All, Thanks for the input. My feelings exactly but I had to ask (-- action item). Their concern is not about the parts falling off but about the reflow of the solder joints again on the 1st side. Especially, any BGAs if populated. joe -----Original Message----- From: Tom Gervascio [mailto:[log in to unmask]] Sent: Friday, August 06, 2004 10:59 AM To: [log in to unmask] Subject: Re: [TN] Double sided boards and reflow Both sides will go into reflow but the surface tension of solder holds the bottomside SMT parts in place. Threads from the TechNet address the component weight versus the surface area of solder (if the ration of the two is less than 30 the part will probably remain in place). Literally millions of boards have been build with a double sided process over decades. Using different alloy will drive you to alloys where you might have to prove it performs as well mechanically, electrically as eutectic tin/lead. IBM did a lot of work with Sn-Bi which melts at 138 C - however if lead is present you might be a ternary alloy with a very low melting point and provide for reliability problems. My $0.02 worth Tom Gervascio Senior Process Engineer Sparton Electronics (352) 540-4040 >>> [log in to unmask] 08/06/04 12:33PM >>> Good Morning, Many of our board designs are double sided with components on each side. When I explained the build process, a few of the design guys expressed concern about the 1st side seeing another reflow when the board goes through again for the 2nd side and asked me to look into the practice of using solder paste with different melting temperatures for the 1 st side reflowed vs. 2nd side reflowed. For example, use a solder paste/alloy with a higher melting temperature for the 1st side reflowed and a lower melting temperature for the 2nd side so the 1st side reflowed does not reflow again (or minimize the impact on the 1st side) during 2nd side reflow. Some of the guys remember seeing CMs changing solder paste for the 2nd side assembly. Would anyone like to share their experience with this practice or comment on its potential value. thanks again. joe --------------------------------------------------- Technet Mail List provided as a 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/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a 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/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a 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/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------