EMI-caused EOS Sources in Automated Equipment


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"Electrical overstress (EOS) is the number one cause of damage to IC components" states Intel in its Manufacturing Enabling Guide1. While most manufacturers are concerned with electrostatic discharge (ESD), not enough attention is paid to the much more damaging EOS phenomenon.

Significant source of EOS in manufacturing environment is high-frequency noise (often called electromagnetic interference, or EMO). High-frequency noise is present in most manufacturing tools such as pick-and-place machines, lead trimmers, wire bonders and many others. This noise is caused by operation of various electric and electronic equipment, both inside the tool and by the tools nearby or even far away. Understanding the sources of EMI and how to mitigate EMI-caused EOS exposure is important for safe handling of sensitive components.

Sources of EMI in Tools

VKraz-OnFILTER-Figure 1.jpg

Every electrical and electronic equipment generates some sort of artifacts on power line and ground during its operation. Different types of equipment generate different types of noise.

Commutation of Power

Figure 1: Power line transient from turning on heat gun.

Commutation of power, such as turning equipment on and off causes strong transient signals. Figure 1 shows typical voltage spike caused by turning on a regular heat gun. The more powerful the load, the stronger the spike. This type of transient noise does not easily follow an observable pattern and often is difficult to diagnose, in part because the spike can originate quite far away from the source and propagate via power lines and ground.

It doesn't take an entire tool to be turned on or off in order to produce a significant transient signal - all it takes is to have heating element, solenoid or other load within the tool to be turned on or off. The resulting transient signal propagates throughout the tool via common power and ground.

Dimmers/Gradual Heat Control

VKraz-OnFILTER-Figure 2.jpgFigure 2: Noise from the dimmer.

Common power line dimmers that control light brightness, such as in microscope light, or temperature of certain heating elements produce periodic spikes synchronized with the power line frequency. Typical waveform of noise from dimmer is shown in Figure 2.

Switched Mode Power Supplies (SMPS)

While many power supplied provide small manageable level of noise, there are enough SMPS in use that skip on noise suppression and serve as a major source of electromagnetic interference. Switch mode power supplies generate DC voltage from AC mains using high-frequency pulses (typically between 40 and 200kHz). These pulses have sharp edges which are the main culprits in generating undesirable noise. This noise may have rather complex waveform as illustrated in Figure 3. It is important to know that SMPS generate noise both on DC output and on AC mains.

Uninterruptable Power Supplies (UPS)

VKraz-OnFILTER-Figure 3.jpgFigure 3: Noise on AC mains generated by several switched mode power supplies.

UPS provide AC power to your equipment when main AC power either fails completely or when its key parameters fall below acceptable levels. In such cases UPS reconstructs AC mains power from its internal battery. The output power in case of mains failure, however, is far removed from the expected sinewave. It is most likely a square wave with sharp edges that cause significant high frequency noise at the output.

Figure 4 shows AC output of a typical UPS (blue trace) and resulting high-frequency pulses on power line (red trace). Power conditioning in absolute majority of UPS does not include noise filtering and, in those that do, it is mostly sub-standard.

VKraz-OnFILTER-Figure 4.jpg

Figure 4: Noise from UPS.

Servo and Variable Frequency Motors

Almost every equipment with moving parts utilizes either servo or variable frequency motors. They are a workhorse of today's automation. Unfortunately, they are also the strongest source of EMI in the tools. These motors are driven by the pulses ranging typically between 8kHz and 20kHz with sharp edges only a few nanoseconds long. This pollutes the entire tool, especially its ground, with strong sharp transients with repetition rate of that of drive pulses.

Figure 5 shows how rising edge of servo motor drive pulse (red trace) is synchronized with ground current inside the tool (blue trace). This current was measured using Tektronix' CT1 current probe and with its 5mV/mA ratio the ground current peak-to-peak measure is almost 2A with width of pulses of under 100nS.

VKraz-OnFILTER-Figure 5.jpgFigure 5: Effect of servo motor pulse on ground noise.

A typical production tool has several servo/variable frequency motors, sometimes more than 10. Combined electrical noise from all motors can significantly pollute the ground of the entire tool.

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