Tom’s Circuits – Using X-Ray Inspection to Reveal Hidden PCB Assembly Defects

BGA and leadless components require X-ray inspection to find hidden assembly defects. X-ray images provide a unique view of modern electronics. The images here are from the X-ray machine at the Tempo Automation PCB assembly QA station. To avoid revealing a customer board, the example board is from the pile of development boards on my workbench. This board has a BGA and other interesting components.

The first thing to catch my eye was the shielded, filtered LAN connector, which has toroids and other components buried inside. Now I can see why filtered connectors are so expensive!

The X-ray machine is 3D, with a rotating table and adjustable camera angle. The table has a 16" by 16" viewing area and a maximum capacity of 28" by 30". The joystick controls are responsive with minimal lag. The user interface feels like flying a flight simulator around a printed circuit board assembly landscape. The X-ray chamber has a window, and looking inside, there is a red laser spot where the beam is centered. This provides a good clue, because X-ray vision is disorienting! While the laser dot shows the region of interest on the top of the board, the X-rays come up through the bottom of the board. This means that the X-ray images are a bottom view. I flipped the side-by-side pictures to make a better comparison with the optical images.

BGA oblique view shows good connections

The BGA connections are the dark circles. The round shape is good, since soldering problems tend to disturb the round shape. This oblique view shows a via dogbone pattern rather than via-in-pad.

A good DC-to-DC converter circuit image yields its inductor coil

This DC-to-DC converter circuit shows a power inductor, resistors, capacitors and IC bond wires. In X-ray images, the metal is dark, while glass, plastic, and ceramics are transparent. The metal plates in the capacitors are dark, while the thick-film resistor is mostly transparent except at the terminals. The inductor coil shows up as darker rings within the mostly-metallic ferrite material.

X-ray of thermal pad detects potential hidden short-circuits

One purpose of X-ray printed circuit board assembly inspection is to find solder bridges like the one shown in the picture. This board works fine, so I drew in a typical defect in the picture above, where the soldering problem would otherwise be hidden by the IC package. 

Bond wires are visible for an IC package with two chips

The bond wires for the switching IC are extra thick to handle more current, and show up well in this picture. Smaller bond wires are also visible at higher magnification. This image also shows thermal vias and some power plane keepouts. This IC package has two chips inside, connected together by bond wires. The high-current pins use multiple bond wires to handle more current.

Higher magnification is required to see network interface bond wires

In this network interface IC, the bond wires are not visible at this magnification. If necessary, the operator can zoom in to see the finer bond wires. This machine has capability for micron-level resolution, which is limited by the X-ray source spot size and the electromagnetic lenses.

This image also shows a coil in the ferrite bead FB1 in the upper-left-hand corner. Again, the X-ray view enables inspection of the exposed-pad solder joints underneath the IC.

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Learn to See with X-Ray Vision!

It takes a while to learn to read these translucent images. It is best to start with a simple image at an oblique angle.

View of an SMT resistor at an oblique angle

Since only the metal is visible, the front-to-back orientation is not obvious like it is in the optical images.

Viewing inner layer plane keepouts around vias show the ground and power plane stackup

While operating the X-ray machine, the 3D structures are easier to understand by flying around and changing the angle of the view. Unlike medical imaging, the operator doesn't have to worry about the radiation dose. This allows time for exploration.

By looking at the inner layer plane keepouts around the vias, it is possible to see the stackup of the ground and power planes. Each copper layer darkens the image a little. There is enough beam energy to penetrate the copper layers and create these grayscale images. Thicker copper or more layers of copper require higher beam energy. At higher power levels, there is some loss of resolution as the X-ray source spot size increases due to thermal effects.

X-ray inspection showing solder voids in pad areas

Solder voids are caused by entrapped gas or flux. Only exotic processes such as vacuum soldering without flux can eliminate all the bubbles. The voids decrease solder joint heat conductivity. If voids cover too much pad area, or cause a part to tilt, they can also hurt reliability. Process control to minimize these bubbles relies on X-ray images. For example, solder paste is kept refrigerated. Water contamination of solder paste can be caused by condensation on paste that has not reached room temperature before being opened. Water contamination causes solder balls and voids. Inspection ensures detection of bad solder joints or short circuits, and it is up to the process engineers to track down the problems and solve the root cause. Without X-ray images, they would be flying blind.

3D measurement of through-hole connectors can uncover pin-hole fill problems

The X-ray machine has 3D measurement software that can make precise coordinate measurements. For example, inadequate pin-hole fill for through-hole connectors is a common problem. This board X-ray shows some pin-hole fill challenges that can be quantified by the software. The selective solder process can solder large through-hole parts, but sometimes changes are required, such as adding thermal relief to the design. The selective-solder process can also be adjusted, for example with dry nitrogen preheating.

More About Tempo's X-Ray Machine

The Nikon 3D X-ray provides a combination of rotation, tilt & magnification

Tempo has the Nikon XT V 160 X-ray machine. It enables inspection from any angle interactively or with a program. Tempo creates a program in advance to automatically inspect and analyze each board. Tempo Automation uses these X-ray inspection programs to achieve rapid turn-around-time while maintaining quality. At less than one second to capture an X-ray image, it reduces labor and machine time and enables more detailed inspection.

This Nikon is the fastest machine available. In addition to automated testing, Tempo QA inspectors examine four different views of every board shipped to find problems with hidden BGA and QFN solder joints, or any other part solder skips, bad solder joints, shorted pins, and lifted pins. With up to 500nm resolution, Tempo detects defects down to the micron level as part of the standard assembly service.

Tempo Automation raises the bar for rapid prototyping quality. Please reach out to Jesse Koenig, Tempo's VP of Technology if you are interested in seeing the Tempo QA process in action!


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