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Nitromiser helps make perfection affordable at Westinghouse Signals

Product reliability is critical for railway signal manufacturer Westinghouse Signals. The Nitromiser nitrogen control system has brought both direct savings and other, less quantifiable, benefits to the company’s state-of-the-art SMT production line.

“Our electronic signalling systems directly affect the safety of millions of railway passengers around the world,” says Brian Samuels, Engineering and Technical Services Manager of Westinghouse Signals Ltd., based in Chippenham, U.K. “Signalling equipment has to work perfectly from the day it’s installed, and it has to keep on working for a design life of perhaps 30 years. So we take product reliability very seriously indeed.”

Many factors influence the reliability of a complete signalling system, among them software engineering, circuit design and assembly, installation and testing. The quality of soldered joints is just one part of the complete reliability chain, but a very important one. Samuels and his team are convinced that eliminating oxygen from reflow soldering ovens is the best way to ensure consistent joint quality when assembling circuit boards using surface-mount technology (SMT). Since commissioning their new SMT line in summer 1996 they have used nitrogen supplied by Air Products to control oxygen levels.

Now Westinghouse Signals has a new tool, the Nitromiser nitrogen control system, to aid the search for perfect joints every time. The Nitromiser has made proven savings in nitrogen consumption, but more importantly it has brought greater understanding and flexibility to the process. “We now have better control of oxygen levels, and full traceability to help our QA procedure,” says Samuels. “That means fewer faulty boards, and therefore better workflow and much lower costs for rework. We also have greater production flexibility because we have a wider ‘process window’. For instance, we can vary oven temperatures to suit different component types, while still being confident that we can make perfect joints. We’re delighted with the Nitromiser.”

Keeping up the quality...

With a history of making railway signals that dates back to the 1870s, Westinghouse Signals is in some ways a very traditional company. Traditional perhaps, but not backward-looking; the new Westinghouse TBS signalling system is the most advanced of its kind, and the company knew from the start that such a complex system would have to be built using SMT. Work on the new SMT assembly line started in March 1995 and the line was up and running by August 1995.

Right from the start the decision was taken to use nitrogen. While other manufacturers disagree about the need for nitrogen, or grumble about the cost, Brian Samuels was convinced that nitrogen would be needed to deliver the quality essential to Westinghouse Signals. The nitrogen system was installed in summer 1996 and Air Products soon became a partner as well as a nitrogen supplier. “All this was new to us,” says Samuels. “We needed Air Products’ expertise as well as their equipment.”

By excluding oxygen from the high-temperature areas of the oven, nitrogen prevents damaging oxidation and so helps the flux to do a better job of creating defect-free solder joints. Westinghouse Signals considers nitrogen a must when soldering the second side of the double-sided boards, and is now considering also using nitrogen for the first pass through the ovens, says Samuels. Better solder quality first time around means much lower costs for rework and re-testing, as well as greater overall reliability.

And for the kind of low-volume, high-value production that goes on at the Chippenham plant the indirect benefits are even greater. Fewer faults means a more predictable workflow, which can have huge advantages when a single faulty board can hold up an entire project. “We still inspect the boards visually, but with nitrogen the process is so reliable that we don’t really need to,” says SMT Process Engineer Peter Gardner. The SMT team members are convinced they have made the right choice.

...while cutting costs...

By the beginning of 1997 Brian Samuels was wondering if there was any way to cut nitrogen consumption to reduce costs. Nitrogen is normally piped to the oven at a constant flowrate that takes no account of process conditions or changes in production. Samuels consulted Air Products engineers, who suggested that it might be possible to adapt a system already used to control the atmosphere in heat treatment furnaces used in the metals industry. Work began on a control system that would deliver nitrogen to the oven only when and where it is needed, eliminating costly waste.

Air Products decided that the importance of the problem justified developing a stand-alone control system that could be used throughout the electronics industry. Westinghouse Signals offered to help with the development work, and the first prototype, dubbed the Nitromiser, was installed in February 1997 for a three-month trial. The advantages of the Mark I Nitromiser were clear, but the control response originally developed for bulky high-temperature furnaces was too slow for the compact ovens used for reflow soldering. An improved version, the Mark II, was installed at Chippenham in July 1997 and has worked well ever since.

The Nitromiser uses an oxygen probe to measure the oxygen concentration inside the oven at the critical point where the solder paste melts, and this signal controls the flow of nitrogen to the reflow zone of the oven. A control system based on fuzzy logic gives a fast response and allows the system to “learn” from experience as the process conditions change. Optical detectors along the conveyor at each end of the oven detect breaks in the flow of work and reduce the nitrogen flow to a standby rate whenever the oven is empty — especially useful for low-volume production lines such as this one.

...and reaping the rewards

Considered purely in terms of nitrogen savings the Nitromiser will pay for itself in five years, Brian Samuels estimates. But there are other benefits which, while harder to quantify, are even more important and yield much greater savings.

Two benefits are confidence and traceability. The railway companies who buy signalling systems such as the TBS have formidable inspection requirements for both the systems themselves and the production processes used to build them. Records, for instance, must be kept for a minimum of 13 years. In this climate of strict quality assurance, the knowledge that oxygen levels are being accurately controlled and recorded provides peace of mind. “We run at 200 ppm of oxygen, but because our measurements are accurate we know that our 200 ppm is probably equivalent to other manufacturers’ 100 ppm or even 50 ppm,” says Peter Gardner.

Next, better control of oxygen levels opens the process window by providing high levels of reliability under non-ideal process conditions. Without nitrogen, issues such as pad placement, flux composition and oven temperatures all have to be exactly right to produce consistently good joints. With nitrogen, oven temperatures, for instance, can be lowered slightly without compromising joint quality. This allows odd components such as through-hole plastic connectors to be added before reflow soldering and so saves a separate stage of manual assembly. Boards that have been in store for longer than usual, boards with fingerprints, boards with incorrectly-sized pads — they all solder much more easily when nitrogen is used.

While SMT may no longer be witchcraft, it is still difficult to get a good idea of what goes on inside the oven. Westinghouse Signals has found that the extra control offered by the Nitromiser gives a good insight into the process. Early on in the development work, for instance, it became clear that air currents around the oven can have a big effect on the oxygen concentration inside. As a result the company has taken steps to improve the sealing arrangements at each end of the oven. The oxygen probe also makes it easy to follow the “outgassing” of each board as it approaches the high-temperature zone, with important consequences for the way the oven temperature profiles are set.

But the most important of all these advantage is product quality. As component densities increase, visual checking for bad joints becomes more and more difficult. Electronic testing backed up by environmental stress testing is a fast and powerful way to detect faults, but it is not a panacea. Once a defect is located it is expensive to repair the board and even more expensive to re-test it, especially on production lines making small numbers of many different board types. The reliability of a repaired board is never as good as one that was properly built in the first place.

In the end it is product quality that matters, and it is best to get this right the first time. “Rework is so expensive that I’m sure nitrogen is the best solution even for high-volume production,” says Brian Samuels, who has extensive experience in high-volume production. “At Westinghouse Signals we have passenger safety to think of, not to mention the reputation of more than a century. Without nitrogen and the Nitromiser our job would be that little bit harder.”

Is it a plane? No, it’s a train

Modern railway signalling has come a long way since the rods, levers and kerosene lamps of the nineteenth century. The TBS system, developed by Westinghouse Signals for the Jubilee Line Extension to the London subway system, uses triple-redundant voting and other computer technology pioneered in the aerospace industry to get more trains onto a given length of track whilst maximising safety. “TBS is probably the most advanced railway signalling system in the world,” says Brian Samuels, Engineering and Technical Services Manager.

Signalling is the limiting factor in determining the passenger-carrying capacity of a railway. By monitoring the position of each train at all times, the TBS system keeps close control over the distances between successive trains. This allows it to control up to 36 trains per hour in each direction — around twice as many as conventional systems allow — without compromising safety. The technology needed to do this includes fibre-optic and radio communications, sophisticated computer programs and ultra-reliable hardware, both on board the trains and at the trackside.

The £56 million contract for the signalling system is a small but vital part of the Jubilee Line Extension project, whose total cost of £2.6 billion makes it Britain’s biggest civil engineering project. The line, which connects central London with the Docklands business area to the east, is due to open in late 1998.

Smaller and smaller: future soldering challenges

Westinghouse Signals has found significant benefits in using nitrogen atmospheres to improve soldering quality, even though many of its boards are less complex than those used in state-of-the-art consumer products such as notebook computers. As component densities continue to increase and pad sizes shrink these benefits will become even clearer, says Pete Gardner, SMT Process Engineer at Westinghouse Signals.

Smaller pads increase the chance of misalignment when screen printing the solder paste or placing components, which in turn makes it harder to ensure good joints. Smaller pads also make it more difficult to provide enough flux to prevent oxidation. In these cases nitrogen helps provide reliable joints, says Gardner, when air atmospheres are useless for practical purposes.

The next challenge to solderability will come when ball grid array technology becomes widely used. To pack more connections into a given package size, a ball grid array chip has connection points on its underside as well as its edges — making conventional visual inspection impossible once the chip has been soldered in place. “At some point everyone will have to start using nitrogen,” says Gardner.

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