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Product Labeling vs. ESD
December 31, 1969 |Estimated reading time: 17 minutes
One of the many potential sources for electrostatic discharge are standard PCB and component bar-code labels that utilize insulative materials able to generate up to 1,000 V or more when removed from the release liner. One answer: new static dissipative materials that allow the charge to dissipate quickly through the adhesive layer to ground.
By Vicki Heideman and Dennis Polinski
Although standardized methods for testing the properties of static-dissipative labels have not yet been developed, test methods that exist for materials other than identification labels can be successfully employed so that label materials can be classified. However, the tests do not fully describe the use of static-dissipative labels in a manufacturing environment. Other factors to be considered include the generation of voltage and the effects of humidity on different types of release liners, the voltage effect on unpopulated printed circuit boards (PCB) when applying and removing a label, and how generated voltage differs between the methods of label application (manual vs. automated process).
How Labels Cause ESD FailureThose familiar with electrostatic discharge (ESD) and its control in the assembly environment may not have considered standard labels as a potential source for damaging events to components or assemblies. Yet, most are familiar with the term triboelectric charging, the phenomenon of creating an electrostatic charge via contact or separation of materials.1
In label usage, several actions can cause voltage build-up from triboelectric charging. Table 1 lists the voltages generated when a standard label is removed from the release liner on which it is supplied (Figure 1). Such an action can generate voltage sufficient to damage sensitive assemblies and parts.
Voltage TestingMeasuring voltage generated during triboelectric charging remains a difficult task. Although several test methods have been published (e.g., an inclined plane test that simulates components charging as they slide through a shipping tube2), none seems to exist for measuring charges generated by labels. Accordingly, a test method was developed internally to measure voltage created by labels removed from their release liners.
Figure 1. Typical profile of a label. ESD may be generated when the label is removed from the release liner.
Method development. Before the test (see sidebar) was developed, measuring in units of coulombs and volts were both considered. Voltage was chosen because these units seem to be more widely used and understood. The test procedure requires the operator to wear a wrist strap attached to ground similar to that worn by most assemblers while at their workstations.
The first step is to remove the label manually from the release liner. Next, the voltage on the label is measured using a static fieldmeter. (It may be necessary to follow the test procedure exactly as described in the sidebar to duplicate the results presented.) The repeatability of the test method is not as good as that of a totally automated test method. However, automation is omitted so that the voltage values generated represent those of a manual process. To improve the accuracy of the values reported, 10 labels are measured and then averaged as one data point.
(With the lack of test methods, there are also no specific industry standard limits for voltage levels permitted on a label. Thus, as a guideline, the definition of a static-safe workstation in EIA-625 is employed, and a static-safe label must generate less than 200 V when removed from the release liner.3)
Basic voltage-testing results. Testing of various label materials is performed to determine typical voltage levels. Three separate lots of each material are tested and the averaged results are presented in Table 2. (The effect of label area on voltage generated is ignored but will be addressed later.)
One specific static fieldmeter to measure voltage has been used almost exclusively for all internal testing. However, in Table 2, some results from an alternate meter (no. 2) are presented for comparison. Although this study is not conducted to compare static meter performance (and it is not apparent which voltage values are more accurate), no. 1 seems to provide more repeatable results while not drifting off of zero as readily. Results indicate that voltage measurements on the static-dissipative labels are lower than those on the standard labels for all three substrates tested.
Area effects. Label area seems to affect the amount of voltage generated. This is explored further by measuring the voltage generated on labels of varying size using the test procedure.
Figure 2. Voltage vs. label size for three materials, measured at 22° to 23°C and 28 to 46 percent rh. Results are presented in a log-scale format so that data from the standard and static-dissipative labels could be presented on the same graph.
After documenting the voltage performance of standard labels, the same test is performed on static-dissipative label constructions. The procedure for measuring voltage is identical and results are presented in Figure 2. The data confirm that for standard labels, the measured voltage increases with size. For static-dissipative labels, the voltage remains at a comparatively low level even as size increases. Even though the exact mathematical relationship between area and voltage is not investigated, the information in Figure 2 is useful in that it pinpoints larger labels as having greater potential for ESD damage than smaller labels.
Surface-resistivity TestingSurface resistivity may not be an interesting performance property to a label user, but it is important to be able to measure it accurately so that label materials can be classified.
Definitions. According to EIA-5412 and EIA-6253, the criteria for classifying materials by surface resistivity are: less than 105 Ω/sq as "conductive"; between 105 and 1012 Ω/sq as "static dissipative"; and greater than 1012 Ω/sq as "insulative."
The test method for measuring labels intended as static-dissipative is EOS/ESD S11.11; test equipment used is a wide-range resistance meter with resistivity probe.4 The method requires that the materials be conditioned at 23°C (±3°C) and 12 percent (±3 percent) ambient humidity. However, the capability to simulate this environment was not available for this test. Instead, the temperature and humidity present during test is recorded; electrification time for all testing is 60 seconds.
Because pressure-sensitive labels have a laminated structure, the surface being measured must be noted. Here, the adhesive surface is of interest because it is in contact with the release liner. (EOS/ESD S11.11 notes that conversion of values to resistivity may not be valid for laminated materials. However, the conversion to resistivity is made here so that the values can be directly compared to the EIA definitions of conductive, static-dissipative and insulative. Also, as is common with other composite or layered materials, occasional insulative resistivity values may be obtained on the static-dissipative materials.5 The frequency of these may be due to many factors including cleanliness of the test probe, surface properties of the material tested, ambient humidity during testing and electrification time.)
Comparison testing of various standard label materials is performed to determine their surface resistivities. Since the adhesives are typically insulative, a different method must be employed because EOS/ESD S11.11 is not a valid test approach for these materials. With the method selected ASTM D 257-936 no special conditioning is performed on the samples, and temperature and humidity during the test are recorded. The equipment: a high-resistance meter* and resistivity cell**. The sample label size used was 4 x 4". Three separate lots of each material were tested, with results shown in Table 3.
Employing these methods for measuring the resistivity of pressure-sensitive adhesives is mostly successful. Adhesive residue that may build up on the test electrode over time may be removed by wiping the electrode with solvent or by rubbing the electrode surface lightly with fine sandpaper. However, because such cleaning procedures may eventually degrade the electrodes over time, it would be prudent to check with the manufacturer for the best cleaning method.
The results in Table 3 confirm that the surface resistivities of standard label adhesives fall in the insulative range, and that the static-dissipative labels meet their EIA definition when measured via the method employed.
Static-decay testing is performed according to EIA-541 Appendix F, "Measurement of Electrostatic Decay Properties of Dissipative Planar Materials."2 The intent of this test is to determine the label material's ability to dissipate 99 percent of a 5 kV charge to ground within a specific time. The material is considered acceptable if the average decay time for each test sample is less than two seconds. The test procedure is used only to evaluate static-dissipative materials and are so checked by an independent testing lab. The results show that all the samples met the time requirement of less than two seconds.
Summary of basic testing shows that it is possible to measure voltage, surface resistivity and static-decay time of label materials using commercially available equipment. The following conclusions can be drawn:
- Standard labels can retain charges on their surface of well over 200 V
- Static-dissipative labels retain significantly less voltage than that of standard labels
- The choice of a static fieldmeter to measure labels will influence results
- Larger labels can generate more voltage when removed from the release liner than smaller labels
- Standard and static-dissipative labels can be differentiated by measuring voltage or surface resistivity. Static-decay time can be measured on static-dissipative labels
- The voltage behavior of a label can be predicted by measuring adhesive surface resistivity.
Voltage Effects in the Manufacturing EnvironmentThe goal is to consider various situations apart from the manual removal of the label from its release liner, i.e., how other aspects of using labels may or may not pose a threat to components or assemblies.
Release liner and humidity voltage testing. One of the assumptions behind tribocharging is that each surface accumulates an equal but opposite charge. This points to the fact that the release liner from which the label is removed may also be accumulating charge on its surface. The two basic types of release liners, film and paper, typically have a silicone-release coating on one side.
It was suspected that voltage measurements on the release liner itself would differ significantly from paper to film liners because the surface-resistivity characteristics of each are very different. In measuring film release liners, resistivity levels were found to be in the insulative range, above 1 x 1013 Ω/sq. The surface resistivity of film liners, both silicone-coated and uncoated, does not appear to be affected by humidity. Due to this high resistivity, film liners can generate significant voltage while being unwound or wound, as in a roll-form printer or applicator.
On the other hand, the surface resistivity of paper liners can be affected by humidity. Table 4 lists surface resistivity data measured on two silicone-release-coated paper liners, of which each is comprised of a different grade of base paper. The test method used is ASTM D 257-93.6 Samples were conditioned at the listed temperature and humidity before test. The equipment used is a high-resistance meter with a resistivity cell.
These data show that the paper side of the release liner has a resistivity in the static dissipative range at 57 percent relative humidity (rh) and very close to the static-dissipative range at 31 percent rh. The resistivity is well into the insulative range at 9 percent rh. Thus, at low humidity, the paper release liner will be able to retain a charge. (One general observation noted during testing: The type of release liner, paper or film, from which a label is removed does not seem to affect the magnitude of voltage that can be measured on the label. However, the voltage on the release liner itself can vary significantly.)
Humidity also affects the voltage measured on paper labels after they are removed from the release liner. The results in Table 2 show that voltage measurements on standard paper labels after removal from the release liner are much lower than those on the standard film (polyester and polyimide) labels (the data were gathered at 30 to 60 percent rh). This is presumably due to moisture in the paper bringing the surface resistivity of the paper into the static-dissipative range. Interestingly, the fact that an insulative adhesive is between the paper label substrate and the release liner does not prevent the paper resistivity from having an effect on label voltage properties. However, the effect is not as dramatic as when a static-dissipative adhesive is employed.
Figure 3. Measured voltage on a panel for standard and static-dissipative polyester labels shows levels to be significantly reduced for the latter.
Voltage effects of manual label application/removal from PCBs. After a label is removed from its release liner, the next step is to apply it to the surface of the object to be identified. The testing in this section was developed to determine how the voltage level on a label affects the surface to which it is applied. To simulate a typical surface, a section of FR-4 laminate (no soldermask) is used. (Testing on a conductive surface is not attempted.) The test method is described in the sidebar.
Figure 4. Measured voltage on a panel for standard and static-dissipative polyimide labels. Note that the static-dissipative, upon removal, will not decrease the voltage generated.
Standard and static-dissipative materials are both tested to compare their performance (Figures 3 and 4). The results indicate that when a label is applied to the panel, the voltage is reduced significantly for a static-dissipative label compared to a standard label. However, using a static-dissipative label will not decrease the voltage generated when the label is removed from the board.
Automatic label removal from release liner. The final area investigated is voltage generated when a machine automatically removes a label from its release liner. Here, voltage on labels is measured by modifying the test procedure for manual label removal, i.e., the machine partially or fully removes the label from the liner, followed either by full manual removal and measurement or a measurement taken while the label remains on the applicator head. Three removal techniques are considered: a device that partially peels the label from the release liner and presents the label for full manual removal (partial removal), a commercially available automatic applicator that uses peel-edge technology and compressed air to remove the label from the release liner (automatic removal A) and a commercially available automatic applicator that peels the release liner away from the label (automatic removal B).
The data in Table 5 suggest that the voltage generated on the standard label will vary with different removal methods. The more automated methods seem to generate less voltage on the standard labels than a completely or partially manual removal process.
Summary of TestingThe testing described shows that measurements can be performed to indicate how much voltage labels may generate when used in different ways in a manufacturing environment. The following conclusions can be drawn:
1. Paper liners retain less charge on their surface than polyester liners at typical indoor humidity levels.2. Paper liners may retain a charge at very low humidity.3. When applied to an insulative surface, a standard label can introduce higher voltage levels than those of a static-dissipative label.4. Static-dissipative labels generate just as much voltage when removed from an insulative surface as that of a standard label.5. Using an automatic applicator to apply labels to PCBs or components may reduce the risk of ESD damage.
Because static-dissipative materials have lower resistivities and rapid static-decay characteristics compared to standard label materials, they are unable to build up significant levels of charge on their surfaces. These advantages also carry over into the manufacturing environment, where the voltage on a label can remain long after it is applied. Accordingly, the use of static-dissipative labels can help users in the assembly environment conform to the requirements of EIA-625 and reduce the likelihood of ESD-induced product failures.
TEST PROCEDURE: Triboelectric Charging on Labels
A. When Removed From Release LinerI. Scope. This procedure provides a method for measuring how much voltage is generated on a die-cut label when manually removed from its release liner.
II. Related Documents. EIA-625, "Requirements for Handling Electrostatic-Discharge-Sensitive (ESDS) Devices."
III. Test Equipment Required. Charge analyzer and supplied accessories: wrist strap, temperature/humidity gauge and scale.
IV. Sampling Plan. One die-cut roll from a known raw material lot should be used for testing. A strip of 10 labels should be selected.
V. Preparation. Using the scale, measure the length and width of the die-cut labels to be tested. Flip power switch located on the static-charge analyzer to "on." Zero equipment by following the manufacturer's instructions. Record temperature and humidity in which samples were conditioned and will be tested (this may be ambient or in a controlled chamber). Put wrist strap on wrist and attach to ground.
VI. Procedure.
- Hold die-cut label sample (on liner) at least 1' away from sensor on test equipment
- Grasp liner with one hand and quickly peel label off liner with opposite hand
- Quickly place label 1" away from unit sensor (keep liner away from sensor).
VII. Test. After each sample is peeled off liner and placed 1" away from unit sensor, record voltage value displayed by unit. If value is dropping, record highest (initial) value displayed.
VIII. Results. Calculate the average voltage generated on the group of 10 labels. The average should be reported as one data point. Per the definition of a static-safe workstation in EIA-625, a label must generate less than 200 V when removed from the liner. Compare each data point to the 200 V limit. If less, report it as a pass; if greater, as a fail.
IX. Reporting. Report the lot number tested, the label size tested, the temperature and humidity at which samples were conditioned and tested, the average voltage values obtained and whether each passed or failed per the EIA definition.
B. When Label is Applied or RemovedI. Scope. This procedure provides a method for measuring the amount of voltage generated on an FR-4 panel when a label is applied or removed.
II. Related Documents. None.
III. Test Equipment Required. Charge analyzer and supplied accessories: wrist strap, two 1 inch high blocks or cylinders made of insulating material, temperature/humidity gauge, 4.5 lb roller with a static-dissipative film secured to the rolling surface, timer, ionizing unit, 2 x 6" FR-4 panel, scale, isopropyl alcohol.
IV. Sampling Plan. One die-cut roll from a known raw material lot should be used for testing.
V. Preparation. Using the scale, measure the length and width of the die-cut labels to be tested. Flip power switch located on the charge analyzer to "on." Zero equipment by following the manufacturer's instructions. Record temperature and humidity in which the samples were conditioned and will be tested (this may be ambient or in a controlled chamber). Clean FR-4 panel with isopropyl alcohol. Place wrist strap on wrist and attach to ground.
VI. Procedure.
- Place the two insulating blocks or cylinders opposite each another and 1" away from the sensor on the top of the charge analyzer
- Use an ionizing unit to neutralize the FR-4 panel to 0 ±1 V; check voltage by laying panel across insulating blocks or cylinders
- After removing the panel from across the blocks or cylinders, test the die-cut label for voltage. Record this value as the label voltage
- Replace the FR-4 panel on the insulating blocks or cylinders
- Quickly apply the label (but do not press on it with fingers) to the panel above the sensor on top of the charge analyzer
- Using the roller, roll over the label once forward and once back.
VII. Test.
- Read the voltage on the meter, record this as initial voltage value
- Start the timer and record the voltage value at 1, 2, 5, 10, 30 and 60 minutes
- After the last reading, peel enough of the label off the FR-4 panel to grasp
- Neutralize both the panel and the label to 0 ±1 V
- While grounded, use fingers to quickly peel the label off the panel at 90°
- Read the voltage on the meter; record this as initial voltage
- Start the timer and record the voltage value at 1, 2, 5, 10, 30 and 60 minutes.
VIII. Results. Data from each trial tested should be recorded in a table or graph.
IX. Reporting. Report the lot numbers and label sizes tested, temperature and humidity at which samples were tested and the table and/or chart for each trial.
- HP 4329A.
** HP 1600A.
This article is adapted from a paper originally presented at Surface Mount International '98, San Jose, Calif.
ACKNOWLEDGMENTSThe authors thank Kriss Kapp, laboratory technician, for her help in formalizing and executing some of the testing described.
REFERENCES1Electrostatic Discharge Control Handbook, Electrostatic Discharge Association, ESDADV-2.0-1994, p. 4.
2 "Packaging Material Standards for ESD Sensitive Items," Electronic Industries Alliance, ANSI/EIA-541, 1988.
3 "Requirements for Handling Electrostatic-Discharge-Sensitive Devices," Electronic Industries Alliance, ANSI/EIA-625, 1994.
4 "Surface Resistance Measurements of Static Dissipative Planar Materials," Electrostatic Discharge Association, ANSI/EOS/ESD-S11.11, 1993.
5 Brian Hohman (private conversation), Electro-Tech Systems Inc., December 15, 1997.
6 "DC Resistance or Conductance of Insulating Materials," American Society for Testing and Materials, ASTM D 257, 1993.
VICKI HEIDEMAN, senior development engineer, and DENNIS POLINSKI, product manager, may be contacted at Brady Worldwide Inc., Identification Solutions and Specialty Tapes Group, 6555 W. Good Hope Road, P.O. Box 2131, Milwaukee, WI 53201-2131; (800) 537-8791; Fax: (800) 292-2289; Web site: www.bradycorp.com.