I think you need to look for metal mask stencil maker.
Or some one that can cut by laser those opening on a
0.15mm stainless steel metal plate. Once you have this
you need to buy a solder paste and appy this on the opening
by using metal squeege.
This process normally is use in SMT manufacturing.
Hi,
you can do it on your own ((-:
have a look here h**p://wwwbode.cs.tum.edu/~acher/bga/index.html
Unfortunately it's in german, but you can see the ideas behind this approach.
If the pin count is low, I can do it with a heat gun. You must mark the PCB with an outline so that you can center the part. Touch a soldering iron to each PCB pad with a little solder and then remove it with solder wick so that each pad is tinned. Place the part on the board and then heat it with the heatgun. I use a wooden toothpick to gently push the part down to hold it in place when the solder reflows. It would be nice to have a paste stencil but sometimes you cannot afford one. This won't work if you have already removed the part and the balls are damaged.