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Emerald Group Publishing Limited
Copyright © 2005, Emerald Group Publishing Limited
Perfect printing with no vibe drive
Perfect printing with no vibe drive
Keywords: Printed circuits, Assembly
A small company in Dorset (UK) is setting new levels of productivity and price performance ratios with the range of screen-printing machines; it is developing for surface mount printed circuit board (PCB) production.
A key factor to the quality of the finished PCBs is the vibration-free, vertical movement of the bare board during the printing process allowing solder paste to be applied with precision. The design team at Reprint Ltd has worked hand in glove with specialist motion engineers from Hoerbiger-Origa to develop the drive system that achieves this.
Reprint was originally set up to support businesses using electronic production machines built by a company that had been taken over and had selected assets moved to the USA. “Initially we were maintaining supply of the consumables and spares,” recalls Financial Director Martin Linford. “Phase Two of our plan was later to move into printer refurbishment, as many machines in the field would by then be in need of a mid-life upgrade. We would then progress (as we have done) into designing and building our own machines.”
Now Reprint is launching its flagship product, a fully automatic in-line screen printer (the “R29 Spectrum”) aimed at medium to high throughput users, packed with highly automated and innovative features. Stylish and compact in design and intuitive to use, it combines a precise servo motion control system with a robust mechanical frame. The machine can maintain high productivity yields while delivering outstanding printing results (Plate 4).
The servo is critical to both the speed and the quality and is based on a belt- driven electric linear actuator from Hoerbiger-Origa, customised with special bearings designed to eliminate vibration and backlash, coupled with a Maxon motor with encoder feedback.
Plate 4 The R29 Spectrum is aimed at medium to high throughput users
In use bare boards are conveyor-fed onto the printing table and positioned to an accuracy of ±25µm by a vision system developed in-house by reprint. The actuator then lifts the whole table up to the printing position. The table has a deadweight of 35kg and to this the actuator can add a squeegee pressure of between 0 and 20 kg, holding the position for a precision specified dwell time. A high-resolution encoder means the pressure can be held exactly to allow optimum printing.
“This lift arrangement was developed especially for the new machine,” says Martin. “Originally we did not think it would be possible to have such an accurate drive in such a compact space. We had expected to have to use a lever mechanism that would only just be accurate enough, expensive and cumbersome. It would also have been difficult and time consuming to build, which would cause training problems for new staff as we expand our engineering department.”
However the Hoerbiger-Origa engineering team analysed the motion profile and the vibration, stiction and backlash patterns of the application. With a bit of experimentation at its base in Tewkesbury Hoerbiger-Origa developed a way to eliminate the unwanted movements, based on modified bearings and a low stiction insert in its otherwise standard 32 mm bore, 150 mm stroke rodded lead screw actuator.
This actuator was chosen because it cannot be back driven by the weight of the load and it also gives a very smooth and quiet movement. It was also decided to build a small belt and pulley drive so that motor and encoder could be mounted piggyback onto the actuator to create a compact self-contained unit.
The system is based on dual cameras and uses a combination of stored and live images to align the PCB with the stencil. It can recognise standard board features or dedicated fiducials to provide perfect alignment every time. The system is fast and flexible and features multi-point alignment to compensate for stencil stretch and board manufacturing variations. Alignment calibration and vision system training are fully automatic to minimise operator intervention.
For further information, please contact: Ray Barnes, Hoerbiger- Origa Ltd, Tewkesbury Industrial Estate, Tewkesbury, Glos GL20 8ND. Tel: 01684 850000; E-mail firstname.lastname@example.org; Web site: www.hoerbiger-origa.co.uk