The solder fillets around the base of the header pins are almost flat - very different from the kind of fillets you'd expect to around a 0.1" header. These kind of fillets are very common on ST's evaluation boards. By the consistency of the soldering, it look like it has been done on a machine. I am interested in knowing the soldering technique involved in this.
Kind of fillet you'd generally expect to see:
I suspect it has to do with the pad geometry. The pad rings look very narrow with a fairly large hole (for the pin size) which prevents the normal solder fillet from forming. Notice there is more of a fillet on the larger square pad for pin 1. There are robots that do the soldering ensuring a measured amount of solder on each joint.
It does not look like paste in pad (using the stencil process to put paste on the through hole pads), as you would end up with paste on the pins if they were inserted after paste was applied.
It was probably not done on a selective solder machine (like a one pin wave soldering fountain) because there is no access on the bottom side and the pins on the top would end up with solder on them if selective soldering was done upside down.
My guess would be that they used paste-in-hole soldering technology (aka intrusive reflow soldering) to solder the headers, but did not do the calculations properly to determine how much solder was needed to provide a fillet (or maybe they intended for there to not be a fillet at all). Here are a couple of links describing this process:
https://blog.samtec.com/post/guidelines-paste-hole-reflow-processing/ http://suddendocs.samtec.com/processing/through-hole-printing.pdf
By using a stencil that is too thin, less solder would be applied to the holes and it would give you the result shown in your image.
