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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?
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!)
Glueing is seldomly needed/used these days. How the process is done exactly will depend on your fab. But most will first populate the high-density side and solder it and in a second run the other side will be done. The components on the high density side will usually be hold by their surface tension. The manufacturer will also slightly change the temperature profile for the second side, so the higher temperature "capacity" of the high density side will keep the components in place.
Some packages can not be reflowed twice, because of their high temperature sensibility. These should be placed on the side soldered last.
But the most important thing is, talk to your manufacturer, they can help you and very often can provide you with their assembly/layout-guidelines. If you follow them it will save you a lot of money and trouble in the end. Proper PCB-assembly is not a a totally straightforward process. It requires communication between customer and assembler and a lot of tweaking.
