As far as I am aware, and please correct me. If we have a 2-sided PCB with SMD components on the top and also bottom, first we shall put components on the top side and solder them and then on the bottom side and solder them. It could be done the other way around but the thing is that, it will be done on one side at a time. The soldering could be done by hand if the component side allows or we could use solder paste with hot air gun.
If we are to do a complex board then we would be using a reflow oven. For prototype board the components would be placed by hand and then the board would be put into the oven. For manufacturing at industrial scale the component placement would be done by a machine.
Now the question in my mind is, is it true that if we have to do 2-sided soldering of such a PCB it would be that once the one side has been soldered in the oven and we place components on the opposite side and put into oven again, the components would fall off from the first side due to heat in the oven? If this is true, how is this mitigated?
Best Answer
As other answers mentioned the light parts are usually not a problem. For heavy parts there is special heat curable adhesive that will ensure that those stay in place. Adhesive can be placed either under the component if there is no bottom pad or can be placed at the corners (for BGA components).
Good SMA house will have machine that dispenses adhesive. For manual process it is sold in syringes same as solder paste and can be (very carefully) applied to the board.
This type adhesive is cured by heat and allows for double sided assembly where you have heavy components on either side. The process will still remain the same though - first reflow one side then reflow the other side. Of course this also requires attention to components maximum temperature. If you have components that require low temp those should be all on same side and that side should be last to reflow to ensure that you don’t damage them by overheating.
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