PCB tinning including VIA soldering

For quite some time I was searching for a solution to protect the cooper on my PCBs from oxidation.
One solution I found for the finished PCBs is to spray them with some sort of plastic coating.

When soldering more complex self made PCBs it can take some time until all parts are soldered. During this time one is touching the PCB a lot leaving fingerprints and all kind of dirt on the cooper. This increases the speed of oxidation.
Also how nice would it be if one could use a PCB without all parts soldered on it for some time and after a while solder on some more parts which one wants to test. This does not work very good if on sprays the PCB with the plastic coating.

The solution I found was tinning. People are using soft solders which is for soldering fittings of water pipes to tin their PCB. If one is looking in the internet one often finds people using ROTHENBERGER ROSOL 3.
And it actually works! Putting the paste on the PCB, warm it up with a heat gun wash it away and you have a very nice tinned PCB, amazing!

So far nothing new.

What I did is I use the paste to solder my vias. As described in my post Flat VIAs for double sided PCBs I do flat vias by pressing wires into the PCB. This gives already some connection, but I’m not sure about the long term stability.
Therefore my solution was to solder also the vias. If you do the solder very thin you often can place a IC on top of a via.
But things become complicated if you have many vias, and specially when you have many vias under an IC which needs to be very close to the PCB.

I discovered this when designing a FPGA board. The FPGA needs contact with the PCB since it has a heat pad in the middle of the package (EQFP-144).

The first step is to drill all holes for the VIAS:

The whole setup looks like this:

Then you place the vias and press them together.

The last step is to put the soft solder on, warm it with the heat gun and remove the past which did not completely melt.
For me it worked best not to worm the solder paste until everything was melted since then I got no smooth surface,
Instead I worm it up until it looks that the flux becomes liquid. After washing one gets a nice tinned PCB.

After having tinned the while PCB one can drill all the other holes needed for mounting through hole parts and mounting holes.
One should to this after the tinning since otherwise a lot of paste is sticking in the holes.

To be really sure of good connection I still solder some of the important vias. Just to be really save. Even though it is probably not necessarily needed.