’s barrel (FIGURE 4). These defects may not be readily apparent and may pass a non-stressed electrical test at the PCB fabrication facility and through the final quality control without notice. Two tests are used to further insure PTH reliability. The first is a PTH Life Curve; the second is a Current Induced Thermal Cycle Test (CITC). The PTH Life Curve test plots a curve that determines when and how a PTH fails during accelerated thermal cycling. CITC inspects and tests the dif18 ferent board materials, finishes, and via construction. Both tests determine through-hole longevity before they begin to show fatigue characteristics and other reliability issues that might not otherwise become apparent until the product is in the field. At Assembly Among the major requirements for maintaining high PTH reliability is a well organized, trained, disciplined, and comprehensive incoming audit and inspection for fabricated boards entering the assembly process. The focus here is on via stability and reliability. Inspectors should insure that sample vias, annular rings and holes are thoroughly plated and within tolerances as specified in the fabrication design documentation. This includes insuring that holes designed to be non-plated aren’t mistakenly plated through. The board is also subjected to different thermal cycles at reflow and wave solder stages. Reflow incorporates multiple heat zones, gradually increasing board temperature then holding it at a peak level, then reducing it, so that the board experiences fatigue as a result of repetitive temperature cycling. Here, the Z-axis expansion of the via barrel alloy needs to match the expansion of the board material composition. Alloy composition is different than the board’s material composition, and this mismatch creates fatigue that can cause cracks in the via barrel when subjected to heat cycling. Temperature spikes between critical zones within reflow ovens also have the potential of creating temperature fatigue at PTH and vias. This fatigue ultimately damages throughhole components when they undergo excessive wave soldering cycles. Special attention must also be given to via size expansion and contraction since the board is exposed to varying temperature ranges. This is especially important for micro vias with thin plating on the via barrels. Via expansion and contraction – which is inevitable in the assembly process – will take its toll on PTH reliability unless all of the previously outlined design, fabrication and assembly guidelines are carefully followed. PCD&F ZULKI KHAN is president and founder of Nexlogic Technologies; zk@nexlogic.com JANUARY 2008 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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