A plan for coordinating 2-D and 3-D x-ray and microsectioning strategies.
Last month I discussed use of “full CT” (FCT), or 3-D x-ray, for analyzing electronic components and boards. This provides a 3-D model of the sample from which “virtual x-ray microsections” can be taken at any plane within the sample for analysis. While this can give excellent information, it does take time for the quantity of 2-D x-ray images to be acquired from 360° around the sample, from which the 3-D model is produced. Furthermore, as electronic features are small, it usually requires a set of high magnification 2-D images to provide the analytical detail within the 3-D model. As I have mentioned, the geometric magnification provided by an x-ray system depends on being able to place the object/field of view (FOV) close to the x-ray tube. As the sample is rotated within the tube-detector axis for FCT, then the larger the sample, the further away the sample must be placed to prevent a collision with the tube during rotation. Although it is possible to create an FCT model by only taking images from 180° around and using careful sample placement to allow an FOV to be placed closer to the tube, detailed information is unavailable for the model because only half the potential data are taken. Looking at an individual component under full CT does permit its placement close to the x-ray tube for high-magnification images. Once that component is on a board, however, the available high magnification for full CT is likely to be lost. At this point, cutting the sample to reduce its size becomes the only realistic option to getting the required detail from the FCT model.
Wisdom comes with a charge. Usually by the hour.
I like old things. Old shirts. Or old soft sweaters. Old test engineers recommend themselves. Old explanations. You know what to expect. Like a long-term, happy marriage. There’s a comfort level. Fewer mistakes, more detail. Less bewilderment. General competence. Analog testing. The ability to answer the question, “Why?” in waveform. Experience and familiarity sometimes matter. Some dare call it wisdom.
Why do we still worship youth? Haven’t we learned?
Old cars provide a certain comfort, too. As of this writing, my current model has accompanied me faithfully for 14 years and 417,000 miles with no engine rebuilds. Sure, the leather in the driver’s seat is ripped, but it’s still comfortable, and the ride remains blissfully smooth. So, I keep it. For now. Cognizant that the risk-reward ratio is tipping left toward risk with each succeeding day. Am I pushing my luck? Probably. But it still works. I know what to expect. So much else in life isn’t that way.
CT takes time, but can virtually microsection and analyze any plane in an entire sample.
2-D offline x-ray inspection of electronics is directly analogous to 2-D medical x-ray imaging, the only difference being image magnification is required to see the much smaller features within electronic devices and boards. (Also, the sample doesn’t talk back!) Medical x-ray imaging also offers CT scanning (also known as 3-D x-ray or computer tomography or CAT scans), and the same technique is available for electronics analysis. From Greek, tomography means writing slices.
CT provides a 3-D density map model of the sample from which virtual 2-D x-ray slices can be taken and examined in any plane within the model. Consider it like being able to take virtual microsections anywhere, at any angle and as often and repeatedly as desired through the sample without having to cut, pot, polish, optically image, repolish, re-image, etc., as when making a traditional microsection. 3-D representations of the sample may also be created and virtually sliced (FIGURES 1 to 3).
Everyone has an x-ray system for BGAs. Why not use it for through-hole too?
I am often asked about the use of x-ray inspection to investigate issues with surface mount components. Quite naturally, there is always a focus on BGAs, flip-chips, QFNs, etc., where the optically hidden nature of the joints lends those parts to x-ray techniques for analysis. I am rarely asked about through-hole joints, however. Is this because no one uses them anymore? I don’t think so. Indeed, I would venture to suggest there probably are more through-hole joints made today than ever. I am surprised PTH joints do not come up in conversations more frequently because they are obviously hidden joints.
IPC-A-610 guidelines qualify the level of hole fill to be expected within these joints. How else can you nondestructively check during manufacture if these conditions have been met? Are all through-hole joints made perfectly with 100% fill, and never with any issues? Or, as I suspect, is through-hole seen as a great, but slightly less fashionable, technique, and do many not consider an x-ray system can also quickly and simply confirm the quality of these joints? In other words, you already have an x-ray system for BGAs, so why not also get the benefits of its quality control capabilities for through-hole joints?
Explaining solder joint defects, one interruption at a time.
“Thank you all for promptly joining this morning’s webmail conference call. Today we are going to review the x-ray images our outside lab FTP’d us yesterday. Please set your phones on mute by pressing *6 if you are not speaking, and set your computers to the link provided by the meeting invite. You can unmute your phones in the same way, by pressing *6, if you wish to contribute to the conversation. You may listen via phone link, or by plugging earbuds into your laptops and activating sound. If you feel compelled to speak, unmute your phone and speak one at a time. We’d like to give equal time to as many members of our several participating engineering groups as possible. We have a big agenda, and many participants, so let’s get started.”
Here we go. Amateur hour.