Machine Design #21: Static Control in Clean Rooms – ESD, ESA and How to Design for Automatic Machines

When designing automatic machines for a clean room, many people pay a lot of attention to dust, stainless materials, covers, exhaust, grease and cable carriers.

But there is a problem harder to see with the eye: static electricity.

Static electricity does not cause failures in a way as easily visible as a mechanical collision or a broken component. It can damage an IC, attract particles onto the product, cause signal noise, or make a sensor or controller run unstably. Worse still, some failures do not appear right at inspection but later become latent failures in the field.

In a clean room, static control cannot just be done by "grounding a few spots" or "adding an ionizer and calling it done".

Correct design needs to be seen as a whole system:

Where does the static electricity originate?
Do you need to protect against ESD or ESA, or both?
Which materials are conductive, which are insulative, which are dissipative?
Is the machine frame properly bonded?
Does anodized aluminum really conduct electricity?
Where is the ionizer placed, what type, and is it maintained?
Do cables, cable carriers, actuators, belts, plastic covers accumulate charge?
Can you measure the effectiveness of static control, or is it done by feel?
Does it relate to the IEC 61340, IEC 61000-4-2, SEMI E78, SEMI E129 standards?

This article records the points to note when designing static control for automatic machines in a clean room, from a machine designer's practical perspective.

1. Static electricity in a clean room is dangerous in two directions: ESD and ESA

In a clean room, static electricity usually causes problems in two main directions:

ESD – Electrostatic Discharge
ESA – Electrostatic Attraction

These two phenomena are different, the failures they cause are also different, so the countermeasures are not entirely the same either.

2. What is ESD?

ESD is the discharge phenomenon that occurs when a charged object contacts or comes near an object at a different potential.

Simply put, charge is accumulating on a person, machine part, tool, tray, jig or product. When discharge conditions arise, a large current in a very short time will flow through some path.

For electronic components, especially ICs, sensors, semiconductor devices, this discharge current can damage the internal circuit.

There are two common failure types:

Immediate death failure

The device is clearly destroyed. The test does not come up, the measurement fails, the product is NG right at the process.

Latent failure

This is the truly dangerous type. The component still passes the test, still runs, but internally it has already been partly damaged. After some time in use, it then fails.

For electronic products, semiconductors or medical devices, a latent failure from ESD can directly affect product reliability.

When designing a machine, you should understand a few ESD models often mentioned:

Model	Meaning	Example in an automatic machine
HBM – Human Body Model	A person is charged then discharges into the device	A worker touches a board, tray, jig
MM – Machine Model	A machine or tool is charged then discharges into the product	Gripper assembly, end effector, machine frame, fixture
CDM – Charged Device Model	The device itself is charged then discharges on contact	IC, board, wafer, small component after transport or separation

For an automatic machine in a clean room, all three can happen. You should not only think of the operator. The machine, jig, robot hand, vacuum pad, conveyor, tray and product can all be a source or a receiving point of ESD.

3. What is ESA?

ESA is the phenomenon of a charged object attracting particles in the air.

In a clean room, this is very scary.

If a product, jig, film, tray, wafer, glass substrate or plastic cover is charged, it can attract fine dust around it. Particles that have stuck to the surface by electrostatic force are not easy to blow off with an ordinary air blow.

ESA can cause failures such as:

Particles sticking on a wafer
Dust sticking on a glass substrate
Pattern or masking defects in the process
Short/open defects due to a dirt particle in the wrong position
Dust sticking on film, label, lens, optical part
The product looks clean but the particle count rises

Unlike ESD, ESA does not necessarily cause discharge. It causes failure by an electric-field attraction that makes particles stick to the product.

You can say it briefly:

ESD damages the product by discharge current. ESA dirties the product by particle attraction.

In a clean room, you must protect against both.

4. Don't just say "static control", know which failure you are preventing

A common mistake is to use the single word "ESD" for every static problem.

But if the product's IC is damaged by discharge, that is an ESD problem.

If the product attracts dust, that is an ESA problem.

If the machine has sensor noise, a controller reset or an encoder signal error after a discharge, that also relates to EMC/EMI.

So when receiving a request from a customer, you should ask clearly:

What is the product type?
Is the product sensitive to ESD or particles, or both?
Is there a maximum surface-voltage requirement?
Is there a surface-resistance requirement for the jig/tray?
Is there an ion-balance requirement?
Is there a factory or SEMI/IEC standard to follow?
Has any position ever had a static-related failure?

If you just install an ionizer without knowing whether the real problem is ESD or ESA, it is very easy to do a lot but not at the right point.

5. Humidity strongly affects static electricity

Humidity is a very important background factor.

The drier the air, the more easily materials charge. Especially plastics, film, belts, trays, cable jackets, polycarbonate covers, acrylic, non-ESD-treated PEEK, PTFE, PE, PVC…

When the humidity is high enough, the material surface has a very thin moisture layer that helps charge leak away. As a result, charge is hard to accumulate too high.

In practice, when the relative humidity / RH is lower than about 40%, charging usually increases sharply.

So many electronics or semiconductor clean rooms try to control RH around:

RH 40–50%

This is a relatively good balance zone between reducing static and avoiding other problems such as:

Rusting of metal parts
Effects on some chemical processes
Effects on hygroscopic materials
Condensation if heat is poorly managed
Effects on specific products

However, humidity does not solve everything.

An automatic machine running fast, moving film, spinning belts, sliding trays, high-speed robot pick & place can still create charge faster than the natural leak rate.

Some processes even require low humidity. Then you cannot raise RH just to control static.

More precisely:

Humidity management is the foundation, not the entire solution.

You still have to combine bonding, grounding, dissipative materials, ionizers, layout and measurement.

6. Grounding and bonding: the foundation for the conductive part

For conductive parts like metal, the most basic static control is grounding / earthing.

If a metal part is grounded properly, charge generated will have an escape path to earth. That part will be hard to charge to a dangerous level.

But in an automatic machine, the issue is not just connecting one ground wire to the machine frame.

You need to distinguish:

Grounding / earthing: connecting to earth.
Bonding / equipotential bonding: connecting metal parts together to the same potential.

If the metal assemblies in the machine are not well bonded, each assembly can have a different potential. When a product or a person touches, ESD can occur.

The design mindset should be:

The entire frame, cover, bracket, motor frame, metal plate needs to be well bonded, and then brought to a suitable ground point.

7. One ground point or many ground points?

In many machines, you should gather the bonding points to an earth bar / ground bar and then connect to the factory ground system in the way specified by the company's or customer's electrical standard.

The goals are:

Reduce potential differences between assemblies
Avoid unwanted ground loops
Easy to check and maintain
Easy to measure continuity resistance
Easy to manage ground wires

The original document mentions the one-point grounding principle to avoid ground loops. This is an important idea, but when applied in practice it needs to coordinate with the electrical design, EMC requirements, customer standards and the factory system.

For machines with servos, inverters, high-frequency equipment, analog sensors, vision systems, pay special attention because grounding affects not only ESD but also noise.

You should not let the mechanical side connect ground by feel without discussing with the electrical side.

8. Bonding on painted frames, anodized aluminum and plated parts

A very common mistake is to think that just bolting into a metal frame gives electrical continuity.

In reality it is not simple.

If the frame or plate has a layer of paint, powder coating, anodizing, an oxide layer or an insulating surface treatment, two metal parts may not conduct well with each other even if bolted very tightly.

With a painted frame

You can use a star washer / serrated washer / external tooth washer / 菊座金 so the washer teeth break the thin paint layer and create metal contact.

But you must understand correctly:

A star washer is more suitable for a paint or soft coating layer. It is not a reliable solution for a hard anodize layer.

With anodized aluminum

Anodize / alumite is normally an insulating oxide layer. If you want it to conduct, you must plan ahead.

Some handling directions:

Leave a non-anodized zone at the bonding point
Machine away the anodize layer at the ground-mounting zone
Use conductive anodize if suitable
Use plating or a conductive metal insert
Design a separate ground point, with a resistance check

Without checking, an aluminum frame that looks very nice but has no ground continuity is entirely possible.

With a moving assembly

A moving assembly like a slide table, robot hand, moving cover, door, conveyor section may not be well bonded if it relies only on the guide or bearing.

When bonding is needed for a moving assembly, you can use:

Braided wire
A flexible ground strap
A dedicated cable with a ground core
A ground brush in some rotating/sliding mechanisms

The bonding wire for a moving assembly must be flexible enough, not obstruct motion, not break easily from fatigue, and not generate dust in the clean room.

9. Ionizer: for insulative materials and objects that are "floating"

Grounding is only effective for conductive materials with a charge-escape path.

But a clean-room machine has very many insulative materials:

Engineering plastics
Film
Tubes
Cable jackets
Polycarbonate covers
Acrylic windows
Plastic trays
Tape, labels
Ceramic, glass
Some non-ESD-treated PEEK/PTFE

Grounding these does not help much either, because they do not conduct.

This case needs an ionizer / static eliminator.

An ionizer creates positive and negative ions in the air. These ions travel to the charged surface and neutralize the charge.

If the object is positively charged, it attracts negative ions. If the object is negatively charged, it attracts positive ions.

As a result, the surface voltage drops.

10. When choosing an ionizer, you must look at ion balance and decay time

An ionizer is not good just because it blows ions.

The indicators to note include:

Ion balance
Decay time
Working distance
Ion coverage area
Whether it uses a fan or compressed air
Whether it is clean-room suitable
Whether it generates ozone
Whether it warns of a dirty needle
Whether it has an auto-balance function
Whether the needle is easy to clean

Ion balance

Ion balance tells you whether the amount of positive and negative ions is balanced.

If the ionizer leans too much to one side, it can charge a neutral object in the reverse direction.

This is a very dangerous fault. The ionizer is installed but the surface voltage is still unstable, and even changes sign.

Decay time

Decay time is the time needed to reduce the voltage from a high level to a low level under certain measurement conditions.

If the line runs fast, the part only passes through the ionizer zone for a very short time, and a slow ionizer will not be effective enough.

So when choosing an ionizer, you must look at the line speed and the real contact time.

11. Comparing ionizer technologies

Technology	Principle	Ion balance	Decay speed	Effective distance	Suitable application
AC	One needle or a set of needles creates alternating positive/negative ions	Fairly good	Medium	Short	Local elimination, mounted in the machine
High-frequency AC	Uses high-frequency AC to create high ion density	Very good	Fast	Medium	Fast elimination, near sensitive products
DC	Separate positive and negative needles, continuous ions	Balance needs adjustment	Fast	Medium to long	High-speed lines, often combined with air assist
Pulsed DC	Positive/negative pulses to reduce ion recombination	Good	Medium	Long	Wide areas, low-airflow environments or greater distance

This table is only for orientation. When actually choosing, you must look at the catalog for distance, air flow, decay time, ion balance and the required clean class.

12. Comparing ionizer forms

Ionizer form	Characteristic	Common application
Fan type	Has a fan, blows ions covering a relatively wide area	Workbenches, assembly cells, manual areas
Nozzle / spot type	Uses compressed air to deliver ions to a small point at high speed	Parts feeders, film, separation points, small areas in the machine
Gun type	Handheld, usually used for manual operations	Cleaning, dust blowing, elimination during handling
Bar type	A long bar with many ion-emitting needles	Film, conveyors, web handling, wide areas across the width

In automatic machines, bar type and nozzle type are common. Fan type is often used at workbenches or manual cells.

Note: if you use a compressed-air nozzle ionizer in a clean room, the compressed air itself must be clean. You cannot take dirty air and blow it into the product zone and call it static elimination.

13. Choose materials by surface resistivity

To control static correctly, you must choose materials by surface resistance, not just by feel.

A commonly used indicator is surface resistivity, with the common unit Ω/sq.

By the classification often used in IEC 61340 and ESD documents, materials can be divided into three zones:

Classification	Reference surface resistivity	Charge characteristic	Example materials	Role in an automatic machine
Conductive	< 1 × 10⁵ Ω/sq	Charge escapes very fast	Metal, plastic with much carbon	Frame, enclosure, ground path
Static dissipative	1 × 10⁵ to 1 × 10¹¹ Ω/sq	Charge escapes in a controlled way	ESD plastic, lightly conductive polymer, ESD mat	Jig, tray, work support, product-contact surface
Insulative	> 1 × 10¹¹ Ω/sq	Holds charge, hard to self-discharge	PE, PVC, ordinary plastic, glass, ceramic	Used when insulation is needed, but must have anti-charging measures

For a jig or tray in direct contact with an electronic product, in many cases static dissipative is safer than conductive.

The reason is that conductive discharges too fast, which can cause ESD if the design is wrong. Insulative holds charge too long. Dissipative lets charge escape gradually and more controlled.

Of course, the specific required value must follow the product spec and the customer standard.

14. Outgas: a clean room fears not only particles

In a clean room, materials must not only be low-particle. Another problem is outgas.

Outgas is a gas or molecule released from a material. It can stick onto a product surface, wafer, lens, sensor, film or optical surface.

Outgas sources in the machine can come from:

Cable jackets
Plastic with plasticizer
Glue, adhesive, sealant
Grease
Rubber
Tubes
Plastic covers
3D-printed material
Paint or coating
Tape, labels

In hot zones or a vacuum environment, outgas is even more of a concern.

So when choosing an ESD material for a clean room, you should not only look at surface resistivity. You also need to consider:

Low particle
Low outgas
Resistance to cleaning chemicals
No peeling
No dust from friction
Whether it suits the working temperature

A material with good static control but high outgas can still be unsuitable for a clean room.

15. PEEK and its anti-static grades

PEEK is a high engineering plastic, heat-resistant, chemical-resistant and with good mechanical properties. In a clean machine, PEEK is often used for jigs, guides, end effectors, test sockets, supports, spacers or product-contact parts.

But natural-grade PEEK is an insulative material. If used in an ESD environment without control, it can charge up.

So there are many anti-static or conductive PEEK grades made by blending in:

Carbon fiber
Carbon powder
Carbon nanotubes
Other conductive materials

Reference table:

PEEK grade	Mechanical property	Reference continuous service temperature	Reference surface resistivity	Application
PEEK natural / unblended	Tough, strong, good chemical resistance	About 260°C	> 10¹³ Ω/sq, insulative	Structural parts, insulation, but watch out for charging
PEEK glass fiber reinforced	Harder, better creep	About 260°C	> 10¹³ Ω/sq, insulative	High-load parts, but still watch out for static
PEEK carbon fiber reinforced	Very high hardness and strength	About 260°C	10² to 10⁵ Ω/sq, conductive	Parts needing stiffness, needing ground, special sliding parts
PEEK ESD / dissipative grade	Balances mechanical properties and static control	About 260°C	10⁶ to 10⁹ Ω/sq, dissipative	IC test sockets, wafer handlers, jigs contacting electronic components

The specific values depend on each maker and material code. When designing, you should not write a generic "anti-static PEEK". You should note the grade, the required surface resistivity and the inspection conditions if needed.

16. Actuators in a clean room also have a static risk

Clean-room electric actuators and air cylinders are usually designed to reduce dust generation. But they have not always fully handled ESD/ESA.

The risks can come from:

A timing belt charging as it turns
A cable carrier rubbing
An insulative plastic cover
A slider or table floating without good ground
Grease or a seal charging
An air tube or vacuum tube made of plastic
An end effector made of non-dissipative plastic

For an electric actuator, you need to check:

Is the actuator body bonded to the frame?
Is the slider electrically continuous with ground?
Is the motor frame grounded?
Is the cable shield handled correctly?
Is the cable carrier an ESD/low-particle type?
Does the cover charge up?
Is an ionizer needed near the moving position?

For an air cylinder, you need to check:

Is the cylinder body grounded?
Is the rod or rod-end assembly floating?
Does the air tube charge up?
Is the floating joint conductive?
Is bonding needed for the fixture pushed by the cylinder?

You should not assume a clean-room actuator already fully controls static by default. You must read the catalog and measure in practice if the product is sensitive.

17. Cables, shielding and EMC/EMI

ESD does not only destroy the product. It can also create electromagnetic noise that causes machine faults.

When a discharge occurs, a very fast electrical pulse can cause EMI. The consequences can be:

A sensor reporting wrong
A camera trigger error
An encoder pulse error
A PLC receiving a noise signal
A servo alarm
A communication error
A PC or controller reset
Analog measurements jumping

So ESD is directly related to EMC/EMI.

Some design points to note:

Separate weak signal cables from servo/motor power cables.
Do not run signal cable and power cable parallel for too long if not necessary.
Use shielded cable correctly.
Handle the shield per the machine's electrical standard.
Good bonding between the panel, frame and motor.
Do not let cables rub and generate static in the cable carrier.
Use a dissipative cable jacket if needed.
For a cable carrier, use a separator to prevent cables rubbing each other.

In a clean machine, cables must be low-dust, anti-static and ensure EMC. These three requirements need to be considered at the same time.

18. Measuring static: you must look at numbers, not just feeling

If you control static without measuring, it is very hard to know whether it is effective.

Two groups of measuring instruments are often used:

18.1. Surface electrostatic voltmeter / field meter

Used to measure the surface voltage or electric field of a charged object, usually non-contact.

Used to check:

Does the film charge up?
After a tray passes through the line, how many volts remain?
Does the plastic cover surface charge up?
Does the ionizer reduce the voltage fast enough?
Which point in the process generates the strongest static?

When measuring, note the measuring distance, angle, environment and the instrument's guidance. If you measure at the wrong distance, the result can be very wrong.

18.2. Resistance meter / megohmmeter

Used to measure:

The surface resistance of a material
Resistance to ground
Bonding resistance
Whether the ESD mat, tray, jig meet the spec
Whether the ground path is broken or has poor contact

This is the way to check dissipative materials and grounding by numbers.

A machine with serious static control should have a measurement standard and measurement records, not just say "it is grounded".

19. Maintenance to keep static control effective

Static control is not a job that ends once installed.

Over time, the effectiveness can drop due to:

A dirty ionizer needle
A worn ionizer needle
A fan collecting dust
A clogged filter
A loose ground wire
An oxidized bonding point
A star washer losing contact from vibration
A cable with a worn jacket
A dirty or degraded ESD mat
Grease/oil sticking on the dissipative surface
Aged plastic material

The ionizer especially needs periodic maintenance. If the needle collects dust, the ion-generating ability drops sharply. Some ionizers have a dirty-needle warning or self-cleaning function, but you still need to check.

Grounding must also be re-measured. The ground wire can loosen from vibration, or the contact point can oxidize. If you only look with the eye, you may not detect it.

The maintenance checklist should include:

Cleaning the ionizer needle
Checking the ion balance
Checking the decay time if needed
Measuring surface voltage at sensitive positions
Measuring resistance to ground
Checking the ground wires and ground bar
Checking the cable carrier and cable jacket
Checking the ESD material in contact with the product
Recording the periodic measurement results

20. Standards to know: IEC and SEMI

If you design for semiconductors, electronics or a customer with strict requirements, you should not do ESD by experience alone.

Some standards to know:

IEC 61340

This is an important group of standards on static control, ESD materials, EPA and requirements related to protecting ESD-sensitive components.

In machine design, IEC 61340 gives a basis for classifying materials, setting up control methods and inspection.

IEC 61000-4-2

This standard relates to ESD immunity testing for electrical/electronic equipment. It is often used to evaluate at what level equipment can withstand electrostatic discharge.

For machine design, this standard helps view ESD not only from the product angle, but also from the angle of the control equipment, sensors and electrical system.

SEMI E78

In the semiconductor industry, SEMI E78 relates to the evaluation and management of ESD/ESA in semiconductor manufacturing equipment.

If the machine goes into a fab, the SEMI standard may appear in the customer's requirements.

SEMI E129

SEMI E129 relates to the static-management program in a semiconductor factory. It helps view ESD/ESA at the factory-system level, not just a single piece of equipment.

In short:

If you make a machine for a serious clean room, use standards to verify, not just rely on "experience says it's fine".

21. Static-control checklist for a clean-room machine

Before finalizing the design, you can check by the groups below.

21.1. Understand the risk correctly

Is the product sensitive to ESD, ESA or both?
Is there a latent-failure risk from ESD?
Do particles sticking from ESA affect the process?
Is there a SEMI/IEC requirement from the customer?
Is there a surface-voltage or surface-resistance limit?

21.2. Environment

At what level is RH managed?
Is there any locally dry zone from air blow, exhaust or heat?
Is there a low-RH requirement from the process?
Is an ionizer needed to compensate when you cannot raise humidity?

21.3. Grounding and bonding

Is the machine frame clearly grounded?
Are the metal frames, panels, covers well bonded?
Is there a ground bar to gather wires?
Are ground loops avoided per the electrical design requirement?
Is the bonding point on anodized aluminum handled?
Does paint or coating remove conductivity at the joint?
Does the moving assembly have a braided ground strap if needed?

21.4. Materials

Do the product-contact jigs/trays use conductive, dissipative or insulative?
Is the surface resistivity correct to the requirement?
Is the ESD plastic low-particle and low-outgas?
Does the PEEK used natural, carbon fiber or ESD grade?
Does the material withstand cleaning chemicals?
Are there easily charging materials near the product?

21.5. Ionizer

Does the ionizer use fan, nozzle, bar or gun form?
AC, high-frequency AC, DC or pulsed DC technology?
Is the working distance correct?
Is the ion balance met?
Is the decay time fast enough for the line speed?
Is the compressed air supplied to the ionizer clean?
Is there a plan to clean the needle?

21.6. Actuators, cables and EMC

Does any part of the actuator float?
Is the slider/table bonded?
Do the belt/cable carrier charge up?
Does the cable have an anti-static jacket if needed?
Are the signal cable and power cable routed separately?
Is the cable shield handled correctly?
Is there an EMI fault from ESD during the trial run?

21.7. Measurement and maintenance

Is there a surface voltmeter/field meter to measure?
Is there a resistance meter to measure surface resistance and ground?
Is there a measurement standard and measurement positions?
Is there a record of periodic measurement results?
Does the ionizer have a cleaning schedule?
Does the ground point have a schedule for retightening and resistance measurement?
Are the ESD mat, tray, jig checked for degradation?

22. Conclusion

Static control in a clean room is not just grounding or installing an ionizer.

Viewed correctly, this is a part of machine design:

ESD can destroy components.
ESA can pull particles onto the product.
Humidity helps reduce charging but is not enough for every case.
Grounding and bonding are the foundation for the conductive part.
Insulative materials need an ionizer or replacement with a dissipative material.
Anodized aluminum, paint, coating can remove the conduction path.
Cables, actuators, belts, tubes, plastic covers can all be a source of static.
The effectiveness of static control must be measured with instruments, not just guessed.
Maintenance decides whether the effectiveness is still kept after a few months.
For semiconductors, you should also consider IEC 61340, IEC 61000-4-2, SEMI E78, SEMI E129.

A short sentence:

A clean room does not only need to be free of dust. It also needs to control charge, the charge-escape path and the way particles are attracted to the product.

Good design does not wait until the particle count rises, an IC fails or a sensor has noise before looking for the cause. Static must be considered right from the material, structure, layout, electrical, air, measurement and maintenance.

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