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Lightning Strikes: ESD and Stencil Wiping
December 31, 1969 |Estimated reading time: 4 minutes
By Claudio Orefice, High-Tech Conversions Inc.
Conventional stencil wiping rolls contain a high percentage of polyester, leaving PCBs exposed to ESD damage. A conventional roll creates more static as it wipes across a metal stencil sitting in the middle of un-grounded nylon mesh. Just as polyester clothes cling and spark when rubbed, fabric cleaning rolls retain static electricity generated in the manufacturing process.
The latest ESD Association (ESDA) roadmap identifies a trend of rising ESD sensitivity in electronics, through 2010 and likely beyond, as higher performance and denser circuit board assemblies are achieved. Not only will this trend continue, the ESDA predicts, but circuit performance will be valued at the expense of ESD protection levels. Many engineers are not aware of the static dangers that conventional wiping rolls can pose. Despite understanding of ESD protection throughout the manufacturing facility – wrist and ankle grounds for operators, ESD-safe storage bags, special flooring and paints – they figure that if the machine is grounded, it does not matter if the paper is not safe from electrostatic discharge (ESD). Conventional rolls contain a high percentage of polyester. Everyone knows how polyester clothes can cling and cause static sparks. The mere winding action of manufacturing a conventional roll of fabric causes large amounts of static electricity, which remains in the roll. The majority of static generated on a busy production floor is triboelectricity. Tribocharging occurs at the contact and separation of materials, as in a stencil wiping action.
Furthermore, a conventional roll creates more static as it wipes across a metal stencil back and forth. The metal stencil sits in the middle of nylon mesh that is not grounded. The action is similar to rubbing a balloon on your hair. Therefore, static is building on the metal stencil. When a board comes close to the stencil, a miniature lightning bolt can hit the board and destroy an electrical component on the opposite side, as most boards are double-sided. One of these miniature lightning bolts also can cause a void in a solder trace. Consequently, a failure may not be apparent until after the board is being used by an unsuspecting customer.
ESD can destroy an electrical component on a loaded board and cause voids in solder traces. One solution is stencil wiping rolls that offer ESD properties as well as antistatic packaging. These must be efficient cleaning products as well, and reduce defects, downtime, and health risks.
Another danger is posed by flammable vapors that are present from solvents commonly used for cleaning. It is not wise to take the chance of a fire or explosion due to non-ESD-safe stencil wiping rolls. Sparks from static electricity build-up can ignite solvent vapors – as warnings at gasoline pumps will tell you.
Cleaning Efficiency
It is important that ESD stencil wiping rolls do not use weak paper or tissue fibers. These materials typically are held together only by weak hydrogen bonds that are quickly destroyed when wet. Additionally, wet paper does not hold up well when cleaning the rough underside of a stencil. Leftover debris from cleaning papers contaminate boards and delay production processes.
Figure 1. Coarse fibers in ESD-safe stencil wiping materials trap waste into cavities in the matrix.
Instead of paper or tissue, thick, coarse rayon fibers should be used, because these materials readily absorb solvents. Because the fibers are coarser, the porosity is higher and more vacuum can be used to further draw air and unused solder paste from the apertures. Fibers trap residues and paste into cavities in the material’s structure (Figure 1).
Another benefit to using coarse fibers on ESD stencil wiping rolls is that they are conducive to an aggressive cleaning surface. There are numerous voids between fibers that collect solder paste as opposed to a smooth paper surface. This aggressive surface is essential in cleaning lead-free solder because it is “stickier” and more difficult to remove.
Conclusion
If quality and safety truly are part of a company’s mission statement, this area cannot be ignored. If cost is part of a company’s concern, then what is the cost of damaging, testing, reworking, and re-testing a board? What is the cost of losing a customer? Or allowing undetected damage from ESD to make it into the marketplace, possibly necessitating a recall? Operators can reduce the risk of inflicting ESD damage at the printing step by switching to ESD-safe cleaning materials (Figure 2).
Figure 2. ESD-safe cleaning materials replace paper and polyester products.
Modern stencil wiping rolls must offer ESD protection properties as well as antistatic packaging. Additional benefits to users of well-engineered stencil cleaning materials include high-efficiency macro and micro cleaning, defect reductions, shorter downtime, and lowered health risks. The ideal material offers low linting characteristics and higher porosity. With worker safety regulations evolving alongside environmental restrictions in the production process, stencil rolls are required to clean efficiently, avoid reliance on hazardous chemicals, and mollify dangers to the board or the operator.
Claudio Orefice, president, High-Tech Conversions Inc., may be contacted at 11A Pasco Dr., East Windsor, Conn. 06088; (860) 370-9829; Fax: 860-627-8370; claudio@high-techconversions.com; www.high-techconversions.com.