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

[Th3_uN1Qu3]

Out of all the above, adding some solder paste to the board pads before attempting to solder a new socket on seems like the best idea.

[petemanuk]

already tried that - still made no difference with balls not connecting to the pads

[diif]

Perhaps your profile isn't hot enough or doesn't hold for long enough.

[petemanuk]

I'll double check my temps and hold times and give it another go.

[b0ba]

add flux

[dellxps15]

for bga rework i use flux..... lots of flux when removing (jetronix eco ir) and when temperature reach 232° celsius in about 7 minutes , i remove the socket (i know 233° and around 7; 7:15 minutes is ok because at around 227° the socket start moving)

for resolder it same procedure... flux under it and at around 232° i give the socket a small move to see if it is soldered properly and all is done.

make sure:

1° socket is centered
2° mainboard is perfectly orizontal (no bend of pcb)
3° use good flux
4 use proper rework that can heat well the pcb
5° give the heating profile the right time to heat the pcb and melt solder (7 minutes on mine)
6° measure with 2 thermocouple bottom and top... bottom heater must heat more then top... heat try to go up ...

that's all and ofcourse lots of practice