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Absolute encoders boost process reliability

Absolute encoders boost process reliability Reliable positioning without a reference run is improving process reliability, reducing unproductive time and preventing expensive crashes on precision micro-electronics assembly lines.

Process reliability is one of the principal factors governing the economic operation of high-value micro assembly machines that produce miniature electronic and optical components. At the same time, so is the ability to commission assembly cells quickly and independently of the operator's vigilance. This is particularly important for micro-electronics and micro-optics applications in the automotive sector, telecommunications and IT industries.

Amicra Mikrotechnologie in Regensburg, Germany, develops and manufactures assembly cells designed for this purpose. These machines are known in particular for their high levels of accuracy and reliability. Depending on the model, they will mount micro-components with an accuracy of up to ±0.5µm, achieving a process capability Cpk of 1.66.

The components to be bonded and mounted, such as active/passive semi-conductors, lenses, MEMS, and processors, are picked up with the aid of linear axes and special grippers from feed stations. The components are then positioned on boards or wafers, where they are bonded with adhesives, soldered conventionally or soldered with a laser beam. Surface mounting on wafers and stack die technology make particular demands on mounting and production technology. Stack die technology is used to construct three-dimensional memory and computer structures; Semi-conductors are not only mounted horizontally next to each other and connected (SoC), but also vertically in several planes (TSV). Further miniaturisation is achieved as a result of a higher packing density.

The NovaPlus series of assembly lines from Amicra has been designed for maximum productivity. Using a large array of horizontal and vertical linear axes, they surface mount wafers with practically no unproductive downtime. While a component is being positioned, a second handling unit is already picking up the next component from a buffer store. At the same time, the work tables and other linear axes with lasers and UV lamps move to the positions required for the soldering and bonding processes. In addition, other axes position the integrated process monitoring cameras. As Amicra managing director Horst Lapsien explains, the high process reliability of these installations is particularly instrumental in their economic operation. A crash between the alternate positioning grippers and linear axes must be avoided. "This can be achieved through the extremely precise programming of the motions. Furthermore, the measuring systems on the linear axes have to detect the current position of the slides reliably and very precisely."

This could only be achieved to a limited extent with the incremental measuring systems used previously. "In the past, starting up the production and mounting cycle after a stoppage was unsatisfactory because the readheads of all the linear axes had to travel to a reference position first," Lapsien continues. "Only in this way was the control system able to detect the actual position of the axes, but that took an unnecessarily long time."

Also, starting from reference positions from an undefined position of the slides represented a significant source of error. If the operator has not analysed the crash paths first and selected the reference travel cycle accordingly, the installations can suffer considerable damage as the result of the collision of grippers or even of the gantries. According to Lapsien, this leads to unnecessarily long unproductive times, unnecessary costs and uneconomical downtimes of the entire machine in the event of a crash.

Thanks to Renishaw's Resolute absolute encoders, Amicra's mechatronic engineers have been able to improve their machines considerably. The main advantage of these measuring systems according to Lapsien is that they detect the absolute position immediately at switch-on, without a reference run. Consequently, the micro assembly cells can start their automated cycle quicker, without relying on the operator's vigilance, after a stoppage or on commissioning for the first time. Process reliability is improved, unproductive times are reduced and expensive crashes are prevented.

Renishaw's Thomas Renner comments: "Compared with previous absolute measuring systems, Resolute has a single track scale that combines both the absolute position and the embedded phase information into one single code. This means that Resolute has very wide set-up tolerances and so it can be installed easily and continues to operate reliably over long-term use, even if the axes settle or move over time." The Resolute readhead also features a unique set-up LED for simple installation and commissioning, providing a simple visual check that the scale is being read correctly.

The RTLA absolute measuring scale can be bonded directly to the substrate or inserted into a special Fastrack guide, both made from stainless steel. As a result, it easily installs compactly inside the Amicra machines, achieving ±5µm/m accuracy. The tough stainless steel tape scale is highly resistant to damage, but if necessary, the Fastrack guide allows easy scale replacement at any time.

Resolute encoders feature a unique position detection method, analogous to a very high speed digital camera, capturing very high resolution images of the scale. These images are then analysed by a powerful DSP that applies cross-checking and error-rejection, to determine position to 1nm. Combined with a built-in position-checking algorithm, the encoder has high immunity to contamination. As confirmed by Lapsien, since their introduction into three-shift production operation, his machines have recorded zero stoppages on account of reading errors of the measuring system caused by dirty measuring tapes.

The advanced detection technique also enables Resolute absolute encoders to achieve high levels of accuracy with just ±40nm cyclic error and jitter lower than 10nm RMS. The result is excellent positional stability and a very low noise level. Therefore the micro assembly cells equipped in this way benefit from higher levels of reliability and performance.

For Amicra, the Resolute absolute encoder has yet another critical advantage; with its BiSS-C open serial protocol, it can be easily integrated into many positioning axes. As Lapsien explained, many industry-standard servo drives and controls now have this interface. Amicra can therefore avoid being tied to a single supplier of motion system components.
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