Circuit boards – printed progress
Computers with more processing power than those used for the mission to the moon – back then, behemoths that filled multiple rooms – now fit neatly into your pocket: today’s smartphones. However, the science of miniaturization is nearly as old as the field of electronics itself. And in no small way, it owes its success to the availability of ever-shrinking circuit boards.
One of the oldest and still active PCB manufacturers in the world is Graphic Plc. The company was founded by Rex Rozario, a protégé of Paul Eisler, the inventor of the circuit board. Over the past 50 years, Graphic Plc has witnessed – and helped shape – the industry's evolution from the early single-sided PCBs to today's high-density interconnect technology: the ultra-compact PCB.
These multilayer PCBs from Devon, in southern England, are at work today controlling power plants, saving lives in medical equipment and connecting millions of people around the world via satellites. Graphic Plc has made a name for itself producing such mission-critical circuit boards, and its customers are mostly high-end users. One industry particularly dependent on highly reliable electronics is aviation, which is why several major aircraft manufacturers are supplied by Graphic Plc.
If it happens mid-flight, a fault in the electronics can cost lives. For this reason, high-end PCBs require high-end coatings. Here, Graphic Plc relies on the ENIG process (Electroless Nickel Immersion Gold), in which the copper conductor paths are chemically plated with a 3–6 µm thick layer of nickel; on top of that, a 50–100 nm coating of gold is deposited through an immersion process. PCBs coated in this way have excellent conductive properties and long shelf lives before assembly.
Even better results are achieved with an intermediate layer of palladium between the nickel and the gold: ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold). This advanced finish produces a contact surface ideally suited for high-reliability gold and aluminum wire bonds.
The changing face of quality assurance
To guarantee reliable solder joints and to insure good storage properties, the thicknesses of the different material layers must be perfectly coordinated. Graphic Plc is a well-known manufacturer of mission-critical PCBs. That means the quality expectations are correspondingly high. “Fischer supplies measuring technology with the precision we need to guarantee that our products are of the finest quality,” says Paul Comer, Technical Director at Graphic Plc.
Graphic Plc also values the reliability of Fischer instruments. For the past 20 years, there has been an XDL® system in use at the company headquarters in Devon. And with good maintenance, it would have continued to provide excellent service in quality assurance. But now the norms have changed.
The high-end users not only need very reliable PCBs but also very homogeneous ones. In order to for all subsequent steps to run as smoothly as possible, the printed circuit boards should be as similar as possible. That's why in 2017 the new IPC standard 4552 regulated the ENIG process very strictly: The gold layer may only be between 40 and 100 nm thick.
Only under such strict conditions does it become clear how important powerful measuring technology is.
To ensure that the specified norm limits of 40 and 100 nm are met at all times, the coating process aims at even narrower limits, e.g. 55 and 85 nm. The width of this "safety distance" depends on how consistent the coating process is. The more the coating thickness varies, the larger the safety distance must be to ensure, that the norm specifications are observed. As a rule, a distance of 3 standard deviations is selected.
However, the measured standard deviation does not only depend on the coating process itself – the measurement technology plays a critical role as well. If a device is not sufficiently precise, the standard deviation will be high even with a very consistent coating process.
For this reason, the new norm makes it necessary to monitor the quality of printed circuit boards with X-ray fluorescence devices with semiconductor detectors such as the silicon drift detector (SDD). The previously-common proportional counter tubes are no longer accurate enough for monitoring this strictly regulated coating process.
Tailored to the industry
Mindful of this update to the standard, Fischer developed its newest X-ray fluorescence device, the XDAL® 237 SDD, which was launched in late 2016. This SDD version of the tried-and-true instrument makes handling large and flexible circuit boards both simple and easy, and it allows for the analysis of layers just 100 nm thick.
One of the first devices of the new series was put into operation by Graphic Plc in December 2016. Before delivery, the system was pre-set and customized. Thanks to Fischer’s long experience with Graphic’s applications and products, it was an immediate match. “This tailor-made instrument fits perfectly into our quality assurance process and helps us to optimize our production,” explains Paul Comer.
But although the implementation is complete, Graphic Plc and Fischer are still working closely together. Paul Cave, Senior Application Manager at Fischer, continues to support Graphic Plc with their measurement tasks. The common goal is to keep pace with the changing requirements for the future.
Paul Comer, Technical Director at Graphic Plc
"At Fischer, the customer relationship doesn’t end with the sale of an instrument – that’s only when it begins! "
In addition to its X-ray devices, Fischer also offers other special instruments for the printed circuit board industry. The various layers of multilayer PCBs are connected via through-holes. Here, Graphic Plc uses the Phascope® to test the coating.