IS YOUR STENCIL ROLL ESD SAFE?
By Pär Gester
An ESD protected area, abbreviated EPA, is an area set up in a facility that is protected from the effects of static electricity. The exclusion of static generating material is crucial to maintaining an EPA. No object should be brought into the EPA that can tribocharge and discharge rapidly.
Companies take this seriously by using ESD safe tools, grounding work surfaces and having staff use special shoes and garments.
However, there is one strange exception to these precautions; the SMT stencil cleaning roll.
What, aren't all rolls ESD safe?
ESD safe rolls, that also clean well and don't shred particles, have been a rarity 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 represent 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 many 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 the large numbers of converters using this kind of paper material.
Microcare FP is a rayon-based product, designed for use without solvent. This makes its bad ESD performance even worse as the solvent would have helped discharge static. With no solvent used, you might have to live with a constant static build-up in your printer.
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. It has a bad cleaning ability, besides non-existing ESD compatibility.
| 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 |
All tested fabrics contain polyolefin (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 conditions as they 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 optimized for stencil cleaning rolls.
For Hyperclean, we developed our own chemicals and process equipment to achieve good results while keeping costs 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