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September 2014 • The PCB Design Magazine 49 bob and me Bob Tarzwell is a PCB consultant who has spent 50 years in the PCB industry, inventing technol- ogy and building almost every type of PCB. He is the co-owner of DB Publishing, the publisher of the PCB 101 and Quality 101 handbooks. visit www.dmrpcb.com. Dan Beaulieu is a well-known industry consultant and co- owner of DB Publishing. His column It's Only Common Sense appears Monday mornings in the I-Connect 007 Daily newsletter. He can be reached at danbbeaulieu@aol.com. A PCB POTPOURRI continues cussing how they planned to make this PCB, and I actually laughed out loud. I blurted out, "Microns?" Yes, unfortunate- ly, my mouth was in gear. "Are you guys nuts? You want to make a PCB that's .009" by .009" with nine .001" holes centered in a .001" pad?" I then held up a blank piece of paper and put a pen dot on the center of the page. "You mean one-tenth as big as this tiny little dot? How do you plan on routing these out?" The reply was, "With our drill router." I laughed again. "That old Excellon 2000? The best you can do is 3–4 mils of tolerance half the size of the board. Any idea how you're going to drill a .001" hole in a .001" pad?" The reply was, "With the same drill." "News flash! At 160,000 rpm, you are not going to drill a .001" hole, and, oh yeah, you can't buy .001" drill bits anyway." At an earlier meeting, our emperor had waved his mighty staff and decreed from on high that we were not allowed to use a laser. I continued unabated, "As you're not allowed to use a laser, how are you going to drill the holes? And what about plating and drill tolerances? You need bigger drills and bigger pads." I got the strange sense that my presence was not appreciated or wanted. Oh well. I did not much like the little emperor or his court of meek yes-men. Time to look for another job! Why the Focus on Polyimide? Bob Tarzwell: I consistently do battle in one arena: Polyimide laminates. It seems that any- time a designer wants a tougher board or higher temperature, he demands polyimide and the laminate of choice. Yet I design and make very high-tech, high-voltage printed circuits all the time and I never use polyimide. Why? Because it absorbs moisture at twice the rate of FR-4, and moisture is an enemy of high-reliability circuits. With polyimide, it is also hard to get the pre- preg to flow just right when we make multilay- er boards, and it leaves micro holes all through the final PCB. Micro holes are also the enemy of high reliability due to trapped moisture, lower- ing the laminate's voltage ability over time. Yes, polyimide can take a higher tempera- ture than any other laminate, but the upper temperature limit is not the only qualifying at- tribute I consider. I need laminates that press out with a minimum of micro holes, flows well, and holds the minimum voltage rating with time and temperature. I like laminates that limit electroless wicking, and polyimide does not do a great job here. I cringe when designers quote glass transition temperature (Tg) as the reason for using a specific laminate. I have not used or worried about Tg for eight years, ever since I discovered the challenge we face each day with PCB and copper thickness in the holes. When I look for really tough laminates for downhole PCBs or very high-current, high-volt- age PCBs, I use Isola 370 HR or Panasonic 1755. I don't look at Tg; I use decomposition tempera- ture (Td), the temperature at which the lami- nate starts to lose mass. After I addressed the main worry about Tg, which is the higher ex- pansion of the Z-axis, I moved up to Td. When I design higher-voltage PCBs, from 1,000–15,000 volts, I laminate in a flex sheet core with its higher voltage per mil rating. The only time I use polyimide is for burn-in boards where it is all about maximum temperature with time. But, mind you, some of the new laminates the guys have been coming up lately with are great new tools, and I look forward to working with some of them to help with the extreme boards I work with. PCBDESIGN