Continuous Monitors and ESD Control
Most of you have probably heard of continuous monitors before but do you know:
- what exactly they do or
- why you should be using a continuous monitoring system?
If your response to one (or both) of the above questions is ‘no’, you’re lucky because we’ll be answering them here today. If you already know the answers, why not read on anyway? You never know: you might be learning something new…
What are continuous monitors?
When talking about wrist straps a couple of weeks ago, we mentioned the need for periodic testing. Every day before each use, wrist straps need to be checked (while worn) to ensure they ground the operator correctly.
So imagine this scenario: you come to work in the morning, you test your wrist strap, it passes and you get to working on your ESD sensitive devices. 3 hours later, when you come back from your tea break, you test your wrist strap again and it fails. What to do? You don’t know if the wrist strap only just failed or if it failed right after your first test in the morning. How do you know if the devices you worked on all morning have been damaged? You don’t – after all latent defects are not visible and failures may only occur at a later time. That’s where continuous monitors come into play.
Continuous monitors provide operators with instant feedback on the status and functionality of their wrist strap. They detect split-second failures when the wrist strap is still in the “intermittent” stage. This is prior to a permanent “open” which could result in damage to ESD sensitive components.
Continuous monitors come in different styles and sizes but are intended to be kept on your workstation. Some units just ‘sit’ on your bench; others are attached to your working surface matting; some can even be attached underneath the workbench so they don’t take away valuable workspace. Operators connect their wrist strap to the unit to allow for real-time continuous monitoring. If the wrist strap fails, the unit will alarm. Many continuous monitors also feature a parking stud providing a means for the operator to disconnect when leaving their workstation.
In our scenario above, had our operator used continuous monitoring while working on those ESD sensitive devices, they would have been alerted as soon as their wrist strap failed. The faulty wrist strap could have been replaced with a brand spanking new model from stock and everyone would have been happy – no ESD sensitive devices damaged and no unhappy customers.
Using continuous monitor 222603
Some continuous monitors even provide the ability to monitor working surface ground connections. “Discontinuity or over limit resistance changes cause the monitor to alarm. Worksurface monitors test the electrical connection between the monitor, the worksurface, and the ground point. The monitor however, will not detect insulative contamination on the worksurface.“1
These days you can even find a number of ‘smart’ monitors on the market. They are connected to a computer and software records and displays failures to notify supervisors or engineers when a device needs attention (see the EMIT software as an example).
When and why you should be using continuous monitors
If your company manufactures products containing ESD sensitive items, you need to ask yourself “how important is the reliability of our products”? Sooner or later a wrist strap is going to fail. If your products are of such high value that you need to be 100% sure your operators are grounded at all times, then you should consider a continuous monitoring system.
The ESD Association produced the ESD TR 12-01 technical report which is entitled “Survey of Constant (Continuous) Monitors for Wrist Straps“. It contains some useful information: “Since people are one of the greatest sources of static electricity and ESD, proper grounding is paramount. One of the most common ways to ground people is with a wrist strap. Ensuring that wrist straps are functional and are connected to people and ground is a continuous task.” “While effective at the time of testing, wrist strap checker use is periodic. The failure of a wrist strap between checks may expose products to damage from electrostatic charge. If the wrist strap system is checked at the beginning of a shift and subsequently fails, then an entire shift’s work could be suspect.” “Wrist strap checkers are usually placed in a central location for all to use. Wrist straps are stressed and flexed to their limits at a workstation. While a wrist strap is being checked, it is not stressed, as it would be under working conditions. Opens in the wire at the coiled cord’s strain relief are sometimes only detected under stress.“1
Types of continuous monitors
The impedance (or single-wire) constant monitor “… uses a detection circuit designed to reduce false alarms and eliminate adjustments. [It] use[s] the phase difference between current and voltage to detect changes in impedance of the cord, band and person. A very low AC voltage is used for constant sensing. Any standard [single wire] wristband and coiled cord can be used.”1
Examples of single-wire monitors
Single-wire monitoring allows the use of any standard, single-wire wrist strap and coil cord. The monitor / wrist strap system life-cycle costs are significantly lower than dual-wire systems. While they would not be suitable for the most critical applications, single-wire continuous monitors are an economical way to monitor both the operator’s wrist strap and/or workstation surface.
Resistance (or dual-wire) constant monitors are “… used with a two wire (dual) wrist strap. When a person is wearing a wrist strap, the monitor observes the resistance of the loop, consisting of a wire, a person, a wristband, and a second wire. If any part of the loop should open (become disconnected or have out of limit resistance), the circuit will go into the alarm state.” “While the continuity of the loop is monitored, the connection of the wrist strap to ground is not monitored.” “There are two types of signals used by resistance based constant monitors; steady state DC and pulsed DC. Pulsed DC signals were developed because of concerns about skin irritation. However, pulse DC units introduce periods of off time (seconds) when the system is not being monitored.“1
Examples of dual-wire monitors
Dual-polarity technology provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards. Dual-wire continuous monitors provide redundancy; even if one dual-wire wrist strap conductor is severed, the operator still has a reliable path-to-ground with the other conductor.
Need help choosing the correct continuous monitor for your application? Make sure you check our continuous monitor selection chart.
1 ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps