Printed Circuit Design & Fab - January 2008 - (Page 16)

PTH RELIABILITY Designing to Improve PTH RELIABILITY Understanding the critical process steps in design, fabrication and assembly can increase the robustness of the PTH and improve reliability. by ZULKI KHAN PCB real estate, along with components, vias, and annular ring sizes are shrinking dramatically. Five or 10 years ago, a via hole of 15 to 20 mils with a 5 to 10 mil annular ring was the norm. Now, via sizes ranging from 7 mils down to 4 mils on highly complex boards are increasingly more common. As through-hole size (via) shrinks, so does the annular ring, posing major challenges to fabrication and assembly manufacturers in the process. In earlier PCB generations, two to three mils of play (or wander) within a 10 mil hole was common. Currently, precision laser drilling allows about 0.5 mil wander, (sometimes less) as smaller drills have reduced via size to 4 or 5 mils. To illustrate, this means that one mil of wander represents 25% of the hole size when a four mil via is created. Consequently, tighter fabrication and assembly controls and tolerances, state-of-the-art equipment and trained technicians must be utilized at design, fabrication and assembly to assure high plated through-hole (PTH) reliability. Laser drilling equipment must be up-to-date, highly precise and accurate to ensure that vias and annular rings remain intact as a PCB undergoes the multiple heat and reflow cycles during the different phases of fabrication and assembly. Sufficient clearance is necessary to avoid creating break out on internal layers. This condition is typically caused by miscalculations of drill size, pad size, incorrect annular ring size or over-etching at fabrication, resulting in weak or broken continuity. Lead-free design, fabrication, and assembly are especially vulnerable areas requiring careful attention to assure PTH reliability. Experienced EMS providers fully understand that proper focus must be placed on selecting the correct PCB material, ensuring proper thermal profile characteristics, and choosing the right flux as part of assembly precautions in 16 order to insure long term PTH reliability. Currently, the lead-free assembly industry is experiencing an anomaly called “hot tear,” which occurs with the use of SAC305 lead-free solder. As shown in FIGURE 1, hot tear is a crack created in a through-hole component joint. SAC305 manufacturers are now investigating this problem. With lead-free projects, PTH reliability is also dependent upon the right choice of flux at PCB assembly levels. Contract manufacturers (CMs) are now dealing with a very narrow window of flux activity at a considerably higher temperature levels. At these higher temperatures, error tolerances are greatly reduced. Without the correct solder and flux composition and proper thermal profile, PTH reliability is severely jeopardized. At Design A major function in DfM of the PCB is to determine accurate calculations for through-holes and plated slots. These calculations apply to determining via diameter in relation to annular ring size; another is setting correct via spacing and consistently maintaining that spacing for good signal integrity. Accurately calculating drill hole and annular ring sizes is critical. A larger hole diameter requires a larger annular ring. Miscalculation in annular ring design creates long-term reliability issues, especially at rework levels. Experienced and disciplined designers refer to IPC610D and IPC221A charts to obtain PTH and annular ring definitions as a basis for making precise calculations. The designer must also carefully select the proper type of copper foil when designing a PCB. Choices for copper foil are typically one-half, one, or two ounces. When one-ounce copper foil is used, each square inch of the board will be layJANUARY 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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