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BR/001 United Kingdom, 2014-09-03

Floating down the line

A major issue in the manufacture of semi-conductors and microchips used in printed circuit boards (PCBs) is particle contamination that can dramatically increase production costs and reduce the operational life of the end-product. Paul Streatfield, an expert in linear motion systems at Bosch Rexroth, examines a potential solution which combines linear drives and magnetic levitation.

The manufacturer of semi-conductors and microchips used in PCBs requires an ultra-clean production environment, such as an inline vacuum deposition process.

Consumer products, from televisions to portable systems, such as mobile phones, handheld game consoles and personal digital assistants, all contain sensitive electronics which are manufactured in this way.

However, the process of production, even in a vacuum, all-too-often lends itself to the creation of tiny particles of dirt which can reduce production quality and the operational life of the end product.

Particles are generated by metal to metal, or metal to grease contact inherent in conventional methods of inline transport. Electronics manufacturers are currently using a variety of methods from chain drives to conveyor belts with linear motors which are complex, expensive and, crucially, particle generating.

Typically, the layout includes a vacuum-sealed process chamber with the carrier inside a vacuum. The problem with this method is that the bearings are also inside the vacuum, which immediately results in metal to metal contact and the potential for particle ingress.

What’s more, particles are not the only problem. This type of production is neither scalable nor flexible. Increases in demand cannot be quickly accommodated and the line will need extensive service and maintenance.

The requirement is therefore to get away from any touching of components during the manufacturing process, which in turn improves product quality and cost of production.

One potential solution, that is currently being extensively tested, is magnetic levitation, such as the Bosch Rexroth LeviMotion concept, which combines inverted linear motion technology with a completely contactless transportation system.

With a standard linear motor system, there is one moving coil with the motion controlled by the switching of the current which activates the magnet. The carrier is then driven down the production line.

The alternative currently being extensively tested is the use of an inverted linear motor with magnets underneath the drive carrier, with the coil units mounted outside the process chamber. This type of system enables large air gaps between magnets and carrier which levitates above the permanent magnet tracks.

In addition, a position sensor, consisting of two hall sensor elements, controls the exact location of the carrier. Magnets moving over the sensor create a sinusoidal wave with the sensors spaced to ensure the phase difference is 90o. Interpolation of the signals gives the exact carrier position.

The carrier is also equipped with an automatic alignment procedure and advanced carrier control which offers full degrees of movement on five axes, including pitch, roll and yaw.

This type of system has two advantages. Firstly, a series of coils can be constructed and up to 32 carriers can be used, rather than just the single carrier with the standard linear motor. Secondly, with the coils mounted underneath the carrier, any ingress particles fall away, rather than onto the carrier, which improves product quality.

In short, with the inverted linear motor having no active parts due to the bearings being located in fixed positions there is much less potential for particle ingress.

With this method there is no friction or wear and the movement of the carrier is contactless and clean, with no particle generation and no lubrication. What’s more, this method of transport is frictionless with no bearing related disturbances like sticking or slipping or fluctuating stiffness.

In addition, only passive or sealed components are located in the process chamber which, in turn leads to lower maintenance costs and a lower cost of ownership.

Crucially, the combination of linear motion and magnetic levitation can also deliver operationally. This type of line can offer high speed high positioning accuracy with constant speeds and low ripple. What’s more, testing has shown excellent planarity over long transportation distances with automatic alignment procedure in the bearings’ air gap.

In terms of production throughput, the carriers can achieve speeds of up to 5 cm/s and can carry loads from 1kg to 1000kg. Most importantly, the carriers are capable of repeat positioning of 10-20 µm along with exceptionally high positioning accuracy and minimal velocity ripple.

Whilst this combination of Bosch Rexroth’s NYCe4000 LMS drive system and Mecatronix’s magnetic levitation is only currently being tested, it has already gained significant interest from electronics manufacturers. It is easy to see why as the combination provides a potential solution to particle generation which has dogged semi-conductors and microchip manufacture for years.

 

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Rachel  Day
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Rachel.Day@boschrexroth.co.uk