May, 2016
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Page 57 Taking Charge of ESD Caused by Tapes and Labels Continued from page 55
Typically, barcode labels are applied for the life of the PCB, however, in cases where the board is made by one contract manufacturer and then shipped to another for final assembly, the label may be removed. Masking tapes are primarily used to cover or protect areas on PCBs during assem- bly and removed at the end of the process. In both cases, the removal can generate additional triboelectric charge. After tapes and labels are applied
to the PCB, they can then become a source for charge accumulation that can lead to electrostatic discharge. Since the top surfaces of both materials are either polyimide or have organic coatings, they are inherently insulate. As a result, when PCBs are handled — and the tape or label surfaces are contacted by a conveyor, human or robot — there is a risk that charges can build up on their surfaces. These charges have the potential of inducing charges on neighboring devices, or dis- charging, both of which can result in ESD damage. The best way to prevent this
charge buildup is to make sure the label and tape faces allow charges to move over their surfaces and to a ground. The charge movement should not be fast, but rather gradual so a large electric current is not intro- duced to the PCB. In addition, after the tape and
label are removed from their liners, the liner itself has a static charge both from the separation process as well as from handling — an often under-considered factor in an ESD control plan. If not managed, the charge could be introduced into the production environment and become a source of induction charging.
Charge Behavior The primary measurement for
charge movement or dissipation is the ANSI/ESD STM11.11 test meth - od which details the measure of sur- face resistance. In the test method, surfaces are characterized by the speed at which charges move across them as follows: conductive (rapid), dissipative (slow), and insulate (min- imal/none). Each type of surface produces a
different behavior when a triboelec- tric charge is introduced to it. If a charge is placed on an insulate sur- face, it will create an immediate high charge density at the point of contact that will have minimal decay over time and be prone to a damaging ESD event in the future. If the same charge is placed on a conductive sur- face, it will not produce as high a charge density because of its rapid dis- sipation across the surface through electrical conduction. This rapid movement of charge has the potential to cause damage to static-sensitive devices if the current it generates exceeds the design limits. On a dissipative surface, the
charge density will be initially lower than on the insulate surface and will immediately dissipate slowly across it. Dissipative surfaces help prevent both high charge densities and rapid dis- charges while safely neutralizing them.
Minimizing ESD Risk The ESD Association develops
the guidelines and test procedures that make up the ANSI/ESD S20.20 stan- dard and its international equivalent IEC 61340. The standards are used to create ESD control plans by defining the requirements for ESD-protected areas (EPA). PCB manufacturers must have an S20.20 ESD control plan in place to qualify to create today’s
sophisticated electronic products. Within the S20.20 standard is ANSI/ESD
S541, which governs ESD levels in PCB packag- ing. This includes process-required insulators such as labels and tapes when used in proximity of ESDS devices. The S541 standard requires that
A robust ESD control plan is an absolute
requirement to mitigate the risks of ESD failures. A key element of the successful ESD control plan is managing the effects of charged insulators inside the production environment. Selecting low- charging and static-dissipative tapes, labels and liners that conform fully to ANSI/ESD S20.20 or IEC 61340 requirements, can effectively eliminate a major source of ESD. Polyonics has developed a family
Three types of surfaces and their resistances in ohms.
labels and tapes must have a surface resistance of >104 and <1011W, as measured per ANSI/ESD STM11.11, to be considered static dissipative. Also, the materials must have accumulated volt- ages of <125V when used within 1in, and <2000V within 12in of ESDS devices.
of ESD-Safe™ polyimide and poly- ester (PET) label and tape materials that comply fully with both standards.
Westmoreland, NH 03467 % 603-352-1415 fax: 603-352-1936 E-mail:
info@polyonics.com
The materials have durable, static-dissipative top surfaces, combined with low-charging PSAs and liners that generate less than 125V with liner removal. Contact: Polyonics, Inc., 867 Route 12,
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Dr. Mike Bixenman, CTO
At KYZEN we go way beyond just getting the CLEANING SCIENCE right. We care enough to thoroughly understand your unique process and needs first, so we create the most effective cleaning technologies for your specific situation. When science and care converge, it makes all the difference.
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