Issue link: https://iconnect007.uberflip.com/i/726276
26 The PCB Design Magazine • September 2016 boards or interpos- ers, but sometimes it's bigger. As long as we're working with Cadence Al- legro or Altium I'm good. (One of the sales reps for Cirexx kids me whenever she sees one of my rigid board designs. "So, you're slum- ming again, design- ing PCBs?") TESS began in a similar way. One of my flex design customer sup- plies the camera detectors for the project, and they asked if I could help. A TESS scientist had designed the first prototype PCBs on a Mac us- ing a program called Osmond, and they needed someone to bring the design into a mainstream CAD tool and make some changes to that ini- tial design. As I dug into the design changes, it turned out they needed a complete redesign, and of course they needed it in a hurry. I com- pleted that design—it worked fine. Then they asked me to design a microprocessor evaluation board, and after that a spin of the two proto- type revisions of the satellite PCBs. And these satellite boards were very chal- lenging designs—over 1,000 components each and very specific shapes to the copper planes. A whole different world (pun intended) from flex circuits! For the third round of prototypes, the designs were transferred to a service bureau with a lot of experience in mil/aero PCB de- sign and designers that had CID certification. I continued to help as a liaison between the TESS folks and the service bureau, transferring the design information and helping review the finished board designs. I also designed a couple of test boards for the scientists to evaluate the flight boards. The satellite itself is scheduled for launch in 2018. Looking back, it was as if I had taken a nine- month sabbatical from flex designing; it con- sumed almost all my time from September 2014 to July 2015, and it's been part-time since then. It was definitely a fun experience working with some incredibly smart people. Shaughnessy: What are some of the unique challenges that come with designing boards for space applications? Woznicki: The workmanship standards specified was IPC-6012 class 3A, which has very challenging via requirements: 1.5 mils plating in the barrels and a minimum 2 mil annular ring on inner layers. We weren't able to use very small vias, even though we had to fit lots of components onto the board. Shaughnessy: How is a board for a satellite different than any other high-reliability board? Woznicki: You have to keep in mind the very cold temperatures of space as well as the shock and vibe of the rocket launch. In many ways, the design techniques we use in flex cir- cuitry apply. I'm always looking at the trace lay- out to see if there is anything that might cause a trace or solder joint to crack under temperature cycling or actual physical flexing as the board vibrates. So adding teardrops at vias and plated holes and running small traces out the corners of SMT pads where possible so as not to create stress concentrators help the board be more reli- able. I also try to do as much signal routing as possible on interior layers to protect them if the board flexes, just like we try to keep traces near the neutral axis in flex design. FLEXDUDE ABIDES: PCB DESIGN FOR SATELLITES Tom Woznicki, aka "Flexdude."