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My first time soldering a PCB. The first ten joints went well and then suddenly the next two didn't. I can't get the solder to fully surround the wire on this joint. One side just won't wet despite use of flux.
Also what size solder should I use for this kind of soldering? I'm using 1.2mm rosin cored leaded solder but it seems way to thick for this work.
This is what we call incomplete wetting of the solder joint: the solder didn't wet the surface well enough. This happens because solder has a surface tension of its own, the solder surface itself isn't always "sticky", and because the bare metal you're soldering to might have a thin layer of oxide or dirt on top that repels the liquid. On top of that, the solder needs to be at the right temperature – if it's too cold, it will not form bonds with metal surfaces.
This is usually solved with cleaning and using flux – flux is a component that reduces the liquid solder's surface tension. Your solder has a flux core (there's solder without, but you'll rarely see it in retail), and that should often suffice. If it doesn't here, it might be worth using a desoldering wick or pump to remove the old solder, clean the contact with a paper towel and a drop of alcohol (Wodka works... but isopropyl alcohol is worlds better and contains no potentially corrosive water) and use fresh solder. Don't overheat it – overheating it not only oxidizes metal surfaces and damages the PCB itself, it also burns the flux, which is the opposite of what you want.
In your specific case,it looks like the solder stuck to a few parts of the contact, that's good! so, remove the solder (heating it with a freshly tinned soldering iron that you removed excess solder from to "wick out" the solder on this joint might be enough), clean the contact, put your soldering iron to the contact and with the other hand feed in some fresh solder. Should do the trick.
Also, what greatly helps is liquid flux, which you can buy in little flasks, that comes with a tiny brush. Really makes wetting contacts a lot easier.
Pads need to be clean before soldering.
If some of the board soldered, it sounds like some were 'good enough', and some not.
You might have finger grease on them, or tarnish/oxidation caused by exposure to contaminants in the atmosphere over time.
There are several methods to clean pads, depending on what you have to hand, and what the problem is. Cleaning with iso-propyl alcohol will remove grease. Scraping with a knife to expose a clean surface will deal with anything.
Most common reason I find for that situation is a bad heat distribution. What I usually do is heat solder pad and pin from one side and put up lead from the other side. Then when solder and flux melt and engage, the area to be soldered is at temperature and the flux is able to do its entire job before evaporating.
By the way: you state that this is the first joint going wrong, but the wire joint above is awful, misadjusted and baked on without a good solder connection.
I find 0.8mm,0.03125", 1/32" solder works well for most smaller work.
My guess is that your issue is insufficient heating. Is it always the far side of the soldering iron that is not wetting? Your tip may be too small and have too little contact area and too little thermal mass causing you to need to to hold it too long on the pad, which is what ends up burning boards. The power supply behind your tip isn't actually powerful enough to heat up the pad on its own, even on a soldering iron station. It needs to pre-store heat in the mass of the tip which gets sucked out by the joint.
Don't use a conical tip if you have alternatives. They have poor contact. Use chisel tips or bevel tips. Some tip with a flat face that can actually make good contact. And use as large a tip that you can get in there. Only use tiny tips if there is not enough room. I know there's a rule that says use a tip that is 60% the size of your pad, but I think that's baloney. You can often use a tip that is 50% larger than the pad, sometimes even double and it works even better.
If a bigger tip doesn't fix it, and it's still not wetting with flux that means there's too much corrosion and you'll need to clean it up, either by heating it up with even more flux until it gets shiny. But then you have to clean away all the gunk. But more importantly, you have to go in short heating sessions so you don't burn the pad off the board. You can try and use a fiberglass burnishing pen to clean it up, just mind the dust produced. Wet it down and clean that up with alcohol so you don't breath any solder dust formed as a result.
It's probably your tip though.
In addition to the other fine answers, you should be aware that liquid solder flows toward heat. That means that, despite what your intuition might say, you cannot use your soldering iron tip to push solder to the place you want it to go. You can, however, pull it.
Oxidation, in order for it to fully wet the pad, your connection needs to be cleaned from all oxidation, before soldering i usually take a steel wool and pass it on the joint I'm about to solder, not to hard though but just enough to remove any oxidation that's built up. The flux in the wire can help for mild oxidation but sometimes there's too much and you needs to clean that off
