For the very same "anti-static brush", whether it can enter your cleanroom is not decided by the brush itself but by your cleanliness class. Once the class is fixed, the usable filament and handle materials shrink to a very small set. Thinking in the reverse order (picking the brush first and then seeing whether it can go in) almost always means doing the work a second time.
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The cleanroom class decides more than cleanliness: it sets the material whitelist
The first step in choosing a material is not to ask "how soft should it be" but "which class of environment will this brush enter". The class decides three things: whether animal-derived materials can be used, whether a wooden handle can be used, and how deep the document specification has to go.
This is why our inquiry process lists "process environment" as a mandatory question and not an optional one. An "anti-static brush" for a semiconductor front-end wafer fab, back-end packaging and testing, a module plant, a PCB plant and an optical lens plant has a completely different correct answer in each case. Customers usually do not volunteer the details of their process, but those details are the variables that decide the material.
The four dimensions that need to be clarified in practice:
| Dimension | What exactly to confirm |
|---|---|
| Industry | Semiconductor front-end / back-end packaging and testing / modules / PCBA / optoelectronics / display panels / general electronics |
| Process environment | Which cleanroom ISO Class? Is it an ESD-protected area (EPA)? Or an ordinary factory floor |
| Object being cleaned | Wafers / ICs / PCBs / gold fingers / SMT stencils / fixtures / the appearance of finished products |
| ESD sensitivity | The corresponding electrostatic discharge sensitivity level |
ISO 14644-1 classes and particle limits
ISO 14644-1 defines nine classes by the maximum permitted particle concentration per cubic meter of air; ISO 1 is the cleanest and ISO 9 the least strict (Setra). Taking the most often cited particle size of ≥0.5 µm as an example, the limits for each class are as follows (ISO 14644-1:2015 Table 1, as reproduced by Cleanroom.net):
| Class | Limit for ≥0.5 µm particles (per m³) | What it means for brush materials |
|---|---|---|
| ISO 5 | 3,520 | Animal-derived materials excluded; handle material and tufting workmanship must shed zero debris |
| ISO 6 | 35,200 | In most cases treated as strictly as ISO 5 |
| ISO 7 | 352,000 | Animal-derived materials usually still excluded; wooden handles cannot be used |
| ISO 8 | 3,520,000 | Fewer restrictions, but EPA areas must still meet ESD requirements |
With each step down in class, the particle threshold tightens roughly tenfold. This scale explains why "shedding just a little debris" is unacceptable in the higher classes: if your brush releases particles during work, they count directly against the particle budget of the environment.
At which class animal hair is excluded, and why
Natural animal hair (goat hair, horsehair, hog bristle) has advantages in feel and pick-up that synthetic fibers find hard to replace, but in high-cleanliness environments it has two structural problems.
The first is shedding and biological particles. Animal hair is a protein fiber. After repeated use and cleaning, fibers come loose, and it is of biological origin. ISO 5 / 6 environments usually exclude it outright, and in most cases ISO 7 does not accept it either.
The second is insulation. This point is often overlooked: the resistance of goat hair itself can reach 10¹²⁺ Ω, which is in the insulating range. In other words, even if the cleanliness is acceptable, in an ESD-protected area it amounts to a break point in the path: the bristle does not conduct, so the charge never gets into the grounding path behind it.
Besides animal hair, several other materials are excluded in particular processes, each for a different reason:
| Material | Reason for exclusion |
|---|---|
| PVA sponge | Dedicated to wet processes (post-CMP); the wrong fit for dry applications |
| Horsehair / hog bristle | Too stiff; causes pressure damage on surfaces with raised structures |
| Carbon fiber | Too hard and too brittle; broken fragments get stuck in the gaps of structures |
| Surface-coated conductive nylon | The conductive layer is on the surface and fails after a few washes |
The last item deserves special attention: surface-coated and compounded are completely different things. The conductive layer of a surface-coated filament is lost through wear and washing, while in the compounded type the conductive particles are dispersed through the body of the fiber and the resistance is stable over its service life.
Three problems with wooden handles, and why they cannot be used at all in a dry EPA
The filament is chosen correctly and the resistance meets the specification, yet all of it is undone at the handle. This is one of the most common failures in practice.
Wooden handles have three problems:
- Particle contamination: wood sheds debris, absorbs moisture and grows mold. Semiconductor and memory processes can tolerate particles least of all, and this item alone is enough to rule it out.
- A broken static path: wood is an insulator. The filament has been made anti-static, but the handle does not conduct, so the charge is stuck at the handle and cannot flow to the operator and the grounding system. This means the ESD protection is incomplete.
- Industry guidance: Gordon Brush, a major American brush manufacturer, clearly specifies that brushes with a wooden handle and natural bristle are for use in liquid environments only; a dry EPA must use handles made of conductive or dissipative material.
Wooden handles keep turning up in the reference pictures attached to inquiries because they are extremely common in the general cleaning products market, and the sample photos that customers get hold of often look like this. But "appearance reference" and "specification requirement" are two different things. We have had customers attach a picture of a wooden-handled fan-shaped brush while also asking for ESD and cleanroom documents, and these two conditions cannot physically be met at the same time. Discussing the appearance requirement and the functional requirement separately usually leads quickly to an alternative handle shape.
Handle material priority and the trade-offs of each
This priority order is not ranked by resistance value. When we actually assess a case, we look at four dimensions together: grounding grade, risk of shedding particles, whether it can be disinfected at high temperature, and whether our existing production line can make it. Aluminum alloy comes first because it holds up on all four at once, not because its resistance figure looks best.
| Priority | Handle material | Surface resistance | Trade-offs |
|---|---|---|---|
| First choice | Aluminum alloy tube handle | ≤10⁶ Ω (conductive grade) | The most complete ESD path, no particle shedding, can be disinfected at high temperature, light and durable |
| Alternative | Anti-static ABS injection-molded handle | 10⁶–10⁹ Ω (dissipative grade) | The grip can be shaped freely, suitable when there are specific appearance requirements; ESD grade is lower than a metal handle |
| Alternative | Stainless steel / zinc-plated steel tube handle | Conductive grade | Can be sterilized at high temperature, suitable for higher cleanliness requirements; heavier |
| Not recommended | Wooden handle | Insulating | For liquid environments only |
The reason for recommending aluminum alloy first is not only the resistance figure: a metal handle sheds no particles, does not grow mold and can be disinfected at high temperature, satisfying both the cleanliness and the ESD dimensions. The complete path would be "filament (dissipative grade) → aluminum handle (conductive grade) → operator's wrist strap → ground", which meets the requirements of ANSI/ESD S20.20 and IEC 61340-5-1 for hand tools in an EPA.
Hand brushes and automated carriers follow two different logics
The above is about hand brushes. If the brush is to be fitted into inline automated equipment, the concept of grounding is completely different: there is no handle held by a person, and the carrier is a mechanical part (a shaft, a support plate, a quick-release coupling).
| Form | Direction for the carrier material | Grounding path |
|---|---|---|
| Roller brush | Aluminum alloy shaft + ESD-safe polymer end caps | Shaft → machine → facility ground |
| Strip brush | Aluminum alloy or conductive POM base | Base → mechanism → facility ground |
| Wheel brush | Aluminum alloy hub + grounding wiring | Hub → wiring → facility ground |
In an automated setting, the machine parameters also have to be matched: downward pressure, line speed and the specification of the quick-release interface. These are not something the brush supplier can decide unilaterally; they must be provided by the equipment side.
Beyond class and material, two more things can overturn the conclusion
The material whitelist is a necessary condition, not a sufficient one. The following two points can make a solution that "meets the class" still unsuitable:
- The customer's internal specification may be stricter than the ISO class. For the same ISO 7, the permitted range differs between a semiconductor front-end fab and a module plant, and some plants have their own list of prohibited materials. The class is only the minimum threshold.
- The surface structure of the object being cleaned can veto the choice of material. If the surface being cleaned has raised structures, the filament diameter must be smaller than the spacing between the structures so that the filament does not sit on top of them; once the diameter is restricted, the set of usable materials shrinks by another round.
So the correct order is to confirm the class and the internal specification first, then the physical characteristics of the object being cleaned, and only then move on to the choice of material. Doing it the other way round usually means a wasted trip.
FAQ
Is it true that natural bristle brushes can never be used in a cleanroom?
ISO 5 / 6 environments usually exclude them outright, and in most cases ISO 7 does not accept them either, because of shedding and biological particles. If the location is also an ESD area, goat hair has the additional problem of being insulating (resistance can reach 10¹²⁺ Ω), which makes it a break point in the path. Environments of a less strict class that are not ESD areas can still be assessed, but the customer's internal specification must be confirmed.
Why can wooden handles not be used in a cleanroom?
Three reasons: wood sheds debris, absorbs moisture and grows mold, which directly affects particle control; wood is an insulator, so the anti-static specification of the filament is interrupted at the handle; and industry guidance also clearly restricts brushes with a wooden handle and natural bristle to liquid environments. A dry EPA must use handles made of conductive or dissipative material.
Should I choose an aluminum alloy handle or an anti-static ABS handle?
Aluminum alloy is conductive grade (≤10⁶ Ω) and ABS is dissipative grade (10⁶–10⁹ Ω). The former has a more complete grounding path, sheds no particles and can be disinfected at high temperature. Consider ABS only if you have specific requirements for grip or appearance. Where high-temperature sterilization is needed, a stainless steel tube handle can be assessed.
What is the difference between surface-coated and compounded conductive filament?
In the surface-coated type the conductive layer is on the surface of the fiber; it is lost after a few washes, and the resistance rises with time in use. In the compounded type the conductive particles are dispersed through the body of the fiber, and the resistance is relatively stable over its service life. For cleanroom and ESD applications we recommend the compounded type, so that the specification does not quietly fail after a period of use.
How is grounding handled for brushes on automated equipment?
There is no handle held by a person, so the path becomes "carrier → machine → facility ground". Roller brushes usually use a groundable aluminum alloy shaft with ESD-safe end caps, and strip brushes use an aluminum alloy or conductive POM base. You also need to confirm the downward pressure, line speed and quick-release interface specification with the equipment side, as these in turn limit the filament diameter and stiffness.
References
Further reading
- How to choose anti-static brush resistance: a three-grade selection guide
- Anti-static brush not working? A troubleshooting guide to the seven links of the dissipation path
- Compounded or surface-coated conductive fiber? Service life and selection explained
- What to prepare for a custom brush quote: the three spec elements
- The complete guide to brush materials
- How to choose the best materials in brush manufacturing
- How to accept industrial brushes for shedding: judging loose filaments, broken filaments and tuft pull-out
