Printed Circuit Design & Fab - January 2008 - (Page 24)

FIGURE 3. Cross-section of PCB with multiple delaminations due thermal degradation and decomposition of the resin. Note: The same section is shown with two different illuminations to bring out the failure details. FIGURE 2. Micro-photograph of PTV with inner-layer separation (ILS) caused by the failure of the interface between the copper foil and the copper flash. the finished product instead of specifying material properties. The argument advanced is that it is not only the importance of properties of the raw materials used, but also the impact of the processing environment that ultimately determines the product’s success. Product capability requirements With 35 years of experience We make our own laminates in the U.S. • Material types in FR-4 Difunctional, Tetrafunctional and Multifunctional • Temperature ratings from 130˚C to 177˚C • Thickness ranges from .0025” to 3.000” • Coppers from 1/2 oz. to 6 oz. in stock • Black, Blue, Tan, Green, Orange & Natural • True Cryogenic G10 • Sheet sizes in 36”x48” • RoHS compliant, UL recognized and IPC approved could be given in terms of the maximum expected temperatures reached during lead-free soldering processes and total exposure time, for example, 280˚C for 3 minutes, inclusive of all soldering processes – HASL, reflow, wave, as well as rework and repair. Alternatively, one could specify solder floats or dips, for example 288˚C for 3 to 5 cycles of a specific time. The coupons would have to withstand these exposures without delamination, barrel cracking and/or inner-layer separation. It has also been suggested that a cyclic “time-to-delamination” protocol be introduced. So, perhaps in addition to or as a replacement for a T-260 test, a cT-260 value could be generated by first conditioning the boards by thermal cycling to 260˚C or some higher temperature for X number of cycles, to simulate the assembly process. For example, 6 thermal cycles could be used to simulate 3 reflow and 3 rework cycles. After the thermal cycles have been completed, the PCB would be subjected to the standard T-260˚C testing until delamination occurs. It has been reported that some “high performance” resin systems degrade with thermal cycling much more readily than from a single thermal excursion to 260˚C or even 288˚C, and that materials with poor cT-260 results usually do not fail because of delamination, but because of barrel cracks of the vias. was no significant impact on barrel stresses when NFLs were removed, as can be seen in FIGURE 4. However, recent testing has shown the removal of NFL to be nowhere near as benign for assemblies undergoing the lead-free soldering process as had been tacitly assumed. FIGURE 5 shows some recent industry experience with PTHs in PCBs that reached the “red flag”-level at an aspect ratio of 4-to-1. In FIGURE 6, the same PCB is shown with a full complement of non-functional lands (NFLs) and it shows no wall separation or resin recession. Copper Dissolution and Plated Copper Thickness It was only recently that the IPC-specification for minimum copper plating was lowered from 1.0 mils to 0.8 mils [25 to 20 µm]. This was done in response to the increased costs and time required to plating 1.0 mils of copper in the center of the hole in high aspect ratio boards. These cost are associated with the lower plating current densities required to plate uniform copper into high aspect ratio plated-through vias The higher soldering temperatures, together with the very high tin content of lead-free solders, leads to a significant increase in the rates at which copper is dissolved during soldering processes. This can be seen FIGURE 7. Reductions in copper thickness by as much as 12 µm [0.5 mils] have been reported. The minimum copper plating thickness may need to be increased in light of these finding to perhaps 1.2 mils [30 µm] to allow for the potential dissolution without a negative affect on PTH reliability. Increasing the copper barrel plating thickness can improve the survival and long-term reliability of PCBs that will undergo the higher soldering temperatures used with lead-free solders, and this increased thickness will need to be specified by the designer. JANUARY 2008 Non-Functional Lands A common assumption has been that the removal of non-functional lands (NFLs) has no impact on reliability, but this assumption needs to be rethought in light of the higher lead-free soldering temperatures, which can reach temperatures of 265 to 280˚C when soldering with some lead-free solders. Previously it was shown that there Same Day Pricing and Lead Times Current, Inc. 30 Tyler St. Ext • East Haven, CT 06512 1-877-436-6542 • Fax 203-467-8435 www.currentcomposites.com Louis S. Owen, Director of Outside Sales 860-350-9607 • Fax 860-210-1748 24 PRINTED CIRCUIT DESIGN & FAB COOKSON ELECTRONICS/POLYCLAD http://www.currentcomposites.com http://www.currentcomposites.com

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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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