When designing double sided assemblies, it is a good idea to aim for laying out a complex side and simple side as far as the Surface Mount is concerned. The intention is to create a three thermal cycle process flow as follows.... SIMPLE SURFACE MOUNT SIDE COMPLEX SURFACE MOUNT SIDE WAVE SOLDER FOR PTH The simple side shouldn't have large fine pitch QFPs, BGAs,Fine Pitch SM connectors, Passives of large mass with few terminations which might fall off during reflow. Hopefully the simple side would leave you with some green spaces which are useful for supporting the assembly during the screen printing of paste. It is vital that custom tooling is designed to give maximum support to the Complex side for quality prints. The temptation for complex sides is to build these first as they can be put on a flat plate for maximum support in the printer, but consider the effect of putting all your complex components through the Reflow upside down on the second thermal cycle. The complex side build may require tooling to support the assembly during the reflow operation. This depends on the nature of the inner layers (copper planes with uneven distribution of copper) or perhaps a multi circuit biscuit, both of which will cause warping in the reflow. The tooling should be designed to hold the assembly flat while the PCB is at it's glass temperature. Gravity should provide the neccessary down force. Tooling should be designed to provide support without sinking heat from critical areas of the top side during reflow.(This is an art in itself). The PTH components should be layed to allow the design of a third piece of tooling for Wave solder. The PTH components should have border areas around the pins of about 3 - 4 mm which have no components (and no test pads if possible) on the solder side. The tooling should mask the Surface Mount components and leave apertures for the wave to reach the PTH leads. The border area allows for a wall to come flush with the PCB and isolate the Surface Mount. Some people are unfortunate and have to struggle with a forth thermal cycle and this involves the use of a Solder Fountain to assemble PTH from the second side. Again a border area needs to be planned in to allow tooling to fit around the components being soldered and protect the Surface Mount. Thanks [log in to unmask] ############################################################## 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 ##############################################################