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Figure 9: Reduction of noise on AC power line using stand-alone CleanSweep AC Filter (note different scales for each trace).
While it is appealing to have inherently low EMI levels generating by electrical equipment, in practice factory specialists have to deal with noise-generating equipment all the time. Sometimes improvement in wire routing can help a bit in some circumstances. However, the most practical way to effectively suppress EMI in the tools is implementation of properly-designed EMI filters1. The difference between commonly-found generic EMI filters and specialized filters for factory level EMI suppression is outlined here9.
Specialized AC power line filters help to stop noise on power lines from reaching the tool as shown in Figure 9. This filter which s "plug-and-play" and requires no special installation effectively blocks noise on power line and ground in both directions.
Figure 10: Ground Line Filter GLE30-01 prevents propagation of noise via facility ground.
Some factories use separate facility grounding throughout the facility. This separate ground serves as a pipeline for noise delivering it to every tool in the facility. Figure 10 shows installation of ground line filter with ground bars. Installation with ground wire under the floor is very similar. Connecting ground filters every 2...3 meters in dense tool environment effectively stops propagation of noise while maintaining required ground quality
Servo and variable frequency motors require specialized servo filters such as one shown in Figure 11. The specific filter shown (OnFILTER' model SF20031) is installed between servo controller (also called "servo pack") and the motor. By processing drive pulses while preserving their driving ability this servo filter reduces ground noise by more than 20dB in this particular example.
Figure 11: Ground current (red trace) with and without servo filter.
The resulting current through the device as was shown in Figure 6 is now reduced even more - 45 times as shown in Figure 12. As a positive "side effect" servo filter also improves reliability of motor's ball bearings by reducing erosion-causing ground current.
Conclusion
Electrical overstress is a serious and increasing threat to sensitive devices. Reduction of EOS exposure in assembly is an important way to improving yield and to reduce latent damage to the devices. High-frequency noise (EMI) in automated equipment is a significant source of electrical overstress.
Proper analysis of EMI environment in the facility and inside the tools, setting EOS limits for your process and proactive mitigation of EOS exposure improves yield and reduces EOS-caused failures.
Figure 12: Current through the device with and without servo filter.
References
1. Intel Manufacturing Enabling Guide, Section 3.2, May 2010
2. How Good is Your Ground? V. Kraz and P. Gagnon, Evaluation Engineering, 2001
3. Introduction to Electromagnetic and Microwave Engineering, By Paul R. Karmel, Gabriel D. Colef, Raymond L. Camisa, 1998 John Wiley and Sons).
4. Electrical Overstress (EOS) : Devices, Circuits and Systems, First Edition. Steven H. Voldman. 2014 John Wiley & Sons,
5. William R. Finley and Robert R. Burke, Siemens Energy and Automation Inc., Motion System Design May 2000
6. Bearing Currents, VFD.com 3/20/2014
7. IPC-A-610-E-2010, Acceptability of Electronic Assemblies, IPC
8. ITRS Factory Integration, 2013 http://www.itrs.net/Links/2013ITRS/2013Tables/Factory_2013Tables.xlsx
9. EOS Damage by Electrical Fast Transients on AC Power, A. Wallash, V. Kraz, Proceeds of ESD Symposium, 2010
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