EMI-caused EOS Sources in Automated Equipment


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How does EMI Turn Into EOS?

EMI-caused electrical overstress occurs when a device, such as an IC, is in a contact with grounded parts of a different potential which causes current through the device. While it is fairly easy to establish equipotential environment inside the tool at DC and at 50/60Hz, at high frequencies it becomes very difficult. One of the reasons for that is that at high frequencies conductors behave differently. Even a straight wire becomes an inductor with noticeable impedance and a phase shift.2,3 Metal parts that are not in physical contact can still conduct current because of parasitic capacitance between them. This creates unanticipated current paths and further phase shift. Actuators of robotic arms with associated wiring have substantial inductance and high capacitance to the tool's frame. The result is that high-frequency voltage on the robotic arms is not the same as on the tool's frame. This leads to a possibility of current through the devices.

VKraz-OnFILTER-Figure 6.jpgFigure 6: High-frequency current path through the device in pick-and-place.

While for DC and 50/60Hz metal-to-metal contact is required for current passage, at high frequencies parasitic capacitance offers low impedance to current even without physical contact. Consider as an example an IC suspended on a nozzle of a pick-and-place machine (or, similarly, an IC handler) as shown in Figure 6.

The silicon die of the IC and its leadframe form one plate of a capacitor and the nozzle forms another. Close proximity of these plates provides big enough capacitance and resulting low impedance between the IC and ground of the robotic arm.

The copper of PC Fab which may not be galvanically connected to the tool's grounded frame nevertheless forms even bigger capacitance with the tool's frame and other metal parts of the tool. 

Once the IC is placed on the copped pads of the PC Fab, the metal-to-metal final contact closes the circuit and the current now can flow. Strong current with high repetition rate is a viable cause of EOS.

VKraz-OnFILTER-Figure 7.jpgFigure 7: Current through the device in an IC handler.

Figure 7 depicts actual measurements made in an IC handler. The blue trace shows drive pulses of a servo motor; red trace - current through the device. As seen, current pulses are synchronized with the drive pulses. Other (non-synchronized) pulses are associated with other servo motors in the tool (there were total of 6).

It is important to note that these current pulses are continuous -they "hit" the device up to 20,000 times a second every second. This weakens the device structure and can cause latent damage which is more pronounced due to EOS than to ESD exposure4.

Not surprisingly, such current pulses weaken not only the device structure but the tool itself - just like dripping water often causes more damage than an occasional pour. The most wide-spread phenomenon is damage to ball-bearings of servo and variable frequency motors5. Figure 8 shows typical damage to the bearings from ground transient pulses caused by EMI. If such damage occurs to hardened steel, what would it do to small silicon structures of the devices?

VKraz-OnFILTER-Figure 8.jpgFigure 8: Damage to bearings due to high-frequency currents6.

Places of Likely EMI-Caused EOS Exposure

Every place in automated equipment where there is a metal-to-metal contact is a possible source of EMI-caused EOS. Among the tools liable to produce such exposure are:

PCB Assembly

  • Pick-and-place tools
  • Lead formers
  • Lead trimmers
  • Wave soldering
  • Testers

Device/IC Manufacturing

  • IC handlers
  • Wire bonders
  • Singulators
  • Lead formers
  • Testers

Acceptable EOS Limits

How would a responsible specialist assess current EMI levels in the tool and how would he specify the "safe" limits? The only document today that specifies maximum acceptable EOS levels in PCB assembly is IPC-A-610-E7. In its section 3.1.1 it states that " equipment must never generate spikes greater than 0.3 volt." For semiconductor device manufacturing ITRS (International Technology Roadmap for Semiconductors - www.ITRS.net) in its Factory Integration Tables8 recommends essentially the same levels today and even lower levels for the near future.

Once your factory has conducted an EMI audit of its tools it would become clear where EMI needs to be mitigated and to what levels.

Other Effects of EMI in Manufacturing

Besides causing electrical overstress noise on power lines (AC and DC) and ground infiltrates data lines. The most frequent result is errors in test. It is not uncommon to test the same board or the device several times until it passes. While such problems are not fatal, they reduce productivity and may lead to a bad board erroneously passing the test and be shipped to a customer.

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