Using the bottom layer, construct a aluminium plate with holes drilled larger than the signal vias. This will provide complete isolation to the signal vias and eliminate the need for thermal isolation by mean of isolative material currently used. This aluminmum plate can then be thermally glued onto the PCB surface using thermal adhesive. 

Using a single adhesive material would removal the isolative material and gaskets and would prevent migration between different materials over time. I am not a process gal but hope this method would work well.

Regards - Wee Mei

Genny Gibbard wrote:

 We have a small PCB with SM components mounted on one side and a heatsink mounted against the other side over the entire surface of the PCB.  The heatsink is grounded, so we use a thermally conductive, electrically isolative thermal interface material in between (0.5mm thick), to protect signal vias.  We also use some strips of electrically conductive gasket in strategic areas to provide good electrical grounding between the two.Migration of some substances out of the thermal material onto the PCB over time has caused performance problems on random, not all, units.I was wondering what other methods might exist for heatsinking a PCB, possibly permanently bonding the metal heatsink to the PCB in some way.  Any sources of information for alternatives is appreciated.  We cannot change the construction materials of the original PCB, which is a multilayer (4 layer) FR4, but we may be able to bond an additional layer onto the PCB, in place of the gasketing we use now.Thanks in advance,Genny--------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------