Magnetic levitation solution for PCB production
14 July 2015
Paul Streatfield, from Bosch Rexroth, examines a potential solution to particulate contamination on PCB production lines that combines linear drives and magnetic levitation.
A major issue in the manufacture of semi-conductors and microchips used in printed circuit boards (PCBs) is that particle contamination can dramatically increase production costs and reduce the operational life of the end-product.
The production of semi-conductors and microchips requires an ultra-clean production environment, such as an inline vacuum deposition process.
However, even in such conditions tiny particles of dirt can reduce production quality and the operational life of the end product.
These particles are generated by metal-to-metal, or metal to grease contact inherent in conventional methods of inline transport such as chain drives or conveyor belts with linear motors.
A typical layout will include 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.
Particles are not the only problem with this type of production as, in addition, it is neither scalable nor flexible which means that increases in demand cannot be quickly accommodated and the line will need extensive service and maintenance.
A move away from any touching of components during the manufacturing process would help to improve product quality as well as the cost of production.
One potential solution, that is currently being tested, is magnetic levitation. The Bosch Rexroth LeviMotion concept, for example, combines inverted linear motion technology with a 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 tested is the use of an inverted linear motor with magnets underneath the drive carrier, with the coil units mounted outside the process chamber. Such a system adds 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 carrier control that 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. This method of transport is also frictionless with no bearing related disturbances such as sticking or slipping or fluctuating stiffness.
In addition, only passive or sealed components are located in the process chamber which leads to lower maintenance costs and a lower cost of ownership.
The combination of linear motion and magnetic levitation can also offer operational benefits with high-speed high positioning accuracy with constant speeds and low ripple. Testing has already shown excellent planarity over long transportation distances with automatic alignment procedure in the bearings’ air gap.
In terms of production throughput, the carriers are able to achieve speeds of up to 5 cm/sec and can carry loads from 1kg to 1000kg. The carriers are also capable of repeat positioning of 10-20 µm along with high positioning accuracy and minimal velocity ripple.
This combination of drive system and magnetic levitation is still being tested, but it has already gained significant interest from electronics manufacturers because of its potential to solve the issue of particle generation which has dogged semi-conductors and microchip manufacture for many years.
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