When I think of packages like some DFN
or 0201 resistors I wonder where they would place glue dots. The UDFN pictured is 1.2mm x 1mm. And glue dots on 0.5mm pitch WLP
seem completely out.
Is it still possible to fix parts at the bottom of a PCB for soldering?
Answer
The glue dot method was first used for bottom-mounted SMD parts which had to be wave soldered. The packages you mention (DFN and WLP) are not suited for wave soldering. Also QFN can't be wave-soldered: the exposed parts of lands and pads are too small and not reachable for the wave.
For two-sided reflow soldering components are first glued and soldered on one side, then the board is flipped, and only then components are placed and soldered on the second side, so the glue dots are only needed for the side which is first soldered. Some components may not be suitable for soldering on the first side, but 0201 resistors, and even the WLP-16, will remain in place even without glue, thanks to the surface tension of the solder.
Further reading:
Loctite document "Working with Surface Mount Adhesives". ("Syringes can dispense up to 50,000 dots per hour". Wow, that's 15 per second!)
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