The problem with C-SAM for BGA underfill is that the sound has to pass
through many layers to get to the underfill. At each interface, some sound
passes through and some is reflected back. If the BGA has a laminate chip
carrier, the sound has to go through all the glass fiber bundles. You are
limited in how deep you can penetrate and the farther you penetrate, the
smallest flaw you can find will get bigger. So depending on you package
design, it may be easy or difficult. Thru-scan may be better at this than
reflective but I don't have any experience with thru-scan. With thru-scan,
the sound passes all the way through and is picked up on the other side of
the PCB.

Similar to the glass slide experiments, we make our own components and use
them for a wide range of testing. We start with a 16 mil thick board with
arrays of pads on one side. It has no Cu planes so you can sort of see
through it. We print flux on the pads and use a version of DEK's solder
sphere printing process to place the spheres with stencil printer. After
reflow, clean, and singulation, I attach to a PCB and underfill. I can see
the flow through ball array and big voids. After cure, I flat-section to see
the smaller defects.

Daniel Blass
Process Research Engineer with the Area Array Consortium
Advanced Process Laboratory at Unovis Solutions

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