BOT SIDE QFN DROP DURING TOP SIDE REFLOW; Bot & Top side solder profile temperatures are almost similar with a 5 Deg variation

You problem will be related to the part type or the design. Standard 40 or 100 pin QFN or LGA parts are not going to drop due to weight as they are just a few grams. If you send a photo of the PCB pad footprint and the component separately I may be able to comment for you. A picture is worth a thousand words bob@bobwillis.co.uk If you are talking about a BTC package module then the weight may be the issue. Poor design of the vias mounted under the package on the centre pad have been known to outgass and flip off packages or cause solder to be spat out causing solder balls or shorts on the edge of the packages

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