Technet A question for bare board gurus. Is there a difference in reliability between high Tg and regular Tg FR-4 ( IPC-4101/24 vs IPC-4101/21) in a vehicle type application? The end use application is less than Tg of /21 material, so not in the glass transition range for either material. Typically - is the CTE different between these two? (I expect that depends on the manufacturer to some degree). If there isn't a high temperature operating requirement, why select the high temp FR-4? Why indeed? Using high Tg materials primarily is for one purpose. That is to extend the board operating temperature range even though the z axis CTE still is in the range of 50 ppm/C for whatever material. Running TMA or DSC confirms this as higher Tg materials, as polyimide or BT, turn to dust after severe delamination - after a prolonged period compared with shorter times for "normal" epoxy resin systems. Expasion rates are the same. High Tg stuff is for higher reliability (long term) applications exposed to higher temperatures for longer times and to fly once into whatever inferno. However, cure requirements can contradict this. Polyimide, taken to full cure at an advertised 270 Tg, can have other problems. This is why it should not be taken to full cure but should be moderated at, say, 250 C in a modified press cycle. Laminate and foil bond strengths have been proven to diminish rapidly with the higher Tg requirement. Everything, again and again, is a compromise. Determine initial quality requirements, as thermal stress and shock, and long term requirements as prolonged temperature excursions. In other words, polyimide can be a good all around choice, but why go the the extra expense when a modified epoxy or BT will do? And on it goes. MoonMan --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d 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 delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail 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 ---------------------------------------------------------------------------------