When do you need Ionisation for ESD Control?
Often, we get asked, "how do you ground an insulator?". Since an insulator is a non-conductor, by definition, it cannot be grounded. So, if they can't be grounded, how do we deal with insulators in ESD control? The answer is ionisation (or neutralisation) with an ioniser.
What is an Ioniser and how does it work?
The primary function of an ioniser is to neutralise electrostatic charges on insulators and isolated (non-grounded) conductors. To reduce the size of electrostatic discharges in the ESD protected area. Point-of-use compressed gas ionisers combat electrostatic attraction (ESA) by neutralising charges on particles that otherwise cause contamination or visual defects on products.
Ionisers create great numbers of positively and negatively charged ions. Air flow, via fans or compressed air, help the ions flow over the work area. If there is a static charge present on an item in the work area, it will be reduced and neutralised by attracting opposite polarity charges from the air.
Ionisation can neutralise static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.
Desco Europe offers a wide product range of ionisers, in the following categories:
• Bench top
• Overhead
• Compressed gas point-of use
Enhancing your ESD Control Programme with Ionisation
Companies will need to improve their once acceptable ESD Control Programmes due to the ever-increasing sensitivity of electronic components. One such way to make enhancements is to include ionisers.
Per the InCompliance Magazine article Now is the Time for ESD Control Programs to be Improved, “Factory ESD control is expected to play an ever-increasing critical role as the industry is flooded with even more HBM (Human Body Model) and CDM (Charged Device Model) sensitive designs.”
Specific to ionisers, it reads "If the ESD control program has not used ionization, that possibility should be considered. If ESDS items become charged, ionization will help neutralize the charge. The primary function of ionizers with regard to ESDS items are:
- to remove/neutralize charges from process necessary insulators, which can charge ESDS items and thus create the potential for a damaging CDM event.
Remember that the PCB substrate is a process necessary insulator and can become charged during automated handling processes.
- to remove/neutralize charges from a charged, isolated/floating conductor which, when grounded, can result in a potentially damaging CDM event.
Remember that during automated handling processes, the ESDS devices on the PCB are isolated or floating conductors."
Per IEC TR 61340-5-2 User guide clause 5.3.3.4.6.1 "The primary method of static charge control is direct connection to ground for conductors, static dissipative materials and personnel. However, a complete ESD control program should also deal with isolated conductors that cannot be grounded as well as insulating materials (e.g., most common plastics) ...Air ionization can neutralize the static charge on insulated and isolated objects by charging the molecules of the gases of the surrounding air. Whatever static charge is present on objects in the work environment, this will be neutralized by attracting opposite polarity charges from the air."
Types of Ionisation
Desco Europe designs and manufactures ionisers primarily using corona discharge. A high voltage gets applied to one or more sharp points, creating quantities of air ions. Fans or blowers get used in the ioniser to assist the movement of the ions and enhance performance. Within corona discharge ionisation there are three technologies:
1. AC Ionisers
AC Ionisers use a transformer to multiply the AC power line voltage. AC stands for Alternating Current, meaning the power cycles from positive to negative sixty times per second. Thus producing both positive and negative ions from the same points or emitters. The drawback of this approach is that many ions recombine due to the cycle frequency being too fast. Causing a longer decay time. For this reason, most AC ionisers rely on high-speed air flow.
2. Pulsed DC Ionisers
Pulsed DC ionisers utilise separate power supplies to generate positive and negative voltages. Usually, each power supply has its own dedicated emitters. The power supply alternates between positive and negative, but usually at a lower frequency than AC units. This reduces ion recombination and increases performance. Allowing for reduction in Airflow, making it more comfortable for the operator. All without sacrificing much performance.
3. Steady-state DC Ionisers
Steady-state DC ionisers also use separate power supplies and emitters. Yet instead, both supplies are on all the time, as the name implies.
As expected, there is some degree of recombination. However, the ion density is still greater, due to the continuous operation of both supplies. The offset or balance voltage at the output will usually be more consistent than pulse units.
"Steady-state DC systems consist of separate negative and positive ion emitters connected by a pair of high voltage cables to their respective high voltage power supplies. The spacing between emitters will vary depending on the design, and DC power is constantly applied to the emitter points. Emitters of opposite polarities are spaced farther apart in the DC systems than in the AC systems. Ion recombination occurs at a lower rate and steady state DC systems will operate at a lower airflow than AC systems. In some situations, it may not be desirable to place sensitive components close to the emitter points. The electric field of the ionizer is used to move ions in the absence of high airflow." [IEC TR 61340-5-2 User guide clause 5.3.3.4.6.7.4 Steady-state DC ionizers]
Most of Desco Europe ionisers use steady state DC ionisation. Experience has found that steady state DC ionisers work well with a modest airflow. Making them suitable for sensitive items, soldering operations, and where operator comfort is a concern. Three of our compressed gas ionisers utilise AC ionisation technology. To compare the range, visit our Ioniser Selection Chart.
Desco Europe’s Bench Top Zero Volt Ioniser roduces almost an equal amount of positive and negatively charged ions. Providing a very low offset voltage balance (± 3 volts). An auto-balancing sense feedback manages the low offset voltage. This is consistent with IEC 61340-5-2, clause 5.3.3.4.6.14.4, "Various feedback and auto-balance methods are available to ensure ionizer performance. ...Providing a sensor for each ionizing device can improve feedback control. Also, an effective monitoring and control method can reduce the amount of maintenance required for proper operation of the ionizer device or system."
Ioniser Maintenance
Per IEC TR 61340-5-2, clause 5.3.3.4.6.16.1.1 "All ionization devices will require periodic maintenance for proper operation. Maintenance intervals for ionizers vary widely depending on the type of ionization equipment and use environment." Companies should ensure their ionisers have serial numbers. The ionisers should be included in their maintenance and calibration schedules. Particularly critical for ensuring the offset voltage or balance is within acceptable limits. If not, the out-of-balance ioniser will put a charge on the insulator or isolated conductor, instead of neutralising the charge. Depending on the value and function of their products, companies must determine the maintenance and calibration frequency.
Auto-balancing closed-loop feedback technology can save money and extend the maintenance interval of the ioniser. How? By the sensor detecting if the offset voltage (balance) has shifted. If so, the circuits will attempt to compensate. The offset voltage shifting in corona ionisers is usually caused by the build-up of particles on the emitter points. Often referred to as "fuzz balls".
To maintain a fan ioniser:
- Case Wipe the case with a soft cloth and deionised water. Squeeze the wiping cloth or sponge to remove any excess liquid. If a stronger cleaning solution is required, dab a soft cloth with a mixture of isopropyl alcohol and deionised water (70% IPA and 30% DI water).
- Emitter Points Clean the emitter points by using a specific emitter point cleaner or a swab dampened with Isopropyl alcohol. Below are general instructions on how to clean emitter points. However, each unit is different so always refer to the ioniser’s manual.
- Turn the unit OFF and unplug the power cord.
- Open the top screen by loosening the screw and swinging the grill to one side.
- Clean the emitter points using an emitter point cleaner or a swab dampened with Isopropyl alcohol.
- Re-attach the top screen.
- Plug in the power cord and turn the unit ON.
Once completed, verify the performance of the ioniser using the compliance verification method.
Ioniser Compliance Verification
Using the Ionisation Test Kit or Charged Plate Analyser, periodically check the offset voltage (balance) and decay times (+ and -) on every ioniser. The recommended procedure is to do this both before and after maintenance. Record the measurements as with all compliance verification testing.
The test method for product qualification and compliance verification of Ionisation is technical document IEC 61340-4-7. Perform product qualification at 12 ± 3 % RH and 23 °C ± 2 °C. The decay time should be less than 20 seconds and the offset voltage should be less than ± 35 V. Use the basic test procedure for compliance verification in the factory. The decay time needs to be less than 20 seconds or user defined. IEC TS 61340-5-4 provides all compliance verification test methods required in IEC 61340-5-1.
To conclude “When do you need Ionisation?”
IEC TR 61340-5-2 explains it well: “Air ionization is not a replacement for grounding methods. It is one component of a complete ESD control program. Ionizers are used when it is not possible to properly ground everything and as backup to other ESD control methods. In clean rooms, air ionization may be one of the few methods of ESD control available.” [Clause 5.3.3.4.6.1]
Improve your ESD control programme using Desco Europe Ionisers to neutralise charges on insulators and isolated ungrounded conductors.