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Due to a careless selection of components (and you could say also bad layout) I ended up having a PCB with surface mount pads on both sides and on the same x,y coordinates.

I have already used my airgun for the TQFNs on one side but I have to solder some more on the other side and I am worried that by applying heat with the airgun, the already soldered chips will drop off from the bottom as I am heating up the top side.

Has anybody dealt with this issue before and if so, how do I ensure that what is already soldered will stay there despite the extra heat.

I've thought about putting Kapton tape or simple tape to hold the bottom components in place while I heat up the top side but then this could trap the heat around the components and make them desolder easier.

What I am planning on doing is simply place the PCB on a table so that the bottom components are sandwiched between the PCB and the table and they will hopefully not move out of place even if the solder melts while the top side is being heated up.

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    \$\begingroup\$ I would try to add a drop of superglue after soldering. I know double-sided SMD components in real manufacturing are held in place with a drop of glue. \$\endgroup\$ – Oldfart May 22 '18 at 10:29
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    \$\begingroup\$ The component glue used in "real manufacturing" is placed under the components. Not needed in the case of parts with a thermal pad with a properly designed footprint because the surface tension of melted solder will keep the part in place. \$\endgroup\$ – Michael Karas May 22 '18 at 10:56
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    \$\begingroup\$ @Oldfart I wouldn't use superglue. That will vaporise with heat from soldering and results in nasty fumes (painful if you get them in your eye). For PCB fab they use high temperature glue. \$\endgroup\$ – Tom Carpenter May 22 '18 at 10:56
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    \$\begingroup\$ Note that the component glue used under a part to hold it in place is designed to cure during the reflow cycle. When the solder paste melts and surface tension between the pads and part leads centers the part the glue must be viscous to allow the part to move but then cure when the part cools down. The glue is not really something you want to add after the fact. \$\endgroup\$ – Michael Karas May 22 '18 at 11:05
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    \$\begingroup\$ Or, get out the soldering iron and solder the tqfn part by hand. That works just fine, unless there's a ground pad underneath. \$\endgroup\$ – JRE May 22 '18 at 11:18
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There are a few ways to do this:

1) Direct the heat flow on the PCB

There are a few ways to direct the heat flow, you'll have to get creative especially if the parts are near each other. You could put a heatsink on the components that you don't want to move around, or cool it down with a cool air flow on the bottom side (which might make soldering on the top side harder. It might be advantageous to keep the flow of heat in mind if you ever rev your board.

Another clever way to direct heat is using aluminum foil around SMT parts to keep the hot air away from other components and the board. The picture below is an example (although they are using an iron, but this also works for hot air)

enter image description here

2) Use solder with different melting points

The way two sided PCB's are manufactured are with solder with two different melting points, a higher melting point for the bottom side of the board (which you assemble first) and a lower one for the top side. Not a great option for prototyping

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  • \$\begingroup\$ I really like the heatsink idea for the bottom, thanks. Concerning the foil, I tried wrapping one of my boards once all around and only exposed the area where an LQFP-100 would be desoldered from. It felt like the whole PCB was baking underneath. I am using Kapton tape since then. Also, two different soldering materials is not an option atm.Thank you \$\endgroup\$ – George May 23 '18 at 20:47

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