Reliable positioning improves process reliability and reduces downtime
March 1, 2013 | By Special to Manufacturing AUTOMATION
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.
German manufacturer Amicra Mikrotechnologie GmbH learned that first-hand when it achieved major improvements with Renishaw’s future-oriented absolute measuring system, Resolute, which allow Micro Assembly Cells to be commissioned more quickly and, in particular, independently of the operator’s vigilance. This is particularly important for microelectronics and micro-optics applications in the automotive sector, telecommunications and IT industries.
Fast feed and very precise positioning
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.
Amicra develops and manufactures Micro Assembly Cells designed for this purpose. These machines are known in particular for their high levels of accuracy and reliability.
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.
Collisions absolutely ruled out
As Horst Lapsien, managing director of the German company, explained, 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,” Lapsien says. “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. Only in this way was the control system able to detect the actual position of the axes, but that took an unnecessarily long time,” Lapsien adds.
Also, starting from reference positions from an undefined position of the slides represented a significant source of error. If the operator has not analyzed 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. This leads to unnecessarily long unproductive times, unnecessary costs and uneconomical downtimes of the entire machine in the event of a crash.
Absolute measurements without reference run
Thanks to the absolute encoders, Amicra’s mechatronic engineers have been able to improve their machines considerably. The main advantage of these measuring systems according to Lapsient 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.
The RTLA absolute measuring scale can be bonded directly to the substrate or inserted into a special guide, both made from stainless steel. As a result, it easily installs compactly inside the Amicra machines, achieving ±5 µm/m accuracy. The tough stainless steel tape scale is highly resistant to damage, but if necessary, the guide allows easy scale replacement at any time.
The 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 analyzed by a powerful Digital Signal Processor (DSP) that applies cross-checking and error-rejection, to determine position to 1 nm. Combined with a built-in position-checking algorithm, the encoder has high immunity to contamination. Lapsien says taht 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 the absolute encoders to achieve high levels of accuracy with just ±40 nm cyclic error and jitter lower than 10 nm 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.