Two brushes both state "surface resistance 10⁶ Ω" on their specification sheets. After three months of use, the resistance of one has climbed into the insulating range, while the other lasts its whole service life. The difference is not in the number but in how that conductivity was built into the filament.
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Compounded and surface-coated: the mechanism decides the service life
There are only two ways for a conductive fiber to give the filament its dissipative ability, and their failure modes are completely different.
| Item | Compounded | Surface-coated |
|---|---|---|
| Location of the conductive component | Dispersed through the body of the fiber | Attached to the surface of the fiber |
| After wear | Conductive paths remain inside, resistance stays relatively stable | The surface layer is lost, resistance rises |
| After washing | Not affected | May fail after a few washes |
| Signs of failure | Changes slowly over the service life | Suddenly "stops working", yet looks normal |
The most dangerous thing about the surface-coated type is that its failure shows no visible sign. The filament looks intact, the density is normal and the resilience is still there, but the resistance has already climbed out of the dissipative range. On the shop floor, all anyone sees is "the static problem is back", and nobody connects it with the fact that the brush specification has changed.
In our selection advice we always list surface-coated conductive nylon as not recommended, for exactly this reason: the conductive layer comes off after a few washes. For cleanroom applications that require regular washing, this material amounts to buying a specification that expires without telling you.
Positioning and resistance ranges of the three conductive materials
The conductive filament materials used in practice fall into three main types, each in a different resistance grade:
| Material | Surface resistance | Grade | Positioning |
|---|---|---|---|
| Copper sulfide bonded acrylic fiber (Thunderon®) | About 10² Ω/cm | Conductive | Active neutralization, used for inspection and dust removal where static sensitivity is highest |
| Static dissipative nylon (PA612 compounded with a carbon-based conductive masterbatch) | 10⁶–10⁸ Ω | Static dissipative | The mainstay on electronics manufacturing floors; a permanent type with no layer to lose |
| Anti-static PBT | 10⁶–10⁹ Ω | Static dissipative | Good resilience, often used as a reinforcing blend |
The gap between the three is not "good or bad" but "what it is for". Lining them up from best to worst is where selection errors begin.
How Thunderon works: not dissipation but active neutralization
The working principle of copper sulfide fiber deserves a separate explanation, because it differs from ordinary dissipative materials.
Thunderon® is a conductive acrylic fiber about 0.0015 inch in diameter, chemically bonded with copper sulfide and produced in Kyoto by Nihon Sanmo Dyeing of Japan (Nihon Sanmo). It works by corona discharge, neutralizing a charged surface without contact (Gordon Brush).
Here is the key difference: ordinary dissipative materials carry charge away through contact, while this fiber can neutralize the electric field without contact. For surfaces that must not be touched, or where touching carries a risk, this is something a grounding path cannot do.
Another point that is often overlooked: chemical bonding is not the same as surface plating. The bonded type does not peel off in whole patches as easily as a plated layer does, but it is still a structure in which "the conductive component is on the surface". That becomes the key point in the next section.
When the conductive grade is actually the wrong choice
This is the most important section of this article, and the judgment that most often carries a price in practice.
In projects for two different customers, we gave advice in opposite directions on the same question. One made encapsulated memory modules, and we recommended copper sulfide conductive fiber as the first choice; the other ran an unencapsulated wafer process, and we explicitly advised against the same material. The difference was not budget or grade, but whether the object being cleaned was exposed.
Two reasons for not recommending it on unencapsulated components:
First, the brush manufacturers' own product pages carry this warning. Major brush manufacturers state clearly on their own conductive brush product pages that conductive fiber is not recommended for use on unencapsulated microcircuits. Exposed circuits fall squarely under this warning.
Second, wear debris from the coating is itself a source of contamination. Wear debris from copper sulfide counts as copper metal contamination. If it remains on the wafer surface, it may affect the downstream underfill and reflow processes. This is a risk that no brush specification sheet will mention, but one that shows up in the yield.
So in this kind of process the right choice is to go back to compounded nylon of the static dissipative grade. Not because it is cheaper, but because it introduces no metal contamination and its resistance grade is safer for unencapsulated components. Conductive does not mean better; in certain processes it is more dangerous.
How water absorption affects resistance stability
Even among compounded nylons, different grades perform very differently, and the key variable is water absorption.
| Material | Water absorption (equilibrium value at 50% RH) | Effect on ESD |
|---|---|---|
| PA612 | About 1.3% | Resistance is little affected by humidity; stable in a cleanroom environment |
| PA66 | About 2.8% | Both dimensions and resistance change after absorbing moisture; humidity must be controlled |
| PA6 | Higher | Absorbs moisture noticeably; an upgrade is recommended for most ESD applications |
The equilibrium water absorption of PA66 is about 2 times that of PA612; DuPont's design data also points out that PA612 absorbs less water and therefore has better dimensional stability and electrical properties (DuPont Zytel design information). This is also why a purchase specification cannot just say "nylon": the grade must be specified, otherwise a supplier who gives you PA6 still meets the description "nylon".
Obtaining documents is a hidden lead-time cost
When choosing a material it is easy to compare only the unit price of the material and overlook the documents.
An existing material source can reuse the upstream raw material supplier's existing RoHS / REACH declarations, which gives the fastest sample and volume production lead times. Introducing a new material means sending it for testing again, and that often takes longer than tooling. If the test result fails, the whole round has to be repeated.
Semiconductor and medical applications have two further tests that are often added: metal ion extraction and outgassing. Neither falls within the standard RoHS / REACH scope. If the customer's internal specification requires them, you need to confirm in advance whether the upstream raw material supplier can provide them, or arrange for additional testing.
The theoretical best is not necessarily the best for the customer
In one project we first worked out that "a certain material is the theoretical optimum in terms of physical properties" and came very close to recommending it to the customer on that basis. Only later did we remember that another customer had already gone into successful volume production with an existing material source in a cleanroom environment of the same class, and that the same specification also had a corresponding item in the main product line of a major overseas manufacturer. In the end the existing material source was used, the sample lead time was shortened, and the documents could be reused as well.
The lesson here is not "do not pursue the best solution". It is that physical properties are not the only item on the scorecard when selecting a material:
- Lead time: sample and volume production with an existing material source are usually several weeks ahead of a new material.
- Documents: the existing raw material supplier's declarations can be reused; a new source has to be sent for testing again.
- Production line experience: quality is far more controllable for a specification that has been made before than for one being made for the first time.
- Supporting cases: being able to cite a track record in an environment of the same class is more convincing to a customer than the numbers on a specification sheet.
The theoretical best solution usually scores zero on all four. The real best solution is the one that scores highest on these four items while still meeting the key indicators.
FAQ
How can I tell whether the brush I have is compounded or surface-coated?
If the specification sheet only says "conductive nylon" without explaining the conductive mechanism, we suggest asking the supplier directly. In practice, the way to judge is to ask two questions: is the resistance guaranteed to stay the same after washing? Is the conductive component a compounded masterbatch or a surface treatment? A surface-coated type usually comes with no guarantee of resistance after washing.
Is conductive-grade filament a higher grade than dissipative-grade filament?
No. The two serve different purposes; one is not a higher grade than the other. On exposed, unencapsulated circuits, brush manufacturers' own product information explicitly advises against conductive-grade fiber, and the wear debris from a metal coating may itself cause contamination. For most electronics manufacturing floors the correct choice is the static dissipative grade.
Why can a purchase specification not just say "nylon"?
Because water absorption differs considerably between grades: at the same 50% relative humidity, PA612 is about 1.3% and PA66 about 2.8%, roughly a 2-fold difference. Water absorption directly affects how stable the resistance is as humidity changes. If the specification only says "nylon", a supplier who provides PA6 still meets the description, but its performance in a cleanroom environment may be completely different.
How much will a change of material delay the lead time?
The bottleneck is mainly the documents, not manufacturing. An existing material source can reuse the upstream raw material supplier's RoHS / REACH declarations; a new material has to be sent for testing again, which often takes longer than tooling. If the customer also requires metal ion extraction or outgassing tests, you need to confirm even earlier whether the upstream supplier can provide them.
Is copper sulfide fiber the same as ordinary metal-plated fiber?
Not entirely. Copper sulfide fiber is chemically bonded, not simply plated; it does not peel off in whole patches as easily as a plated layer, and it can induce a corona discharge to neutralize the electric field without contact. Its conductive component is still on the surface of the fiber, however, and its wear debris counts as metal contamination, which is a risk that has to be assessed in processes with unencapsulated components.
Further reading
- How to choose anti-static brush resistance: a three-grade selection guide
- How to choose cleanroom brushes: 3 red lines where the class decides the material
- Can wafers be cleaned with a brush? Topography decides feasibility: a 5-level guide
- How to submit brushes for testing: four document types and when testing is needed
- The complete guide to brush materials
- How to choose the best materials in brush manufacturing
