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When 2-D X-ray Isn’t Enough
December 31, 1969 |Estimated reading time: 4 minutes
For most applications, non-destructive 2-D X-ray inspection is sufficient to detect manufacturing defects. In some cases, however, 2-D analysis does not give a complete enough picture, and destructive testing is needed. With advances in computed tomography (CT), this is no longer necessary.
By David Moller-Gunderson, phoenix|x-ray Systems and Services Inc.
In the few cases where non-destructive 2-D X-ray inspection is not sufficient enough to detect defects, the standard practice has been to cut out the part of interest, encase it in a plastic cylinder, and grind it down slowly to see the interior using standard optical microscopy. The two major downfalls to this technique are time and cost; it can take days to section a part completely, and when the sectioning is done, the part is destroyed. For high-cost applications, this can be prohibitively expensive.
CT techniques with sub-micron resolution* have created more possibilities. Parts that were too expensive to destroy, were time critical, or may have been falsely damaged by the sectioning process can be sectioned in a matter of hours. Computer-aided design (CAD) data tie-ins and accurate measuring allow for automated inspection of part placement. CT is a powerful tool, but it is too powerful in many cases. Users who are initially awed by the ability to fly through a part are soon put off by the scan time and amount of effort compared to 2-D, and revert back to the machine’s 2-D inspection capabilities. There is a time and place for CT. Users can easily fall into a one-tool mentality, and forget that they have both CT and 2-D capabilities. While most defects can be easily seen using the quicker 2-D method of X-ray inspection, there are several cases where 2-D is not enough.
Multilayer ICs
In many PCB design situations, board space is critical. Fitting in one more IC can be problematic. One solution is to stack multiple ICs into a single package, thereby shrinking the board footprint for a minimal height increase. While this technique saves space, it makes X-ray inspection difficult. Voids in heat-transfer material between chips may be easy to identify, but their vertical location becomes both important and impossible to detect.
Figure 1. High-resolution CT cross-sectioned image of a multilayer IC, showing voiding between layers.
With CT, users can take a 3-D scan of the chip, allowing voids to be identified and located, and letting users recommend changes to reduce or remove voids. Although the same result could be achieved by cutting the part, it would take longer and cost significantly more. A single IC might only cost $10, but when you stack eight of them together, you make an $80 chip. While this is not terribly expensive compared to the application it may be used in, the cost of sectioning several good chips to catch the defective ones can slowly begin to diminish profit margins. Submicron CT can also be used to determine whether a given multi-layer chip will fail due to improper heat transfer at some point in its life, while keeping the chip intact for use.
Multilayer BGAs
Another space-saving measure that has been widely used and can cause several problems with 2-D X-ray inspection is multi-layer BGA solder joints. Defects in single-layer BGA joints can be difficult to see - voiding tends to pop out; cracked or lifted pads can sometimes evade detection without manipulating the object. But when a second layer of BGA is involved, detection is nearly impossible. Using CT, however, can alleviate the problem.
Electrical testing will detect a problem with a certain chip quickly, but finding that defect using standard sectioning methods when 2-D inspection fails means one must physically cut a piece out of a PCB. Removing the chip for sectioning is not good enough, because the defect will probably be destroyed in the process. Instead, the entire board must be destroyed to search for a single defect, when replacing the defective chip would probably make the board salvageable.
Using CT, BGA solder joints appear very clearly, and detecting flaws in them is simple. With techniques such as 180º scanning, even BGAs on large boards can be scanned easily. Chips may not be costly by themselves, but PCBs tend to become costly quickly, so destructive testing becomes expensive; with CT, it also becomes unnecessary.
Dense Electrical Connectors
As electronics become more complex, boards need more communications capacity. While serial communications can be used to save space, speed and power requirements can sometimes necessitate bulkier connections. As these connections grow, they become increasingly difficult to check for both defects and potential trouble spots, particularly in the solder connection holding them down. Optical inspection can be physically impossible if there is an opaque plastic insulator, and 2-D X-ray is difficult because of part density.
Figure 2. High-resolution CT image of solder joints in a microBGA. One solder joint is virtually sectioned to show different solder phases.
Using CT with 180º scanning permits large connectors on larger boards to be three-dimensionally represented. Flying through or virtually sectioning the resulting 3-D representation allows easy detection of cold, broken, or missing solder joints; poor fits; misalignment; or other potential problems. Being able to do this without destroying the PCB holding the connector can save time and money, and allows for spot-checks on the line without reducing inventory.
Conclusion
The majority of manufacturing defects can be seen easily using standard 2-D X-ray techniques. In some cases, however, 2-D analysis does not give a complete enough picture, and destructive analysis is needed. This is not the optimal solution; destructive testing destroys the part, which, in turn, reduces both potential profits and the number of other tests that can be run.
Advances in CT have reduced the necessity of destructive testing to find physical defects significantly. By combining the potential of 2-D X-ray inspection and submicron CT with other nondestructive testing techniques, costly destructive testing may become a thing of the past.
*nanoCT, phoenix|x-ray, Wunstorf, Germany.
David Moller-Gunderson, customer support manager, phoenix|x-ray Systems + Services Inc., may be contacted at: dmgunderson@phoenix-xray.com.