Selecting X-Ray Inspection Equipment

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2. How many ‘Ds’?

The D, of course, stands for dimensions. There are three kinds of systems:

• 2D, which provides a straight top-down view

• 2.5D, which allows top down and tilted or angled views

• 3D, which is a three-dimensional re-construction of the assembly. This might use such techniques as tomography, laminography or (for the full 3D effect) computed tomography, or CT.

JJS-Fig2-SMTJan2018.jpgFigure 2: In this example, voids in the solder are highlighted in green.

Of course, the more you get to see, the slower the inspection is. Complex CT scans for example can take hours to make. If the aim is to look at missing solder balls under a BGA for example, or shorts between them, then 2D can be fine. However, tilting can help get a better view if there are components obscuring the area of interest. 3D might be used for detailed quality investigations.

3. Ease of use

Some systems allow a degree of automated inspection, for example by programming sequences of inspections with pass/fail criteria. This of course makes repeatable inspection and operation very easy, and allows an in-line process if required. Setting it up though, or performing ad-hoc inspections, does require some skill.

While modern X-ray systems are easy to use, the inspector does need to understand what all the settings do (e.g., the voltage and contrast settings we mentioned before) and be able to interpret what they are seeing, which does require a reasonable knowledge of PCB assembly. There can be features that make image interpretation a bit easier, for example, by applying color.

4. Maintenance

It is worth remembering that the Health & Safety Executive (HSE) must be notified before using X-ray equipment. Obligations may also exist around creating rules or procedures for use, and engaging radiation protection supervisors and advisors. The equipment supplier should be able to provide advice, and it is a good idea to have them perform an (at least) annual health check on the system. It is also worth mentioning that there are different types of X-ray tube. ‘Open tube’ types are relatively quick and inexpensive to replace—perhaps a few pounds and a couple of hours—but need doing so after every 200-300 hours or so of use. ‘Closed tube’ types can last for many years but are much more expensive, maybe thousands of pounds. So perhaps the best choice depends on how much you will use the system.

The X-ray detectors tend to be standard or high-definition flat panels. The X-rays will cause these to degrade over time, typically around 20% after 10 years. While still usable, replacement after eight to 12 years could be advisable.

It is also worth finding out the common failure modes on systems, as while the component parts are similar, they can be assembled in different ways. For example, power supplies, connectors or cables can need replacing in time. Hopefully, this article has helped you understand some of the areas to focus on when looking to invest in X-ray inspection equipment.



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