I am attempting to reball some PBGA's with no luck. The problem is getting the solder spheres to sit in the stencil apertures and removing excess spheres. I apply light amount of flux to pads of a BGA, align a stencil over a BGA, then pour solder spheres. Some spheres get into the apertures, some apertures are empty . . . using a brush I gently try to removes excess ones but it just does not seem to work.
Any suggestions?
My mini stencil is 5 mils thick, apertures in the stencil are larger than the sphere diameter by approx. 10%-20% so that the stencil may come off after reflow. Also, the pads on the BGA are flat in some areas (after applying solder wick) and slightly spherical where the excess solder was removed but solder wick has not been applied. I have placed the stencil flat on the BGA . . . I also tried creating a standoff between the BGA and the stencil by applying Kapton tape to edges of a BGA so that the spheres would "fall in" the apertures. Nothing seems to work. The BGA is 256 ball count 1.27mm pitch with spheres 0.030" in diameter. I never tried to reball a component . . . in theory the process is simply in practice, not so (well, at least as it stands right now).
BTW, another issue that I noticed is that spheres tend to clump up with even a slight trace of flux. Trying to separate the balls is almost impossible in that case unless you have a lot of time.
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