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Is your stencil roll ESD safe?

By Pär Gester, SMT Express

An ESD protected area, ESDPA or just EPA, is an area set up in a facility that is protected from the effects of static electricity. Exclusion of static generating material is crucial to maintain an EPA. No object should be brought into the EPA that can tribocharge and discharge rapidly.

Companies takes this seriously by using ESD safe tools, grounding work surfaces and having staff use special shoes and garment.

However, there is one strange exception to these precautions; the SMT stencil cleaning roll. 

ESD safe rolls, that also cleans well and don't shred particles, have been a raritey on the market. Is there a solution? Let's see how some typical products behave!

We tested the ESD properties of five different stencil cleaning rolls that represents fabrics typically used stencil cleaning rolls on the market.

Hypeclean PX3800 is the latest generation of Hyperclean roll by SMT Express. Hyperclean is a Polypropylene (PP) based fabric. 

Hyperclean PP4200 (the same product is sold by a few distributors in Europe under various names) is the old generation Hyperclean, 100% PP.

68 gsm Cellulose is a standard fabric used (in variants) by mainly Chinese manufacturers. Even if we know the name of the manufacturer of the tested product, we have chosen to not disclose the identity as they are only one of large number of converters using this kind of paper material. 

Microcare FP is a rayon based product, designed for use without solvent. This makes it's bad ESD performance even worse as solvent will help discharge statics.

Sontara is a popular "high end" fabric used by many roll manufacturers (or they use clones of Sontara). It's a Rayon-based product and is found in DEK / ASM standard rolls among others.

Here are the results:
 

Rank Material RV (Ohm) RS (Ohm) Tribocharge HBM Comment
1 Hyperclean PX3800 8.82 x 107  4.19 x x108 <15 Volt Dissipative, low charge
2 Hyperclean PP4200 1.75 x 1010 1.75 x 109 <15 Volt Dissipative, low charge
3 68 gsm Cellulose 1.75 x x109 1.52 x 1010 229 Volt Dissipative
4 Microcare FP 2.15 x x1011 1.34 x 1012 354 Volt Isolative
5 Sontara 1.07 x 1011 6.12 x 1012 838 Volt Isolative

Rv = Vertical resistans, Rs = Surface resistance, HBM = Human Body Model
 

All tested fabrics contains polyoleofin (plastic) fibres. Such fibres build up static electricity if they are left without finishing or mixed with antistatic fibres. Cellulose fibres will help in the right direction under high humidity condition as the absorb water from the air. But this is not by design and is of no or little help under normal conditions.  

There are many methods available to make a fabric antistatic. However, the process to apply antistatic finishing is hard to combine with the high-volume manufacturing lines that normally are used to produce fabrics used for stencil cleaning rolls. 

For Hyperclean, we developed our own chemicals and process equipment to achieve the good results while keeping cost at a reasonable level. 

The figures in the table will of course vary depending on fabric production batch, air humidity and measuring methods. But the big picture is clear, very few stencil rolls on the market are ESD safe. Hyperclean is, by design.

Equipment used:
ProStat PAS853B Ohmmeter
ProStat PFM-711A Electrostatic Fieldmeter
ProStat PHT-711 Psycrometer
ProStat PRF-911 Concentric Ring Set

Standard:
IEC 61340-5-1

Test performed by Magnab Eurostat.

Contact us for the full report and more information on Hyperclean.

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