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Mitigating ESD Risk in Barcode Labeling and Masking Continued from page 68
the tapes and labels use a high-temperature film, such as a polyimide or polyester. Typically, the label will have a white coating that allows ink to adhere and provides contrast for scanning the bar- code. Similarly, the tape may have a func- tional coating on the surface of the poly- imide film. Labels and tapes produce similar
static discharge concerns. Before being applied to the PCB, both materials are removed from their release liners. This can be done manually or automatically with a machine. By its nature, the adhe- sive and liner are poorly bonded, which allows for easy separation. This poor bond is achieved by using a release coating on
the liner that is chemically dissimilar to the adhe- sive.
The dissimilar chemistry causes a poor “wet-
ting” of the adhesive that yields a poor bond. However, the dissimilar nature of the adhesive
and liner can produce a significant triboelectric charge when separated. During the application process, the charged adhesive will induce a charge in conductive items (including circuitry). Measurements of charge potential of labels and tapes removed from their liners can be greater than 5 kV, depending on size. This charge potential on the adhe-
Barcode label and masking tape cross section.
sive can be amplified by a charge intro- duced by the human or applicator machine used to apply the material to the PCB. This increases the discharge risk. In addition, the label/tape being a charged insulator can create a secondary problem. This charge can polarize a neutral PCB or device. This “field-induced” charge imbal- ance on the PCB or device creates a risk of discharge if the PCB is subsequently grounded at an inopportune time.
Dangers of Handling Another concern with tapes and
labels applied to PCBs occurs after application and during the life of the PCB. When the tape or label is
When the tape and label surfaces are either
polyimide or have organic coatings and the PCB is
handled or moved, there is risk of charge building up on the surface when it is contacted by a human, conveyor or robot.
applied to the PCB, it can be a source for charge accumulation. As men- tioned previously, the tape and label surfaces are either polyimide or have organic coatings that are inherently insulative. As a result, when the PCB is handled or moved, and the tape or label surface is contacted by a human, conveyor or robot, there is a risk of charge building up on the sur- face. This charge has the potential for enabling the polarization process, which may lead to induction charging and discharging. Typically, the barcode label is
applied for the life of the PCB. However, in some cases, the label is removed. For example, when a PCB is made by a contract manufacturer and the PCB is shipped to another company for final assembly in a device, the label may be removed. In the case of a masking tape, its func- tion is to cover and protect an area on the PCB during assembly. At the end of the process, the tape is removed. The removal of the tape and label provides an ESD concern as charge is created during its removal. The PCB has dissimilar triboelectric charge potentials to the adhesive and a charge can be created on the PCB and the adhesive during removal. This needs to be considered in an ESD con- trol plan.
Combating ESD An ESD control plan should con-
sider “specialized” tapes and labels that have low-charging and static- dissipative properties. Both proper- ties will reduce ESD risk.
Low-Charging Adhesive Systems. A key ESD concern is the static charge that is created on the adhe- sive when it is removed from the liner. The dissimilar nature of the adhesive and liner allows for the adhesive to easily release, but also generates a significant triboelectric
See at SMTAI, Booth 415 Continued on next page
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