Steve, This sort of problem has been addressed before. Here are some personal observations from a few examples like this that I have seen work through high levels of vibration and thermal cycling. All designs call for two single sided surface mount boards bonded to a center heatsink. I am from the low board CTE camp as a method to minimize thermal cycling solder fatigue issues and have been privy to very long duration testing programs (5000+ -55C to 85C 2 hour thermal cycles) that show that this method works for even lead less chip carriers. Here are some points to consider: 1. Custom die's to form leads. Fancort Industries, Inc. NJ 201-575-0610 did a great job with the tooling. 2. Low CTE board material. I have seen four different Kevlar (tm) board materials used. Woven Kevlar, and non-woven (Thermount :Arlon 85NT polyimide Kevlar, Arlon 55NT epoxy Kevlar and Hitachi CIL-???(cant find the call out at this time)). For commercial space imaging electronics I currently specify Arlon 55NT because of its low out gassing and relatively short raw material lead-time. I can talk more about the board material selection choices off line if you like... 3. Bond MCM to board so that some of the vibration and thermal cycle loads are carried by the bond line. Options here are epoxies or elastomers. Epoxies have lower CTE but are stiffer (not necessarily a good thing). Elastomers have higher CTE but are more flexible. I have seen both used in thermally and or electrically conductive flavors. 4. Bond surface mount boards to heatsink with thermally conductive elastomer. For al applications I have worked on we used Arlon Thermount. Its extremely low shear modulus (50 psi) and very good damping (Q less than 20) are some of its best traits. The low shear modulus material allows heatsinks to have higher CTE (e.g. aluminum heatsink) than boards with little increased effective board surface CTE. 5. Low CTE heatsinks with rigid adhesives to constrain high CTE boards (i.e. AlSiC or BeBeO heatsinks with polyimide glass boards) can suffer from high bond line shear stresses that CAN cause board to heatsink delimitation. This approach has worked for some but I would test this approach thoroughly before committing to a design. 6. Center core heatsinks add stiffness and conduct heat to mounting rails. I have seen Al, AlSiC, AlBeMet, AlB, BeBeO and CuMoCu to name a few. The new high K graphite products (e.g., TC1050) look great too. The trades that you need to make are $, schedule, weight, K, etc. I have seen low CTE heatsinks lower the CTE of Kevlar boards to a point that there is almost not CTE mismatch between the ceramic packages and the board surface. 7. Don't trust some of the heritage solder fatigue equations for low strain (e.g. flexible leads, low CTE mismatch) situations. They are too conservative and can steer you away from designs that have been proven to work better than the heritage analysis would tell you. My point of all of this is that there are ways to engineer the problem and come up with a design that can meet the requirements. The solutions are often not cheap and require materials that have long lead times. The solutions are also controversial. For every person you find in one camp, there are others who have completely different approaches. Testing of a design using your candidate manufacturing processes is the best way to prove out a design. This requires time to do it right. If you have any other questions about this topic, please reply. -- Karl Sweitzer voice: 716.47.77546 Eastman Kodak Company pager: 716.25.33681 800 Lee Road fax: 716.47.77293 Rochester, NY 14650-3118 mailto:[log in to unmask] ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################