PTH solder joint inspection: automated x-ray laminography ensures through-hole joints meet barrel fill specs.

Although plated through-hole package production is a mature process, it still results in unreliable and poor quality solder joints. IPC-A-610 specifies that an acceptable criterion for through-hole solder joints is that at least 75% of the barrel is filled along the board thickness and that a bottom fillet is present. Various testing technologies exist to ensure a sound electrical connection is created, but automated x-ray laminography is the only available technology that can ensure the through-hole solder joint meets the IPC-A-610 specifications of barrel fill.

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The widely used technique of wave soldering the bottom side of a board produces an effective solder joint with a strong bottom fillet. At times, however, solder will not flow properly, falling short of the required 75% of the barrel. This occurs if a pin is bent, contaminated or an insufficient amount of solder paste is deposited. This is also common when using Pb-free solder formulations, as the lower wetting force of Pb-free solder decreases the ability for solder to fill the hole sufficiently.

The "paste-in-hole" soldering technique (also known as "pin-in-paste") is a relatively new process of simultaneously printing solder paste for both SMT and through-hole packages. This technique is used when dealing with a double-sided SMT assembly in which components sensitive to high temperatures are used on both sides. This technique also may leave an undesirable solder fill because of poor wetting, insufficient amount of solder paste and no bottom fillet.

Electrical tests may be used to ensure that a connection is made from the joint to the board. On some packages with visible joints, visual inspection confirms that solder appears to have flowed through the barrel. X-ray laminography is the only testing methodology permitting measurements to be made within the barrel to ensure the solder fill within the joint meets the required IPC specifications.

X-ray laminography inspects specific regions or "slices" along the barrel and tests for solder presence. For each "slice" taken, a region of interest is created around the barrel. Within this region of interest, an average gray level value is measured representing solder presence. The average gray level is then compared to a model average gray level previously determined by the user. This results in calculating a percentage to represent how close the measured solder joint is to the known nominal average value. A pin is indicated when the calculated percentage is less than a set threshold. For example, anything lower than 90% of the nominal average gray level will be indicated as insufficient.

Using this technique, an automatic test can be set up to inspect several regions within the barrel and on the board surface. Figure 1 shows an x-ray laminographic image taken at 75% barrel (right side) and an image taken at the bottom of the board (left side). The system determines the missing solder and joint for pin 49. For pin 51, a joint is present, but the solder did not flow properly up the barrel, creating an insufficient joint. The system measured at 75% barrel fill that pin 51 is within 84% and pin 49 is within 30% of a known satisfactory solder joint's average gray value. For both of these joints, a present end fillet can be identified. Depending on the manufacturing requirements, this automated test could be arranged to catch missing and inadequate solder fill, or just the missing joint, by adjusting the allowable percent of the measured nominal average.

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Bibliography

IPC-A-610D, Acceptability of Electronic Assemblies, February 2005.

Stig Oresjo, "Test and Inspection as Part of the Lead-Free Manufacturing Process," ECWC 10 Conference, IPC Apex Proceedings, February 2005.

Dr. David Bernard and Bob Willis, "Measurement Variation in PIHR Pb-Free Solder Joints," "Using X-Ray Inspection, the Efficacy of Pin-In-Hole Reflow with Different Board Finishes is Studied," SMTA International Proceedings, September 2005.

Jeremy Jessen is technical marketing engineer at Agilent Technologies (agilent.com); [e-mail].