Hi! Don't apologize for disagreeing with what I wrote - disagreements can be good things too. Yes, you are correct that there are some designs that the pwa is too thin or the internal signal construction is such that achieving a temperature less that 183 C is just not possible. However, we routinely run pwas in which many of the components do go above 183 C and due to the surface tension of the solder stay positioned right where they should be (no fixturing required). We do some profiling on these type of pwas to insure that we don't generate excessive intermetallic layers in our solder joints. Also, some of the larger SMT components, coupled with gravity, negate the surface tension factors. We have pwas that I do not hit reflow temps on the bottom side throughout the entire reflow run - it just takes a robust oven design and some extra profiling effort. Dave Hillman Rockwell Collins [log in to unmask] [log in to unmask] on 11/26/97 08:35:03 AM Please respond to [log in to unmask]; Please respond to [log in to unmask] To: [log in to unmask] cc: (bcc: David D Hillman/CedarRapids/Collins/Rockwell) Subject: [TN] Double Sided Assembly Dave you wrote|- Hi Lee - two items to think about as you venture into double sided assembly: your reflow oven capabilities and the robustness of your pwb surface finish. Reflow ovens that have both top and bottom side heating provide some recipe flexibility to keep your "first pass and now bottom side" of the pwa from reaching reflow temperature. Surface tension will keep many components attached to the pwa so avoiding bottomside reflow isn't a requirement but it does keep the solder joint intermetallic thickness minimized. Secondly your "second pass and now top side" of the pwa has seen one thermal exposure - the surface finish (OSP, tin/lead, gold/nickel, etc.) needs to maintain its solderability so that successful soldering can be completed during the second pass. Good Luck. Dave Hillman Rockwell Collins This simply isn't the way I see things. Peak reflow temperatures are usually around 212 - 222 centigrade. Reflow temperature of solder 179 - 183 degrees. That's 40 degrees difference. Copper vias the thickness of the board 50 - 60 thou. How do we create 40 degrees differencial across this distance? Even if we do, vias with direct connections,( such as vias in pads ) to the top side pads will be cold, while other pads will have no problem achieving reflow. Assemblies need to be heated up completely, especially multi-layer with lots of copper planes. The issues of tombstoning are currently being discussed. Thermal differences for whatever reasons, (maybee inadequate heating/convection in IR ovens( are likely to be the cause. Surface tension usually does hold components on underneath, but if it doesn't then some means of support is needed while it reaches reflow temperatures. We have used tooling jigs during reflow on our complex assemblies, while adhesive is simpler solution. Sorry to argue Dave but I do feel passionate about this point. Thanks ############################################################## 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. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ############################################################## ############################################################## 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. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ##############################################################