From ESD Electrostatic Discharge (ESD) protective packaging sourcing is not restricted to U.S. and European sourcing. Products from the Asian-Pacific Region are providing end users the opportunity of securing static control materials and packaging over the web and through new channels of distribution. In recent years, contract manufacturing (CM) organizations and suppliers may have been sacrificing quality for price. Some issues which were resolved in the early 1990s for ESD packaging are resulting in corrective actions for ESD protective packaging and materials nonconformance.
Figure 1 represents an ESD corrugated regular slotted container (RSC) which would in Major League Baseball terms be called “bush league;” it represents a worst case scenario, but it is being used by a major CM. Aside from the poor physical characteristics of carbon rub off which can bridge the gaps of circuit lines and cause a short, the box exhibited poor attenuation (shielding) from an electrostatic field. Relying on a supplier to do that right thing without defining your ESD requirements could be costly.
Figure 1: ESD-Corrugated Regular Slotted Container
There are some very reputable suppliers of ESD packaging located throughout the world. However, the qualification of a product based upon a supplier’s specification sheet is not enough in insuring that company ESD requirements are being met.
Organizations need to follow a formalized materials qualification process. ANSI/ESD S541-2003, Packaging & Materials Standard, represents one tool which provides invaluable testing methods with defined upper limits. ESD program managers, packaging engineers, quality assurance and procurement professionals should require suppliers to submit their products for in-house testing or 3rd party evaluation.
This article will serve as a general overview in outlining some test methods aimed at the qualification process for ESD packaging and material so that the reader will gain a greater understanding of what differentiates one ESD product from another. The article will explore mechanical, electrical, and environmental considerations as we search for the “perfect package.” This editorial will also address some basic design considerations in conjunction with testing procedures.
Definitions Since many of the readers do not have the opportunity to attend ESD Association Standard Meetings, several ANSI/ESD S541-2003 (an ANSI/ESD S20.20-1999 document) definitions are important to review so that the reader is on the same page with the authors.
Resistance of Dissipative Materials: A static dissipative material shall have a surface resistance of greater than or equal to 1.0 x 104 ohms but less than 1.0 x 1011 ohms, or a volume resistance of greater than or equal to 1.0 x 104 ohms but less than 1.0 x 1011 ohms.
Resistance of Conductive Materials: A surface conductive material shall have a surface resistance of less than 1.0 x 104 ohms. Volume conductive materials shall have a volume resistance of less than 1.0 x 104 ohms.
Resistance of Electric Field Shielding Materials: Within the conductive materials classification per ANSI/ESD S541-2003, electric field shielding materials shall have a surface resistance of less than 1.0 x 103 ohms or a volume resistance of less than 1.0 x 103 ohms. Other methods may also define the electric field shielding classification.
Resistance of Insulative Materials: An insulative material per ANSI/ESD S541-2003 shall have a surface resistance of greater than or equal to 1.0 x 1011 ohms, or a volume resistance of greater than or equal to 1.0 x 1011 ohms.
ANSI/ESD S541-2003 Classification of ESD Packaging Material Properties: Materials and packages that are useful in preventing damage to sensitive electronic devices exhibit certain properties. These properties include:
1. Low Charging (antistatic) 2. Resistance: a. Conductive b. Dissipative c. Insulative 3. Shielding: a. Electrostatic Discharge b. Electric-field
ESD Test Procedures—Surface Resistance versus Relative Humidity (RH) One misconception by organizations is the assumption that ESD tests at high and low relative humidities will produce similar measurements. Arguably, this may be true for conductive materials and humidity independent technologies, but materials which are dependent upon humidity to facilitate electrical conductivity such as antistatic polymers, untreated kraft and bleached corrugated containers and cleanroom papers will produce significantly different readings at 50% RH compared to ESD test results performed at 12% RH.
The ESDA standard for surface resistance (ANSI/ESD STM11.11-2001) requires the evaluation of planar static dissipative materials at 12% +/-3% RH @230C +/-30C after 48 to 72 hours of preconditioning. At this RH, lab-to-lab correlation is better facilitated by exposing the specimen to low RH that simulates a realistic exposure potential.
Table 1 shows the measurement differences of the same 6.0” x 6.0” planar specimens at both low and high RH. For example, an antistatic blue planar sheet (see Figure 2) is measured for surface resistance per ANSI/ESD STM11.11-2001 at 11.6% RH and 52.5% RH using a concentric ring fixture. At 11.6%RH or standard conditions, the specimens failed the examination.
Table 1
Figure 2
Surface resistance testing may be the most important test in evaluating materials. In 1993, the ESD Association adopted ANSI/EOS/ESD-S11.11-1993 [Rev. ANSI/ESD STM11.11-2001(ohms)] to replace ASTM D257 (in ohms/square), which measured surface resistivity for DC conductance on insulators. In the insulative range, materials become nonconductive and may hold static charges for several seconds or more. Oftentimes, the surface resistance of antistatic materials rises and falls when relative humidity fluctuates. Per ANSI/ESD S541-2003, 1.0 x 1011 ohms is the standard cutoff for retention of static dissipative properties. In practice, however, a lower cut-off is often desired for packaging materials because dry air may be encountered in shipping and handling. In cold and dry climactic conditions, relative humidity can reach 4% or below. As a reference, Table 2 illustrates the resistance ranges as outlined in ANSI/ESD S541-2003.
Table 2
ESD Material & Packaging Requirements In the past several years, a significant number of ESD packaging products which have not been in conformance to ANSI/ESD S541-2003 are vacuum formed antistatic polymer trays or work carriers, ESD corrugated, antistatic polyethylene foam, antistatic polyurethane foam, bubble wrap antistatic cushioning, pink poly and static shielding bags. Often, the corrective actions stem from so-called “knock offs” of brand named products (Figure 2). Moreover, do not assume that a popular trade name constitutes adherence to ANSI/ESD S541-2003 or an understanding by their technical staff of the ANSI/ESD S20.20-1999 standards.
Moreover, requirements for recyclability, Amine free and the European Union’s (EU’s) Restriction of Hazardous Substances (RoHS) Directive requirements by organizations may assist in improving adherence to requirements. The EU’s RoHS Directive (2002/95/EC) was adopted in February 2003, and it took effect on July 1, 2006. While it is not a law, only a directive, many North & South American, European and Asian organizations have already implemented the initiative. This Directive restricts the use of six hazardous materials in the manufacture of various types of electronic and electrical equipment. Each EU member state will adopt its own implementation and enforcement policies using the Directive as a guide. Therefore, there could be as many different versions of the restriction as there are states in the EU (Wikipedia Encyclopedia).
The RoHS Directive is often referred to as the “lead-free” directive, but it restricts the use of the following 6 substances:
1. Lead 2. Mercury 3. Cadmium 4. Chromium VI 5. PBB 6. PBDE
Being involved in several ESD packaging-related corrective actions over the years has made one thing clear. It is important to know what your organization has qualified or specified. Some organizations will purchase and receive non-qualified ESD products resulting in customer complaints. The supplier’s customer calls his vendor to account for their product’s poor performance. The ESD product in question was qualified off the supplier specification sheet and, when subjected to analysis, it tested “out of compliance.” In some cases, a supplier’s specification sheet makes allowances (in their favor) for readings defined in ANSI/ESD S541-2003 as out of range. In essence, the supplier has built-in a safety factor. Consequently, the packaging supplier’s customer, who qualified the ESD packaging materials based solely on a vendor’s specification, may have no recourse for reimbursement. If the customer had clearly defined their requirements according to industry standards or internal company requirements, the supplier would “comply or not supply.”
Figure 3
The corporate ESD program manager, ESD site coordinator, packaging engineer, quality assurance personnel or buyer should be purchasing ESD materials and products which have undergone a formalized materials qualification process. Qualified products are referenced on a Materials Qualification List (MQL) and with the issuance of a purchase order; the referenced document states an organization’s requirements as seen with the Certificate of Compliance (COC) in Figure 4. If a supplier adheres to a COC program, a defective product would be pulled from the manufacturing run before being sent to the customer. Believe it or not, this is a concept which is novel to some suppliers. By incorporating a COC program in the purchasing process, the customer and supplier have done their “due diligence” and the process becomes a “win-win” for each party.
Figure 4: Sample ESD Foam Material COC
ESD Roadmap Matrix Table 3 lists a matrix which may be typical of an organization’s culture. Test methods can be more stringent depending upon specific industry requirements. The reader is encouraged to develop procedures which are suitable for their individual applications. One should try to trace a company requirement to an industry standard.
X=Tests which are performed per Category
Table 3: ESD Test Methods
ESD Testing Methods—ESD Corrugated Test Considerations In this and the following section, two material types will be examined to illustrate an ESD testing methodology: ESD corrugated and vacuum-formed trays.
Several organizations consider key properties for ESD corrugated to include attenuation (static shielding), surface resistance, minimal triboelectric charging, negligible reducible sulfur, favorable abrasion resistance, static decay and repulpability.
Static Shielding at 1,000 to 5,000 volts Static discharges can damage ESD-sensitive items through the wall of corrugated packaging. During the taping application for a regular slotted corrugated (RSC) container, for example, an electrostatic field can be induced through the box as it travels on a grounded conveyor as the metal guides assist in the flap closure process before the triboelectrically charged tape (measured upwards to 25,000 volts at a distance if 2’) seals the container.
To evaluate an ESD corrugated container for shielding effectiveness, several major organizations use a controversial test consisting of a 3M static event detector (SED) that is configured into a modified capacitive sensor placed inside a fully enclosed ESD box. This battery-powered-SED is approximately a 1.0” square reusable device to detect low-level electrostatic discharge events at 100 volts. Said detector is housed inside a plastic case with a clear window on top and a metal back plate. In addition, the detector has a high impedance detection circuit, reference antenna and LCD display.
When placed in a modified capacitive sensor, the SED can sense the rapid change in potential during an ESD event. According to 3M, the difference in potential between the antenna and the back plate triggers the LCD, which will then change from clear to red or blue in color. This finding indicates that an ESD event has occurred. An ESD simulator set between 1,000 to 5,000 volts or greater touches the package to represent a charge carried by a person. At the same voltage, blunt tools will throw a longer and more pronounced miniature lightning bolt (see Figure 5).
Figure 5
Table 4 displays the results of various ESD corrugated technologies. Moreover, this finding indicates that the lower the surface resistance of the shielding barrier, the better the shielding effectiveness. Space restrictions limit showing the evaluation process.
Table 4: Kolyer’s High Voltage Discharge Resistance Test
A static shielding container using a foam or thermoformed insert should not tribocharge the circuit board. The ESD corrugated should not become a static generator in low relative humidity (Table 5). A static shielding barrier is needed to attenuate (shield) the container from a discharge. Therefore, conventional corrugated without shielding layers or barriers would be a poor package in certain environments outside the ESD Protected Area.
Table 5: Resistance vs. RH
Kraft Corrugated Depending upon the porosity of the paperboard and moisture content, Kraft corrugated will remain static dissipative above 15%+/-6% relative humidity (RH). However, if Kraft paper is tested at ESD Association Standard conditions (12% RH +/-3% RH) using ANSI/ESD STM11.11-2001, the material measures insulative and is a potential charge generator. Extensive testing with paper and the relationship between relative humidity and surface resistance is well known. Table 5 illustrates regular Kraft (brown) paper’s performance at low relative humidities. (Note: white paperboard (bleached corrugated liner) becomes insulative at or below 20% to 23% RH, “A Corrugated Study,” Larry Fromm, PE and R. Vermillion CPP, 1999.)
Some companies will not allow untreated paper in their ESD environment. In flight and during dry conditions, conventional corrugated containers can charge by friction (shock and vibration) between the Kraft interior surfaces and the circuit assembly. Thus, in low RH, charge can be induced or transferred on the product.
ESD Corrugated Requirements A static shielding barrier of <1.0 x 103 ohms reflects the shielding barrier’s benchmark resistance value as reflected in a growing number of electronic organization’s adherence to ANSI/ESD S541-2003. The ESD-protective finished surface of the corrugated sheet or box should be rub resistant. An ESD static shielding corrugated container’s outer surface (when employing a static dissipative finish over a buried <1.0 x 103 shielding barrier) should not generate static charges at 12%+/-3% RH.
The surface resistance standard test method per ANSI/ESD STM11.11-2001 (Figure 6) measures the resistance of a corrugated material’s surface as measured on an insulative substrate. Volume Resistance per ANSI/ESD STM11.12-2000 requires grounding of the conductive surface ring and test plate to measure through the material.
Figure 6
Reducible Sulfur Levels Reducible sulfur of less than 8 parts per million (per TAPPI 406 om-94) is considered safe for ESD shielding corrugated. This method involves the reduction of various forms of sulfur to hydrogen sulfide and the development of a dark spot for lead sulfide on the filter paper impregnated with lead acetate. The intensity of the spot is compared with spots developed from standards and is proportional to the concentration. Sulfur combines in the pulping process of paper making to break down the wood fibers. Sulfur can be a contaminant to sensitive electronic components. Overall, recycled liners have low levels of sulfur as compared to virgin fiber.
Rubbing Abrasion Resistance ASTM D 5264-92 using a Sutherland Rub Tester has the specimen mounted on top of the rubber pad of the Sutherland base and the receptor is cut to fit the 4-LB weight. The receptor is mounted to the weight. The test duration is determined by 50 strokes; a stroke is one back-and-forth cycle, traversing over a specimen (Figure 7). The number of strokes desired is preset on the Sutherland timer. The weight is mounted on the Sutherland and the machine is turned on. The Sutherland will shut off automatically when the desired number of strokes is completed. Sloughing of conductive particles could bridge the gap between circuit lines and short out a board. Note: Non-protected Rod-coated or post-printed ink conductive coatings can be removed very quickly (Figure 7).
Figure 7: Exposed carbon or graphite can bridge the gaps of circuit lines and cause a short (Mexico Carbon Coated Material)
Triboelectric Charging This controversial test is erratic and difficult to reproduce. One method is rubbing two objects together and reading the values. An incline triboelectric test for ESD corrugated sheets has not provided consistent readings. The said method employs a 1.0” quartz and Teflon cylinder released on a 17 inch long elevated incline at 150 on top of the affixed ESD corrugated material. Unlike thin-filmed materials, corrugated is porous on the surface. A microscope will illustrate the porous and wavy nature of corrugated.
NASA Triboelectric Test Robot The operation of a robot is controlled from outside an environmental chamber with monitoring by television cameras inside the chamber. The display panel displays the position of all moving systems on the robot. The scientific concept used in this third generation test device is basically similar to the first manual test device reported in the ESD Symposium Proceeding of 1984 (Gompf 1984). The 7.5-inch sample to be tested is held in a metal sample holder (similar to a large photographic slide) and the slide is placed into the maneuverable sample carrier rack. NASA’s track record of 23,000 evaluations over the year has occurred without incident according to Dr. Ray Gompf, PE, Kennedy Space Center (Ret.).
The sample to be tested is positioned alongside the sample carrier system that moves the test specimen to the front of the rubbing wheel. The 5-inch diameter rotating wheel surface is normally a soft felt-Teflon surface backed with 1-inch of soft foam. This wheel retracts backwards and forward, making intimate contact onto the sample with a force of 3 pounds. It is rubbed at 200-rpm for exactly 10 seconds before the rubbing wheel is quickly retracted and the sample is slid in front of the sample electrostatic detector head. The electrostatic detector head measures the electrostatic voltage generated and records the voltage versus time over an 8 second period or less. The sample carrier shuttle then returns the specimen into the sample carrier rack whereby and the carrier rack advances to the next specimen. This cycle is repeated again until all samples have been tested (Figure 8).
Figure 8: NASA Test
Electrostatic Decay This test method measures the rate of decay of a charged isolated object to 10 percent of its original value. Federal Standard No. 101, Test Method Number 4046, specifies that the charged object at +/- 5000 volts should drain the voltage to +/- 500 volts in less than 2.0 seconds. Recently, a more common voltage range at +/-1000 volts to +/-100 volts has been incorporated for a decay time of less than 2.0 seconds. Such tests have difficulty with materials of complex construction. Such materials are ESD convoluted foams, vacuum-formed polymers and small items that could fall below the measuring range of the testing equipment’s fixturing. Therefore, this test represents a material’s ability to dissipate induced voltage with proper grounding.
According to section 30.5 of the Military Handbook-263A, Appendix, H, this test does not always typify real world events. ESD corrugated that has thin layers or a conductive surface acts differently from homogeneous or layered products. However, it is relatively effective for correlation purposes in a controlled environment.
Other Performance Considerations It is no longer acceptable in Tier Level 1 European countries to receive disposed cartons at a recycling center without proper recycled content and safeguards. In evaluating scrap corrugated or used ESD corrugated, a large paper mill with several recycling centers utilized a method of cutting ESD corrugated specimens into pieces, mixed with hot water in a high speed disintegrator (blender) and blended for a five minute period. This sequence of tests resulted in 1-1/2% solids and water. The mixture was poured through a screen to simulate the Fourdrinier or Cylinder-making paper process and allowed to dry for evaluation. Paper technologists can determine from hand sheets the acceptability of used paper for repulping. As seen in Asia, the continued recycling of products raises the concern that shorter fiber lengths can result in decreased mechanical strength.
ESD Testing Methods—Vacuum-Formed Trays Antistatic Transfer Antistatic transfer of surfactants (containing amine agents/fatty acids) can cause stress cracking of the polycarbonate structure of a FR4 circuit board. Mirror fogging, reduced soldering capability and discoloration can take place over time. One method of testing is to press an antistatic material (over a given time) against insulative plastic. It has been estimated an acceleration factor for the MIL-Spec accelerated aging test to be approximately 17 that is one day at 1600 F equals seventeen days at room temperature. An antistatic polymer containing amines can be a cause for concern. Inherently conductive polymers contain no amines and are subjected to other evaluations for contamination.
ANSI/ESD STM11.13-2004 for Small Profile Materials It is important to discuss the released ANSI/ESD STM11.13-2004 standard test method for small profile ESD materials that fall outside the measurement range of a concentric ring fixture as specified in ANSI/ESD STM11.11-2001 (Figures 2 and 6).
ANSI/ESD STM11.13-2004 constitutes an evaluation method (Figure 9) for measuring the draw of a vacuum-formed tray. Figure 10 illustrates what happens in the vacuum forming process. A draw is produced that elongates the material to reduce material thickness, which can lead to a loss of favorable electrical properties. Table 6 shows a “passing” material tested at 11.5% RH, 73.40 after 52.1 hours of conditioning.
Figure 9 (photo courtesy of Noveon)
Table 6: ANSI/ESD STM11.13-2004 (Limit: <1.0E+11 ohms)
Figure 10
Non Contact Voltage Measurement The technique for pin-pointing hidden charges (hot spots) is accomplished by using a non-contact voltage measurement device as seen in Figure 11. ANSI/ESD S20.20-1999 limits voltage at the workstation to <+/-200 volts. For example, a disk drive company may limit the voltage to <+/-10 volts, whereas a microprocessor manufacturing organization will set its threshold at <+/-100 volts.
Figure 11
Typical results are captured in Figure 12 and summarized in Table 7 of this article. The voltages are well in excess to +/-200 volts per ANSI/ESD S20.20-1999.
Figure 12
Table 7
Static Decay Again, this test measures the rate of decay on a charged isolated object at 10 percent of its original value. Federal Test Method Standard No. 101C, Test Method Number 4046 specifies that the charged object at +/- 5000 volts should drain the voltage to +/- 500 volts in less than 2.0 seconds (Figure 13). Recently, +/-1000 volts to +/-100 volts has been incorporated to measure static decay in less than 2.0 seconds.
Figure 13
A modification of the Fed Std. 101C, Method 4046 for non-planar materials (which falls outside the fixturing of a static decay instrument as seen in Figure 13) for the disk drive and semiconductor industries is the use of a charge plate monitor (Figure 14). It is not a standard, but representative of an industry practice. Tables 8 and 9 illustrate the results using a disk drive decay parameter of +/-1000 volts to +/-10 volts in less than 2.0 seconds. (Note: The Inherently Conductive Polymer (ICP) Tray is of complex construction with electrical continuity through the material.)
Figure 14
Table 8: Fed Std. 101C, Mtd. 4046 (modified)
Table 9: Volume Conductive Tray
Overall, the tray performed within expectation levels after 72 hours of preconditioning at 10.5% RH and 72.50F.
Faraday Cup Measurement ESD Adv. 11.2-1995 All trays were low charge generating upon being dropped into a Faraday Cup after being charged to +1000 volts on a charge plate monitor (see Figure 16).
Figure 15
Figure 16
Table 10: 12.3%RH, 73.10F after 72 hours conditioning
Table 11: Many electronic organizations set an upper limit of <1.0nC to be considered passing. This table illustrates the tray’s passing performance.
Conclusion In short, the requirement to ensure your organization’s adherence to sound ESD practices is essential. Table 12 supports technology advancements since ESD protection at the chip level could be sacrificed for performance and speed. The control of electrostatic fields and events is a most difficult task. However, the use of ESD testing protocols and understanding ESD material characteristics can assist in the development of an ESD Preventative Program that incorporates sound procedures for qualifying static control products and materials. Utilizing ANSI/ESD S541-2003 represents a good first step in the implementation of an ESD materials qualification program. g
Table 12: ITRS Technical Requirements—Electrostatics
Figure 17
References 1. ESD from A to Z, Dr. John Kolyer and Donald Watson, 2nd Edition, Chapman & Hall, 1996
2. Robert B. Rosner, 3M Electronic Handling & Protection Division, Austin TX 78726
3. Mil Handbook 1686C-1995
4. Mil Handbook 263A, 22 February 1991/2 May 1980, p. 46
5. Mil Handbook 263B-1994
6. EIA 541 1988 Appendix C, “Triboelectric Charge Testing of Intimate Packaging Materials,” Electronic Industries Association
7. ESD Association Standards:
8. “An ESD Corrugated Study,” Larry Fromm, PE, EE and Robert Vermillion CPP, EOS/ESD Proceedings, 1999
9. Dr. John M. Kolyer, Ph.D., Rockwell International, Telephone interview in 1996
10. The Charged Device Model & Work Surface Selection, John Kolyer and Donald Watson, October 1991, pp. 110-117
11. EIA 541 1988, “Packaging Materials Standards for ESD Sensitive Items,” Electronic Industries Association
12. “Humidity & Temperature Effects on Surface Resistivity,” John Kolyer and Ronald Rushworth Evaluation Engineering, October 1990, pp. 106-110
13. Tappi-T-406, 1983, Reducible Sulfur to be less than 0.0008% & Nontarnishing to Silver, Solder & Copper
14. “Packaging for High-Voltage Discharge Protection,” John M. Kolyer and Donald E. Watson, Evaluation Engineering, March 1992, pp. 96-100
15. “Triboelectric Testing at KSC Under Low Pressure and Temperature,” Dr. Ray Gompf, PE, ESD Association Proceedings 2002
16. ITRS 2006 Factory Integration Chapter, 8 July 2006—The ITRS is devised and intended for technology assessment only and is without regard to any commercial considerations pertaining to individual products or equipment. For more information, go to www.sematech.org.
About the Authors
Bob Vermillion, CPP, is a Certified ESD & Product Safety Engineer-NARTE and holds a US Patent with several patents pending. One of his recent developments has been approved for a NASA Mars Mission. Bob is a member of the ESD Association Standards Committee and ESDA Packaging Working Group 11 coauthoring ANSI/ESD S541-2003. Bob conducts ESD Seminars in the USA and abroad as well as California State Polytechnic University, San Jose State and Clemson Universities. RMV Technology Group, LLC, a member of the American Council of Independent Laboratories, provides ESD materials testing, training, cleanroom and facility troubleshooting/auditing. Bob can be reached at 925-673-0225 or bob@esdrmv.com.
Albert Escusa is a Packaging and Supplier Quality Engineer at Texas Instruments Semiconductor Division. Albert leads a worldwide team which is responsible for packaging design, quality improvement and supplier management. He is a certified ISO/TS16949 Lead Auditor and holds a US patent with additional patents pending related to packaging. Since joining the company in 1987, he has also worked as post test equipment engineer, process engineer and R&D engineer. He has a bachelor’s degree in Electronics and Communication Engineering from Saint Louis University-Philippines. Albert can be reached at 972-917-2670.
The authors also wish to thank Kurt Edwards of Noveon Static Control for supplying vacuum-formed specimens.
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