Lasers give the cutting edge

The flexibility, speed and precision of laser technology makes it an attractive solution for many production problems. When first invented, lasers had no practical use. Now, new applications are appearing all the time and over the next five years just one the dual-laser drilling system is expected to sell in thousands. This application from Oxfordshire […]

The flexibility, speed and precision of laser technology makes it an attractive solution for many production problems. When first invented, lasers had no practical use. Now, new applications are appearing all the time and over the next five years just one the dual-laser drilling system is expected to sell in thousands.

This application from Oxfordshire company Exitech has resolved the problem of drilling 100 micron diameter holes.

Making such tiny holes, the width of a human hair, was a big challenge to the printed circuit board industry, where size is everything. In an effort to shrink printed circuit board size by packing more components into smaller areas, all the features on a board are being miniaturised, including the holes which electrically connect one board layer to another.

These holes, or microvias, are traditionally drilled mechanically, but the process is more uneconomical the narrower the hole.

‘Mechanical drills required for holes of 100 micron diameter are extremely expensive and they wear out after only a few thousand holes,’ says Malcolm Gower, chairman of Exitech, which has pioneered a new laser drilling solution.

The technique, which Exitech is selling to the PCB industry, combines the capabilities of a Yag laser and a carbon dioxide laser (see box) to produce a high-speed, accurate and economical tool.

‘We use the Yag to drill the top layer of copper while the CO2 drills the dielectric underneath. The 10 micron wavelength of the CO2 laser means that it makes no impression on copper, so it does not damage the layer of copper underneath once it has penetrated the upper layer of dielectric,’ says Gower.

While Exitech worked with the laser manufacturers to develop specifications that best suited the microvia application, the main innovations behind the Series 9000 machine were in the machine control software.

‘The clever part was to move the beams with accuracy at high speed,’ says Gower. When he talks about high speed, it really is high up to 20,000 holes per minute, depending on spacing.

This gives massive benefits compared to mechanical drilling. For instance, Exitech says that when drilling microvias of 100 micron diameter in a typical application, the dual-laser machine operates with a speed of 10,000 holes per minute compared to 500 per minute with a mechanical drill. Gower says the cost is less than 1p per 1,000 holes using the laser, and about £14 with the mechanical drill.

Growing world market

Exitech is just one of the contenders for a share of the growing world market for dual-laser drilling systems. Two or three other companies now also offer systems, though Exitech claims to have been the first on the scene. And the company continues to develop the dual-laser technology further.

Exitech’s current machine can drill holes down to 50 micron in diameter. Gower is confident that this will come down to 20 micron soon to offer greater miniaturisation potential.

The Yag and CO2 lasers which form the basis of Exitech’s machine are already well accepted in the manufacturing arena.

They could soon be joined by diode lasers, whose use, until recently, has been limited. This technology, which originated in the electronics industry, is used in everyday applications such as compact disc players, laser printers and barcode scanners. The power of the laser diodes required for such products is measured in milliwatts.

But in the past couple of years, big developments in the technology have taken place. They have resulted in high-power versions with applications for welding thin sheet metal foils and thermoplastics, according to Dr Christoph Ullmann, managing director of German firm Laserline.

The diode laser’s rise to high power levels has been rapid. Two years ago, Laserline demonstrated the prototype of a 500W device. Today, it can offer a 3kW device.

As Ullmann explains: ‘No one development is responsible for this step-change in power. All the features of the laser have been enhanced, not just to increase the power but also to reduce the spot size of the beam.’

Ullmann claims to have reduced the spot size from 2 x 4mm to 0.6 x 2.5mm in two years.

A diode laser is composed of stacks of laser diodes: the larger the stack, the greater the power output. It sounds simple, but the cooling system is complex, as is the optical technology used to focus the beam.

However, there are many benefits compared to other types of laser. More than 40% of the energy input to a diode laser is converted into useful energy while the figure is less than 10% for other lasers, says Didier Boisselier, research and development manager at Irepa, a research institution in Strasbourg, France.

Diode lasers also have the advantage of being lightweight (about 5kg), and of only requiring electricity and water to function, he says.

For plastic welding applications, the diode laser competes with conventional ultrasonic and vibration welding techniques, says Ullmann. ‘Overall, it is cheaper than vibration welding and more expensive than ultrasonic welding.’

However, the diode laser is most likely to be adopted when these two other solutions fail, such as for welding boxes containing electronic components. ‘With ultrasonic or vibration techniques, there is a risk of destroying the electronics, so the laser is a much better approach,’ Ullmann says.

Robot-mounted lasers

The laser also wins over the other tools for complex 3D plastics welding tasks because it can be mounted on a robot or gantry system. Where seams are difficult to access, the laser may be the only solution available. It is also more economical for short-batch runs, as dedicated tooling is not required.

Apart from plastics, the other great potential for the diode laser is the welding of metal foils or thin sheet up to 2mm thick, Ullmann says. He points out that most European car manufacturers are investigating the technology for possible use in body-in-white applications, to weld aluminium or thin coated steel.

‘The diode laser is an alternative to MIG/MAG welding for overlapping joints. The surface quality is excellent and production stability is very high. There are no nozzles, which degrade with time and have to be maintained, as there are with the MIG/MAG process,’ he says.

High-power diode lasers are probably at the stage excimer lasers were at five years ago. In that period, excimer lasers have made the transition to use as an industrial tool. They have become as much a workhorse in some sectors as Yag and CO2 lasers are in others. But their high cost still limits them to relatively high-value products, says Gower, and they have very few critical applications outside the electronics industry.

Big printer manufacturers now use them for drilling holes with a specific tapered shape in ink-jet nozzles. The excimer laser achieves the accuracy and repeatability that is required by this demanding application.

In the manufacture of silicon chips, excimer lasers are increasingly replacing longer wavelength lamp sources for the photolithography process because of the ever-present search for greater miniaturisation. According to Gower, excimers are starting to be used for 64Mbit memory chips and for the latest Pentium II microprocessors.

A third electronics product which takes advantage of excimer technology is the type of flat-panel display used in laptop computers. Each pixel in the display includes a transistor. The thin film of silicon around the transistor needs to be annealed heated up and allowed to recrystallise.

By using an excimer laser to apply the heat, rather than an oven, the heat effect is localised, and the glass substrate does not get hot.

As a result, the manufacturers can use a cheap glass substrate, with a low melting point, instead of a more expensive one with a high melting point.

Shining the light

Laser stands for light amplification by stimulated emission of radiation. A gas, liquid or solid medium is stimulated by electricity, causing it to emit light. Some of the light is reflected back, exciting the medium further and producing an intense beam of light of a single frequency. The description of a laser ‘carbon dioxide’ or ‘Yag’ usually refers to the medium.

Carbon dioxide lasers provide the greatest power and are widely used for cutting and welding.

Yag lasers use a yttrium aluminum garnet crystal as the medium, and are less powerful but more precise.

Excimer lasers are the least powerful but most precise type and use inert gases such as krypton as the medium.

In the diode laser, the beam is generated by a microelectronic diode which transforms electrical energy directly into laser energy.