Petteri Aimonen Turns Scrap Razor Blade Plastic Into a Smart Periscope for Easy BGA Inspection

Embedded systems consultant and self-described electronics hobbyist Petteri Aimonen has come up with a smart way to check whether the solder joints of ball-grid array (BGA) packaged parts are properly made — by literally shining a light on the problem.

"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," Aimonen explains. "The gold standard would be X-ray imaging, but short of that you can use optical verification with simple prism tools."

In a BGA-packaged surface-mount part, the underside of the chip is covered in flat contact points — typically, but not always, arranged in a grid across the entire lower surface. Small balls of solder are positioned between the chip and the circuit board to which it is to be fitted, then the entire assembly is heated to melt the solder and make the connection.

It's one thing to inspect the solder joints on the very outer edge of the part, but quite something else to check the ones closer to the middle. Aimonen's solution, in the absence of an affordable hobbyist X-ray machine, is to shine a light across the chip and look for shadows that would indicate a short — and, where other components mean that's not possible, to create a pair of prisms to act as a BGA-inspection periscope.

"By cutting and polishing [a] 45° angle in a transparent piece of plastic, total internal reflection makes it act like a mirror," Aimonen explains. "The light from a[n] LED will be directed under the chip, and any light that gets through is mirrored upwards towards the viewer."

"I made my first prototypes from 2mm thick acrylic," Aimonen continues. "But then I found out that many PCBs leave only about one millimeter of space around BGA chips. I didn't have any flat sheets of plastic that thin, but searching through my shelves I figured that Gillette shaving blade cartridges are made of thin transparent plastic."

Using this scrap plastic, Aimonen created the prisms by cutting rectangles then sanding them smooth to 1,200 grit before switching to a one-micron polishing paste and a specialist plastic polish. 11 surface-mount green LEDs turn one prism into a light source, and the other prism serves to redirect the light after it's passed under the part on test — making the hidden solder joints visible to the naked eye.

Aimonen's full project write-up is available on his blog, Essential Scrap.