Hi Jim! The FR stands for Fire Resistant, and the 4 is part of the NEMA designation. IPC 4101 has a wonderfully tabulated breakdown of each of these base materials. The specification sheets do not indicate (to me at least) a correlation between the number, like FR-2, FR-3, or FR-4 and any of the physical properties, but this is a guess. I think it has more to do with which came first, as the FR-1 is a "Phenolic, Flame Resistant", FR-2 is a "Modified Phenolic, Flame Resistant" and FR-4's are all epoxy and vary by MIL spec. Hope this helps, and I would love to know if there is a physical reason for the numbering system. Jonathan Meigs Product Development Oak-Mitsui -----Original Message----- From: Jim Marsico 516-595-5879 [mailto:[log in to unmask]] Sent: Monday, March 29, 1999 1:56 PM To: [log in to unmask] Subject: [TN] LAMINATE DESIGNATION Can anyone tell me what the F,R and 4 means for FR-4 material? ################################################################ 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's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################ ################################################################ 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's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################