Test & Measurement

Sophisticated X-ray ensures hassle-free PCBA inspection

18th November 2014
Barney Scott

Subcontract manufacturers of PCBAs generally use a number of different tools for quality control, such as flying probe testers, camera-based automated optical inspection and X-ray equipment. Very few UK manufacturers can boast such sophisticated X-ray inspection capability as Newbury Electronics, following the company’s purchase of a Nikon Metrology XT V160 in January 2014.

BGA and QFN interconnections require close inspection. Modern BGAs are very complex, with tighter pitches and smaller ball diameters, meaning that they are mounted too close to the body of the PCB for visual inspection to be practical. Newbury previously used an endoscope to access and view areas of interest, which was effective for small, basic devices, but time-consuming and becoming ever more impractical, as it was increasingly difficult to position the endoscope without it being blocked by the components surrounding a BGA. The smaller sizes also meant that the inspection equipment could sometimes just not be used at all.

Even if the device could be well positioned, seeing far enough down the rows of balls was becoming more challenging. Some faulty boards would be returned and had to be sent to a bureau for X-ray analysis, typically taking three days and costing around £100, including fault finding, fixing and confirmatory testing.

Philip King, Managing Director, Newbury, commented, “As a subcontractor, we handle 10 to 15 different product lines every day and often do not know what we will have to produce next. PCBAs should be designed for easy inspection, but they rarely are. Accurate inspection is important not only after repair, but also during series manufacturing so that results can be used for process control to maintain quality. The larger the production run, the more potential there is for faulty boards, wasting time and money.”

An X-ray machine was the only realistic solution for quality control of larger board volumes produced by Newbury. Nikon's XT V160 provides a 400x400mm scan area, large enough for most PCBAs, and its comprehensive specification includes control and analysis software.

Mr King continued, “Low-end X-ray equipment, while cheap, provides such a poor image as to be virtually useless, especially given the increasing miniaturisation of BGAs. We knew we would only buy one X-ray machine in the foreseeable future, so decided to select a very capable model from Nikon with variable magnification, a tilting flat panel detector and a powerful, 160kV/20W X-ray source, which is at the higher end commonly used for PCBA inspection.”

The machine must be operated at capacity to generate a useable image only on the densest multi-layer PCBAs, such as those with multiple copper layers or high copper weight. With 2,500 parts, the boards controlling the hydrogen fuel cells on an in-development electric motorbike are particularly sensitive to quality issues, and are thus 100% inspected using the Nikon Metrology system. The XT V160 also offers nanofocus X-ray spot source and advanced image processing capabilities. Focus of the electron beam is maintained by a computer-controlled, electromagnetic lens that ensures the target does not overheat whilst maintaining a nanometer spot size, even at high kV settings.

When inspecting BGAs, a clear view of the ball interconnect is normally achieved by combining tilt and rotate movements and scanning down the rows, ball by ball. The XT V 160 enables this function using single-axis control, rather than the usual method of having to manipulate three axes, allowing operators to focus on the inspection process rather than on machine guidance. True concentric imaging makes it easy to rotate the view point through 360° around the area of interest, which is intelligently locked onto. Once a region on the PCBA has been identified and positioned in the centre of the screen, it remains fixed in position no matter what tilt, rotation or magnification is applied.

The X-ray source is designed for a PCBA to be placed within 250μM of the focal spot, allowing magnification up to 2400 times. The board can be viewed at angles up to 75°, enabling clear views of solder joints and through-holes while maintaining sufficient energy. Special analysis functions are available for inspection of semiconductor package voids, wire bonding and BGA solder bumps.

The machine's open-tube design means that the electron beam-producing filament can be replaced every six to nine months for under £10 each time. With closed-tube designs, the manufacturer has to be called in to renew the tube every couple of years at a potential cost of up to £15,000. Carrying out filament replacement in-house reduces downtime to minutes rather than days.

The machine was supplied fitted with optional CT inspection capability. This re-constructs a 3D image from multiple 2D X-rays taken from a controlled angular rotational scan, allowing the operator to virtually rotate and slice the 3D image. The extra functionality future-proofs the investment in case full 3D graphic displays are required, or if PCBA complexity becomes so high that a standard 2D image on the screen is not sufficient for quality control purposes.

Dave Roe, Senior Production Technician, Newbury, said: “We have not had a single board with a mounted BGAs returned as faulty since we started using the XT V 160. The system is just as good at picking up QFN solder joint faults and shorts, which are difficult to check as the leads are hidden under components.”

“Our engineers were initially sceptical but became converts very quickly, as they were able to see results in seconds rather than spend days waiting to get results from a bureau,” added Mr King.

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