Comparing Antistats versus Inherently Conductive Polymer Coated Type I Moisture Barrier Bags for Humidity Dependence & Charge Generation at Low RH
Electrostatic Discharge (ESD) protective packaging sourcing is not only restricted to the USA and Europe. Products from the Pacific Rim provide end users with the opportunity to secure 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 sacrificed quality for price or purchased packaging that is “suspect counterfeit.” Figure 1 illustrates a non-conforming aluminum Type 1 moisture barrier bag utilized inprocess.
Some critical issues resolved in the early 1990s for static control packages are now resulting in corrective actions for ESD non-conformance, contamination, or suspect counterfeiting.
As recent events dictate, reliance upon the U.S. Federal Government for oversight must be supplemented by user due diligence. Democratic lawmakers boldly stated that the U.S. Food & Drug Administration (FDA) has insufficient funding and power to maintain appropriate oversight and regulation of imported products.1 Medical device packaging engineers need to design ESD control packaging that will protect medical devices from Tribolectrification while ensuring attenuation (shielding) from high voltage discharges, and, at the same time, are contamination free. Semiconductor and disk drive manufacturers define their ESD packaging requirements for contract manufacturers to ensure validation by means of annual on-site assessments. Consequently, ESD certification training combined with 3rd party supplier package testing is practiced before products are placed on a materials qualification list (MQL).
Approximately sixty percent of the author’s testing of offshore ESD safe packaging, especially polymers, has fallen short in meeting minimal requirements set forth in ANSI, EIA, IEC, ASTM, Military Standards, and JEDEC requirements. Moreover, reference to ESD protocols for ASTM Flexible Barrier Packaging and ISO requirements appear minimal, at best.
Sound ESD safe medical device packaging needs to be a component of an overall risk assessment program:
Risk Assessment Categories2
- Material Qualification
- Supplier Quality
- Package Design and Testing
- Equipment and Systems
- Process Design and Control
Space agency and disk drive environmental requirements for cleanliness (sterility) or chemical compatibility are similar. ISO Class 3 to 5 cleanrooms utilize ESD/ESA3 (Electrostatic Discharge/ Electrostatic Attraction) compliant materials that are non-contaminating and free of antistats. However, many ESD protective moisture barrier bags contain antistatic overprotective coatings. Topical antistats do not always maintain favorable properties and can pose problems with “Electronic Part Damage by Antistat Vapor.”4 Figure 2 illustrates droplets and damaged areas on stored devices for a 26 micrometer wide substrate.
As recently as 2008, the aerospace sector has seen launch delays and issues in space due to non-compliant materials. Since the business model now includes outsourcing with contract manufacturers or subcontractors, in-house validation has been replaced by trust upon the supplier to do the right thing. Consequently, a vendor technical data sheet can prove meaningless unless subjected to in-house or 3rd party validation.
To compound the matter one is more than likely to see Class 0 devices (an ESD sensitive device <250>® Itanium® processors (codenamed Tukwila) in 2009, figure 3.
Amine surface resistance data from Dr. Marv Havens’ “The Chemistry of Dissipative Plastics” established that antistats become insulative on the fourteenth day. This finding calculates to a life expectancy or shelf life of about 238 days (approximately seven months).6 Other more severe test methods cut this figure down to only a 90 day life expectancy to maintain a surface resistance value below the upper cutoff limit of 1.0 x 1011 ohms per ANSI/ESD S541 (preconditioned for 48 to 72 hours at 12% +/-3% RH, 73°F +/-5°F). Dr. John Kolyer, (Boeing, retired) in his book, “ESD from A to Z” discusses the problems that antistats pose by facilitating tribocharging at less than 10% RH. It should be noted that MIL-PRF-81705 accelerated aging tests equate one day at 160°F to seventeen days at room temperature.7 Antistatic transfer of surfactants (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. If the plastic becomes antistatic, then surfactant transfer has taken place. Inherently Conductive Polymers (ICPs) contain no antistats so there is low potential for outgassing.
During air transport, RH can drop to 9.33% after 20 minutes in-flight (Figure 4). A 7"x 7" sheet of corrugated Kraft paperboard (cardboard) charged up to -4403 volts. At 50% RH, the same sheet charged <10>
Several ANSI/ESD S541-2008 (an ANSI/ESD S20.20- 2007 document) definitions are important to review.
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 S541-2008, 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 S541-2008 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 S541-2008 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:
- Low Charging (antistatic)
- Shielding a. Electrostatic Discharge b. Electric-field
In order to duplicate moisture barrier bag conditions, testing is conducted at 3.5%RH after 72 hours of pre-conditioning which is below 12%RH, +/-3%RH at 73°F, +/-5°F.
ESD TEST PROCEDURES: SURFACE RESISTANCE VERSUS RELATIVE HUMIDITY (RH)
One misconception by organizations is the assumption that ESD testing at high and low relative humidities will produce similar measurements (Figure 5). Arguably, this may be true for conductive materials and humidity independent technologies, but materials that are dependent upon humidity to facilitate electrical conductivity such as antistatic topical coatings will produce significantly different readings at 50% RH when compared to ESD testing at 12% RH.
Table 1 constitutes measurements for a humidity independent static dissipative coating while Table 2 illustrates findings for a topically treated Type 1 bag structure.
Figure 6 is representative of actual field testing of a moisture barrier bag in a production environment. Note: This article will focus on resistance measurements, charge generation (simulated circuit card pull test, electrostatic decay, and limited electrostatic shielding findings).
Despite company requirements for a static dissipative Type 1 bag, at 40.9%RH, the reading was 3.5 x 1012 ohms—an insulative or failing result. Table 3 represents a production assembly 3-seam non-conforming Type 1 bag evaluated at 11.9%RH after 48 hours of preconditioning—also a failing result.
In addition, a field sample, in which six new bags were secured from an ESD sensitive area, produced insulative results at 12%RH. At 3.5%RH, which realistically represents the relative humidity seen inside a moisture barrier bag, the antistatic over-protective coating in Table 2 was rendered ineffective at low RH.
A bag qualifier needs to consider how a static shielding structure will be utilized. For example, long-term storage requirements compared to external transport where temperature and relative humidity will fluctuate. The requirements may need to be more stringent than the existing pre-conditioning requirements of ANSI/ESD S541-2008. Thus, in special cases, testing a Type 1 bag at <4%rh>
BAG DESIGN FOR ESD SOUNDNESS
A 3-seam Type 1 ESD bag is widely used in the market even though it does not facilitate electrostatic decay. Moreover, this design does not promote top to bottom electrical continuity or charge bleed-off. Bag seam adhesives are rarely static dissipative so that the top surface acts as a floating isolated conductor. “Knock offs” of brand name products will most likely use a 3-seam structure (Figure 7). However, do not assume that a popular brand name constitutes adherence to ANSI/ESD S541-2008. Considerations for recylability, amine free, and RoHS requirements must be considered. The Restriction of Hazardous Substances Directive (RoHS) 2002/95/EC is not a law; it is a directive. This directive restricts the use of six hazardous materials in the manufacture of various types of electronic and electrical equipment. Each European Union member state will adopt its own enforcement and implementation 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.9 RoHS is often referred to as the “lead-free” directive, restricting the use of the following six substances: Lead, Mercury, Cadmium, Chromium VI, PBB, and PBDE. These distinct policies will make it increasingly difficult to find consistency in ESD packaging and validate materials.
In the October issue we will continue the discussion including Electronic Decay and Static Sheilding.
- NewsInferno.com, Thursday, May 15, 2008
- Hal Miller, Pace Solutions, LLC HealthPack on 4–6 March 2008
- Electrostatic Attraction
- J.M. Kolyer, Ph.D., A.A. Passchier, Ph.D. and E.G. Peterson, The Boeing Company, ESDA Symposium 2001
- Intel website
- EOS/ESD Symposium in 1989 by Dr. Marv Havens
- EOS/ESD Symposium in 1989 by Dr. Marv Havens
- ANSI/ESD S541-2008 [Packaging and Materials Document]
- Wikipedia Encyclopedia
- ESD from A to Z, Dr. John Kolyer and Watson, 2nd Edition
- Mil Handbook 1686C-1995
- Mil Handbook 263B-1994
- Packaging Materials Standards for ESD Sensitive Items, EIA-541, June 24, 1988, Appendix C, "Triboelectric Charge Testing of Intimate Packaging Materials"
- ANSI/ESD S20.20-2007
- ANSI/ESD STM4.1-2006
- ANSI/ESD STM11.11-2006
- ANSI/ESD STM11.31-2006
- ANSI/ESD S541-2008; EIA 541-1988 and ESDA Adv. 11.2-1995
- Humidity & Temperature Effects on Surface Resistivity, John Kolyer and Ronald Rushworth, Evaluation Engineering, October 1990, pp. 106 -110
- Triboelectric Testing at KSC Under Low Pressure and Temperature ESD Association Proceedings 2002, Dr. Ray Gompf, PE
- ITRS Technical Requirements – Electrostatics, The ITRS is devised and intended for technology assessment only and is without regard to any commercial considerations pertaining to individual products or equipment
- Intel Website, Moore’s Law
- A Formalized Materials Qualification Process is Essential in Preventing Non-Conforming or Suspect Counterfeit Packaging that Can Lead to ESD Hazards During the Parts Inspection Process, Long-Term Storage Issues and Failures in Manufacturing, Bob Vermillion, Interference Technology, May 2010
- MDA Lessons Learned, The Use of Pink Polyethylene Antistatic Bags, GIDEP
- Electronic Part Damage by Antistat Vapor, John M. Kolyer, Arie A. Passchier and W. Greg Peterson, The Boeing Company , 2001
- Non-compliant or Suspect Counterfeit Materials Can Lead to ESD Hazards and Long-Term Storage Issues, Bob Vermillion, NASA QLF, Kennedy Space Center, 18 March 2010
Author Note: Special Thanks to Daibochi Plastics for providing Humidity Independent Type 1 Moisture Barrier Bag Specimens and Selected Illustrations and Data
Bob Vermillion, CPP/Fellow, is a Certified ESD & Product Safety Engineer-iNARTE, is a leading international ESD materials application expert. One of Bob’s ESD developments has been Mars Mission approved. Bob conducts instrument driven assessments for inprocess systems and robotics for the aerospace, disk drive, medical device and pharmaceutical, automotive, and semiconductor sectors. A co-author of several ANSI level ESD documents, Bob serves on the BoD with iNARTE, is a member of the ESD Association Standards Committee and conducts ESD Seminars in the USA and abroad, including guest speaker engagements for California State Polytechnic University, San Jose State University, University of California at Berkeley and Clemson University. November 17 - 19, 2010, Bob will conduct a Seminar at Oxford University on how Non-compliant or Suspect Counterfeit Packaging Materials Can Lead to ESD Hazards and Long-Term Storage Issues. Bob is Chief Technology Officer of RMV Technology Group, LLC, a 3rd Party ESD Materials Testing and Consulting Company located onsite at NASAAmes Research Center, Moffett Field, California. You can reach Bob at 650-964-4792, or email@example.com