BGA (Ball Grid Array) packaging for microchips is getting more common as devices become smaller. It’s quite feasible to solder with a basic home reflow setup, but verifying the results is difficult. The gold standard would be X-ray imaging, but short of that you can use optical verification with simple prismatic tools.
The idea is to shine light from the side, so that it passes between the ball rows beneath the chip. This will indicate any short-circuits between the rows, but will not show any missing balls. It will, however, show insufficiently melted solder paste or uneven distribution of solder. So if the chip began with all balls present and the solder paste was applied evenly on the PCB, you can be reasonably confident that any faults would be visible.
However, practical PCBs don’t have much space around the BGA chips so this is rarely possible. Instead, we can use a thin prism to redirect light in 90° angle around the corner of the chip. By cutting and polishing 45° angle in a transparent piece of plastic, total internal reflection makes it act like a mirror. The light from a LED will be directed under the chip, and any light that gets through is mirrored upwards towards the viewer.
See more including how to make the prisms in the post here.
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