BGA solder balls not connecting - help pls

[petemanuk]

So I though in my spare time I’d set myself up repairing motherboards mainly pin damaged lga1151 sockets types.

I’m using an achi ir sc pro rework station (gave up on my Achie ir6500 - don’t recommend)

I’ve managed to get my rework profile somewhere in the correct region (after a lot of trial and error) to get the old sockets off and reflow new ones back on again, the problems I’m getting is not all of the solder balls are re-attaching when re-flow occurs. I know this because when socket is removed pads still look clean and have no solder on them.

I'm also pre-baking the boards for several hours before refolwing them to avoid delamination.

Here’s what I’ve tried so far:

1. Made sure all the old solder is removed and pads are clean
2. Tried different makes of flux’s.
3. Tried different thickness of flux between the board and new socket.
4. Nudging the socket at reflow and you can see the surface tension pull it back into place.

I’m getting my new sockets for aliexpress and they are coming pre-balled with unleaded solder. I’ve had a look around the net and noticed some people using a stencil and adding more solder paste to the new pre-balled socket, has anyone had experience of this?

Also I’ve noticed that when I’ve been cleaning the motherboard bga pads that some of the are slightly sunken with the solder mask around them, I don’t think this is helping either.

Anyway enough rambling if any one could give me some tips/guidance it would be much appreciated