CircuiTree - September 2008 - (Page 29)

tions. Table 2 and 3 compare the behavior of two different halogen-free materials immediately after fabrication and after one month of uncontrolled storage.1 Under identical conditions Material A showed no change in thermal reliability while Material B showed a marked decrease in thermal properties when stored for one month in uncontrolled conditions. This drop was thought to be from the resin degradation or perhaps excessive moisture absorption; retain samples were sent back for testing and the supplier conﬁrmed Multek’s results. Again, while one halogen-free material performed poorly, another performed quite well. As for reliability in manufactured PWBs, the Center for Advanced Life Cycle Engineering (CALCE) tested numerous halogenfree materials as benchmarked to similar brominated materials.2 CALCE’s goal was to see the effects of lead-free processing on a laminate material’s ﬁnal properties. The materials had to survive 105 seconds above the eutectic point of a lead-free solder with peak temperatures below 245 °C. A typical reﬂow proﬁle is found in Figure 5. mability, and others—before subjecting them to different reﬂow proﬁles and comparing the post reﬂow properties to the prereﬂow properties. What CALCE found was that by and large there was little difference in the change in properties of the halogenfree materials as compared to traditional brominated materials. Table 4 summarizes CALCE’s ﬁndings. You will notice in the last row that in CALCE’s study, the halogen-free materials generally performed worse for CAF failures and that this trend was exacerbated with further thermal cycling. While these results may be true for this study, it should also be remembered that CAF resistance is highly dependent upon processing conditions, especially drilling where microcrack could develop leading to a pathway for CAF failures. At Isola, very different performance was observed with our products, both traditional and halogen free, at different board shops. All of the major laminate suppliers, and indeed a fair amount of fabricators, know this and recommend adjusting drilling parameters when using halogen-free materials. with the reactive use of TBBPA as found in epoxies for use in PWBs. That being stated, make no doubt our industry is moving toward halogen-free materials even though to date there are no health and safety issues associated with any brominated ﬂame retardants that are currently allowed under RoHS. Still, that hasn’t stopped the European Union from further reviewing materials for possible inclusion to RoHS and reviewing some of the current exceptions. Further, in early 2008, the ÖkoInstitut was commissioned by the European Union to review a list of 46 other substances, of which TBBPA and Bisphenol-A are a part, for possible inclusion to RoHS. Still, for better or worse, whether based on sound science or pseudoscience, we are all headed toward halogen-free materials. The good news is that for these new halogen-free materials, their time has ﬁnally arrived. ■ References 1. Multek, IPC-Sponsored Intel Halogen Free Symposium, Jan. 15-16, 2008, Scottsdale, Ariz. 2. Sood, B., “Comparison of Halogen Free and Halogenated Printed Circuit Board Laminate Subjected to Pb-Free Soldering Conditions,” IPC-Sponsored Intel Halogen Free Symposium, Jan. 15-16, 2008, Scottsdale, Ariz. 3. SCHER Opinion on the TBBPA Environment Risk Assessment, http://ec.europa.eu/health/ph_risk/committees/04_scher/ docs/scher_o_071.pdf Future Considerations By far the most commonly used ﬂame retardant used in PWB applications today is TBBPA, and its health and safety concerns have been extensively tested. In fact, in January 2008, the European Commission’s Scientiﬁc Committee, SCHER, adopted its opinion and considers that “the environmental part of the risk assessment of tetrabromobisphenol-A is in general of good quality.”3 That is, there are no issues Table 4 Summary of CALCE’s Observations Figure 5 Typical Lead-Free Oven Profile David Bedner is a principal scientist with the Isola-Group USA. Email: david.bedner@ isola-group.com CALCE’s experimental method was to benchmark certain material properties— DSC Tg, CTE, electrical performance, ﬂam- Property Flammability DSC Tg CTE Expansion TGA Decomposition Time to Delamination Rockwell Hardness Loss and Permittivity (Dk/Df ) Surface and Volume Resistivity CAF Performance HF Versus Non-HF (Pre-Reflow) HF materials had slightly higher combustion times than non-HF materials. Contribution of flame retardant not observed. HF materials had lower CTEs than non-HF materials. DICY cured: HF materials have higher Td than non-HF. Phenolic: HF materials have lower Td than non-HF. HF Versus Non-HF (Post-Reflow) An increase in combustion times was found for all materials. All materials behaved similarly. HF materials showed more variation than non-HF materials. All materials behaved similarly, Effect of type of flame retardant was not observed. All materials behaved similarly. HF materials had higher rates of CAF failure. Trend toward more CAF failures with more reflow cycles. circuitree.com • September 2008 29 http://ec.europa.eu/health/ph_risk/committees/04_scher/docs/scher_o_071.pdf http://circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - September 2008

CircuiTree - September 2008 Contents My Line Industry Review Tech Talk Flexible Thinking New Halogen-Free Materials: Their Time Has Finally Arrived Asian Section IPC Issues PCB and Package Convergence Ask the Flexperts Market Outlook IPCA Showcase Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - September 2008

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