Magnetic levitation for PCB production
A combination of linear drives and magnetic levitation can be used to alleviate particulate contamination on printed circuit boards’ (PCB) production lines, explained Paul Streatfield of Bosch Rexroth, for Control Engineering Europe.
When manufacturing semi-conductors and microchips used in printed circuit boards (PCBs), particle contamination can dramatically increase production costs and reduce the operational life of an end-product.
The production of semi-conductors and microchips requires an ultra-clean production environment, such as an inline vacuum deposition process. However, contamination can occur even in such conditions.
Tiny 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 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.
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. 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 method currently being tested is the use of an inverted linear motor with magnets under the drive carrier and the coil units mounted outside the process chamber. Such a system adds large air gaps between magnets and the 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 90-deg. 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. First, a series of coils can be constructed and up to 32 carriers can be used rather than just one carrier with the standard linear motor. Second, with the coils mounted underneath the carrier, any ingress particles fall away from the carrier rather than onto the carrier, which improves product quality. The inverted linear motor has no active parts. With bearings located in fixed positions there is much less potential for particle ingress.
This method has no friction or wear, and the movement of the carrier is contactless and clean, without particle generation or lubrication. This method of transport also eliminates bearing-related disturbances, such as sticking, slipping, or fluctuating stiffness. Additionally, 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 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 an automatic alignment procedure in the bearings' air gap. In terms of production throughput, the carriers can achieve speeds of up to 5 cm/sec and can carry loads from 1 to 1,000 kg (about 2,205 lbs). The carriers are also capable of repeat positioning of 10 to 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-conductor and microchip manufacture for many years.
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