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Can one place large packages (QFP-100) on top and bottom side of (same) PCB?

Of course I could hand-solder them but since I also have some small QFN packages I suspect will need to use reflow soldering, in which case I'm afraid the larger packages will fall off the board or at least will have trouble aligning property.

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  • \$\begingroup\$ The thickness of your pcb matters. Thinner and it will transfer more heat faster, causing issues. And do your qfns have large center ground pads? \$\endgroup\$ – Passerby Apr 17 '14 at 15:32
  • \$\begingroup\$ The QFN have a center thermal pad but are rather small 3x3mm. The thickness will be standard 1.6mm \$\endgroup\$ – Arne Apr 17 '14 at 15:39
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Yes, this can be done. However, boards with components on both sides will cost more to build up because it requires extra steps. As you say, you don't want the chips on the bottom to fall off while the ones on top are being soldered.

This is usually dealt with one of two ways. When the part has low weight to solder pad area, then it will stay in place just from the surface tension of the molten solder. For heavier parts, usually a kind of glue is used.

Double-sided boards are common enough, but when doing something like this it is still a good idea to talk to your assembly house first to get some idea what their processes are and what you can do on your end to make things simpler for them, and therefore cheaper for you.

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  • \$\begingroup\$ Since this is a research project with little funding we will probably manufacture in-house (hobbyist-like reflow oven, I'm afraid). Nevertheless, when glueing the heavier parts, how can I assure they 'align' to their pads? \$\endgroup\$ – Arne Apr 17 '14 at 15:00
  • \$\begingroup\$ @Arne First solder, then glue? \$\endgroup\$ – arne Apr 17 '14 at 15:06
  • \$\begingroup\$ First glue then solder I suspect. Or do you suggest reflowing two times - top and bottom side one after another? \$\endgroup\$ – Arne Apr 17 '14 at 15:09
  • \$\begingroup\$ got confused about your nicknames... anyway I think you should go with a double reflow. \$\endgroup\$ – Vladimir Cravero Apr 17 '14 at 15:23
  • \$\begingroup\$ +1 reflow twice is really the only option frankly \$\endgroup\$ – vicatcu Apr 17 '14 at 16:05

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