CircuiTree - September 2008 - (Page 28)

we started with 7,628 prepreg, 42 percent resin content, and then aged the prepreg in a humidity chamber at 35 °C and 85 percent relative humidity. From there prepreg properties were tested. Next, with the aged prepreg, we prepared three-ply laminate samples on 1 oz copper and evaluated the cured laminate properties as per standard IPC test methods. What we found was that while all the halogen-free materials did pick up more moisture than our brominated FR-4 control, it did not necessarily have a negative impact on the prepreg’s shelf life or cured laminate’s thermal reliability. Figure 1 and 2 show the change in minimum viscosity and moisture take-up of various halogen-free materials compared to a brominated control. After lamination, all samples saw a depression of DSC Tg, even the brominated control, but all at the same relative amount. Further, the same trend was seen as before. While two of the samples showed a decrease in T260 performance, the other two samples showed no decrease at all; again, this after being exposed to 35 °C and 85 percent relative humidity for more 12 days (Figure 3 and 4). Figure 3 Change in DSC Over Time As Measured By IPC TM-650 2.4.25 Figure 1 Change in Minimum Viscosity Measured on an RDA at 3 °C/min Figure 4 Change in T260 Values Over Time As Measured By IPC TM-650 2.4.24.1 Figure 2 Moisture Pickup By TGA From 50 to 220 °C Run at 10 °C/min What this shows is that even though all the halogen-free samples showed a slight increase in moisture pickup, two samples showed less of an increase in minimum viscosity (a good indicator of prepreg shelf life) compared to the brominated control. What we have shown is that there are halogen-free materials available that have equal or better thermal reliability than standard FR-4 laminates even after exposure to high temperature and humidity storage conditions. So far so good, but testing and qualifying new materials can be a very long and expensive task. Along the way some unexpected material issues can show themselves that you normally would expect to see with traditional materials. Take for instance the thermal reliability of a processed multilayer board that was stored in uncontrolled condi- Table 2 Fresh PCB (Day 1) Reflow cycle to delamination, reflow profile: 255 °C; 10 set for each condition Material HF Material A HF Material B 2x 3x 4x 5x 6x 7x 8x 4 9x 6 1 Table 3 After One Month in Storage (18 to 24 °C, 30 to 60 percent RH) 10x 11x 12x 13x 14x 15x 5 3 1 Reflow cycle to delamination, Reflow profile: 255°C; 10 set for each condition Material HF Material A HF Material B 28 September 2008 • circuitree.com 2x 3x 4x 5x 6x 7x 8x 5 9x 5 10x 11x 12x 13x 14x 15x 1 2 3 4 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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