Hello Phil- Sounds like you've got a tough one here. In trying to visualize the proposed set up, I had a few questions: 1. Is the heat dissipating component a PQFP? A metal or Kovar case, or plastic? 2. As mounted, does this component come in contact with the PWB, or is there a gap under the component body? About how big a gap? 3. Just a thought: so-called thermal vias under hotter running ICs are fairly common; they are usually just PTH vias, essentially solid plugs of Cu, running the entire thickness of the board. Silver filled epoxy is a matrix of epoxy and silver particles [or possibly silver plated particles?]. It might be possible that Cu vias are lower thermal resistance paths than typical, as-applied and as-cured conductive epoxy plugs! Also, beware of unevenly distributed particles in conductive epoxies; insufficient mixing could cause drastically reduced performance. Thermal conductivity values for Cu vias and for "ideal" [and also, perhaps, "typical"] conductive epoxy plugs should be compared, in order to help decide a prudent approach. 4. Cu vias, or epoxy plugs, will be part of a total thermal efficiency based on how they interface [mechanically] to the bottom of the hot component. Combinations of both types of vias, combined with interface materials such as solder [the option you've already described], or thermal grease, or a thermal interface gasket, or just mechanical contact, should be accounted for when comparing the cooling efficiency of various options. A poor interface to any type of via will torpedo the whole cooling scheme! 5. Speaking of poor thermal interfaces, I'm skeptical of solder wetting to epoxy, though I have no experience with this combo. Regards, Michael Alderete Aerojet Phil Bavaro wrote... ------------------------------ Date: Thu, 12 Aug 1999 09:14:25 -0700 From: Phil Bavaro <[log in to unmask]> Subject: Filled vias Our design group just brought a new technology (to me at least) out yesterday and I find it hard to believe that this is proven technology in the industry. In order to transfer heat away from a part which generates quite a lot of heat (upwards of 4 watts?), they have placed a matrix of vias below it and then filled them all with silver filled epoxy, expecting us to then screen solderpaste on top of this in order to connect the supposedly solid barrel of the via with the belly of the component. The assembly consists of essentially a CCA mounted inside of small chassis and the only cooling transfer is through the boards which sits on a boss in the chassis. Is there published reliability data on solder connections made by interfacing silver filled epoxy and eutectic solder? If so, please direct me to it. We do have a very small percentage of silver in our alloy. The solderability of silver filled epoxy when exposed to solderpaste on top of a nickel gold substrate does not leave me with a warm and fuzzy feeling. I am confused how the intermettalics (I know, I know, which one?) are to be formed, or even if the silver filled epoxy wets with solderpaste under normal reflow temperatures. If there are other concerns I need to focus on as well, I'm open to all suggestions. Someone mentioned galvanic interactions...... My hunch is that we will have large voids in the connections below the part and this will reduce the heat transfer ability of the assembly when compared with a solid solder filled via that wets all the way up to the belly of the component. But then again I could be wrong..........please enlighten me. Phil Bavaro ------------------------------ ############################################################## TechNet Mail List 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://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ##############################################################