Absolute Optical Encoders Boost Micro Assembly Operations

Sept. 5, 2013
Amicra Microtechnologies replaced its incremental measuring systems with absolute optical encoders to better manage the complex machine motions in its NovaPlus surface mounting and stack-die assembly machines.

Amicra Microtechnologies of Regensburg, Germany, develops and manufactures precision assembly systems that bond and mount active/passive semiconductors, lenses, MEMS and processors for microelectronics and micro-optics applications used in the automotive, telecommunications and information technology industries. The company manufactures a series of machines known as NovaPlus that is designed for surface mounting on wafers and stack-die assembly.

Surface mounting on wafers and stack-die technology are used to construct three-dimensional memory and computer structures. Semi-conductors are mounted horizontally and connected (for system on chip [SoC] manufacturing), as well as mounted vertically in several planes (known as Through-Silicon Via or TSV). Amicra’s micro assembly machines use linear axes and special grippers from feed stations to pick up components and position them on boards or wafers where they are bonded or soldered conventionally or via laser beam. The machine simultaneously positions a component while a second handling unit picks up the next component from a buffer store. Meanwhile, work tables and other linear axes with lasers and UV lamps move into position to solder and bond. At the same time, additional axes position the machine’s integrated process monitoring cameras.

“These machines have a lot going on at the same time, so process reliability of these installations is particularly important,” said Horst Lapsien, managing director of Amicra. “More importantly, crashes between the alternate positioning grippers and the linear axes must be avoided. We achieve this through very precise programming of the motions, so the linear axes measuring system’s ability to detect current position of the slides reliably and accurately is paramount.”

Incremental position sensors used in the machines report an incremental change in position and, when powered up, do not report position until they have a reference point. Therefore, starting the NovaPlus’ production and mounting cycle upon commissioning or after a stoppage took an unnecessarily long time. The readheads of all the linear axes had to first travel to a reference position in order to detect the actual position of the axes, and moving to the reference positions from an undefined slide position also represented a significant source of error. If the operator had not first checked the crash paths and selected the reference travel cycle accordingly, the machines could suffer damage as the result of gripper or gantry collisions.

To address the issues Amicra encountered due to the performance characteristics of the incremental position sensors used in the NovaPlus, the company looked to upgrade its machines with different encoders to manage the machine’s motions. The company selected Renishaw’s Resolute absolute optical encoders because they allow the NovaPlus axes to deliver absolute measurement without reference runs. Equally important was the fact that the Resolute encoders’ position detection method eliminated axes collisions in the NovaPlus machines. An added benefit achieved by Amicra, since switching to the Resolute encoders, is that the NovaPlus now achieves an accuracy of ±0.5µm and a process capability Cpk of 1.66.

Resolute encoders can detect the absolute position immediately at switch-on, without a reference run, thereby enabling the NovaPlus machines to start automated cycles much quicker upon commissioning or after a stoppage. Resolute’s single-track optical scale combines the absolute position and the embedded-phase information into a single code to eliminate de-phasing problems associated with dual-track scales. The single-track scale also reportedly gives the encoder a wide set-up tolerance and allows for easier installation and reliable operation, even if the axes settle or move over time. An integral set-up LED on the readhead provides a visual check that the scale is being read correctly.

The position detection method used by the Resolute encoders is said to operate like a high-speed digital camera. High-resolution images of the scale are captured and analyzed by a digital signal processor that applies cross checking and error rejection to determine position to 1 nm.

Lapsien notes another advantage of switching encoder systems has been the use of BiSS-C open serial protocol, which allows the Resolute encoder to be more easily integrated into a variety of positioning axes. “Many industry-standard servo drives and controls use this interface,” he says. “This means that the open protocol allows us to avoid getting tied to a single supplier of motion system components.”