I have many components at the both side of the PCB.
could you please explain more about the oven process?
I mean once I should mount the components at the top layer and put into the oven and once again I do the same for the buttom layer?
how much time does it take during a oven process?
Thanks
My board has only passives on the bottom side:
So I place bottom side parts first and then reflow them. Once that is done, I make standoffs using M3 screws with some nuts, and then place components on the top side (take note of screws I used to make standoffs):
As you can see, the top side has many parts, so by the time I placed them all the board will long be cooled down to ambient temperature. So I wouldn't worry about that too much. Again, my comment was mostly referring to manual soldering of thru-hole parts after reflow.
I don't use any glue to hold bottom parts in place during second reflow, and so far no part has came off - I had everything from tiny 0201 to quite large "type D" (size 2917 IRC) sized parts (see first photo). Surface tension of molten solder is quite strong and keeps all these parts in place.
Granted, I didn't try doing the same thing with parts in BGA package - for those you will probable need some glue. More experienced colleagues will hopefully pitch in and share their experience in that area - I would love to know that as it would allow me to make my boards more compact. For now I try to avoid placing anything other than passives on the bottom side - mostly out of concern that they would fall off/move and create shorts underneath, but also because I don't want to subject sensitive (and expensive) parts to second reflow if I can help it. At the end of the day, even if some of passives would come off, I can always solder them back into place manually, but this isn't so for things like QFN, or BGA.