CHARGED DEVICE MODEL
It may seem to some that CDM has newly arrived as a problem for ESD control programs. However, the ESD Association first published ANSI/ESD STM5.3.1 in 1999 – ESD Association Standard for Electrostatic Discharge Sensitivity Testing – Charged Device Model (CDM) – Component Level. Basically, CDM testing has to do with “testing, evaluating and classifying the electrostatic discharge (ESD) sensitivity of components to the defined charged device model (CDM)” … “to allow for accurate comparisons of component CDM ESD sensitivity levels.”
JESD22-C101C Field-Induced Charged-Device Model Test Method for Electrostatic-Discharge-Withstand Thresholds of Microelectronic Components Table 3
Devices shall be classified as follows:
CLASS I <200 volts
CLASS II 200 to <500 volts
CLASS III 500 to 1000 volts
CLASS IV >1000 volts
The importance of CDM came about primarily because of the increased use of automated component handling systems. The Foreword of ANSI/ESD STM5.3.1 states “In the CDM a component itself becomes charged (e.g., by sliding on a surface (tribocharging) or by electric field induction) and is rapidly discharged (by an ESD event) as it closely approaches a conductive object.”
In November 2002, Roger Peirce published an article entitled “The Most Common Causes of ESD Damage”. There were actually 23 causes. As the founder and president of ESD Technical Services, Roger had investigated hundreds of companies for over eight years. All 23 causes were CDM failure modes. So CDM is really not so new, it has just received a lot of attention in the last few years.
So, what are the things companies should look at to improve their ESD control program regarding CDM? It would seem to be easy: don’t slide ESDS devices and assemblies unless grounded at all times, keep insulators at least 12” away from ESDS, and don’t allow ESDS items to make contact with a conductive surface. Seems simple, but in actual application . . . not so easy.
If the ESD control program has not used ionization that should be considered. If the ESDS items becomes charged, ionization will help neutralize the charge. The primary function of ionizers with regard to ESDS items are:
- To remove / neutralize charges from process necessary insulators, which can charge ESDS items, thus creating the potential for a damaging CDM event
- Remember that the PCB substrate is a process necessary insulator and can become charged during automated handling processes
- To remove / neutralize charges from a charged, isolated/floating conductor, which, when grounded can result in a potentially damaging CDM event
- Remember that during automated handling processes, the ESDS devices on the PCB are isolated or floating conductors
Use an Overhead Ionizer to neutralize charges at your workstation.
The ESD Standards Committee has a Working Group (WG-17) which is currently involved with developing a Standard for Process Assessment to help the electronics community assess their manufacturing and handling processes to determine what levels of devices their process can handle. Once one fully understands where their process is with regards to ESDS devices and assemblies, they will have a clearer picture on what actions need to be taken to further improve the ESD Control Program.
If ionizers are already in use, the company should consider reducing the ionizer offset voltage limit of ±50 volts (the required limit in ANSI/ESD S20.20) to ±25 volts and maybe less, depending on the application and device sensitivity. Discharge times are user defined and should be considered for reducing the time required to neutralize a ± 1,000 volt charge to ± 100 volts.
The required limit for worksurfaces per ANSI/ESD S20.20 is less than 1 x 10^9 ohms with no lower limit. Most companies handling electronics should be following the recommendation of Worksurface standard ANSI/ESD S4.1 that the lower limit be 1 x 10^6 ohms. To combat CDM failures, all surfaces that might come into contact with ESDS items should be dissipative at the 1 x 10^6 to less than 1 x 10^9 ohms range used for worksurfaces where possible. Items such as Static Shielding bags will have a higher resistance on the interior & exterior surfaces, but it still must be less than 1 x 10^11 ohms.
Use a Statfree Dissipative Mat on your workstation.
From published article “Now is the Time for ESD Control Programs to be Improved” by Fred Tenzer and Gene Felder. See full article at InCompliance Magazine- September 2012
ANSI/ESD S20.20 Foreword states:
- “This standard covers … electrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 volts Human Body Model (HBM).”
- “When handling devices susceptible to less than 100 volts HBM, more stringent ESD Control
Program Technical Requirements may be required, including adjustment of program Technical
Element Recommended Ranges.”
HMB Classification Class 0 is:
Per ESD-STM5.1 Human Body Model (HBM) Table 1 Class 0 has ESD Voltage Range < 250 Volts
Basically, to control the environment to decrease the probability of ESD damage in “Class Zero”
situations, involves increasing ESD protective redundancies and periodic verifications to those ESD
Control technical elements.
- Personnel: Decrease Wrist Strap and ESD Footwear upper limit permitted (The ESD Association has test data showing charge on a person is less as the path-to-ground resistance is less) The use of continuous monitors, smocks, use / increased use of ESD flooring, sole or full coverage foot grounders (HBM & CDM)
- Worksurfaces: Dissipative (CDM) i.e. change < 10^9 to a requirement of 10^6 to 10^8 ohms
- Bonded grounds – Carts, shelves, equipment
- Conductors: Minimizing isolated conductors like devices on PC Boards (CDM)
To see examples of Wrist Straps capable of dealing with class zero environments Click Here
To see examples of Grounding Cords capable of dealing with class zero environments Click Here
Minimize Charge Generation
The best form of control is to minimize charge generation. Grounding and ionization eliminate charges once generated. Shielding protects from generated charges.
- Personnel – Low Charging floor finish
- Surfaces – Use of low charging (anti-static) topical treatments
- Eliminate as best as possible all non-process necessary insulators
- Topically treat where ever possible insulators that cannot be removed
- Consider use of ESD Chairs or treat to reduce charge generation
- Shield charges on clothing by using ESD Smocks
To continue reading ESD Control Program Considerations when Dealing with
Class Zero Items Click Here
Desco Industries Inc. (DII), Employee Owned
Reproduced with Permission, EE-Evaluation Engineering, November, 1999
I. Introduction to ESD Control Programs
ESD Control programs are an essential part of a quality process and are always needed when handling ESD sensitive electronic/semiconductor devices. The extent of the ESD Control program is determined by the ESD Sensitive (ESDS) devices themselves and how they are handled. Refer to article “How to Set Up an ESD Control Program”  for additional information.
One of the main reasons that companies deploy ESD Control programs is to save money. Increased throughput and decreased scrap can yield a Return On Investment (ROI ) of up to 1,000% per . A secondary reason is to comply with their customers’ and ISO 9000 type programs’ requirements. Whatever reason, setting up and implementing an ESD Control program will almost always produce favorable financial results.
II. Cost Reduction via ESD Control Programs
Having ESD awareness and following through with an ESD Control program is essential in reducing quality failures due to ESD. ESD can affect product reliability with catastrophic damage which is readily apparent to latent degradation. Latent degradation is particularly expensive requiring costly inspection and rework cycles in-house or product failure in the field. Maintaining good ESD controls will improve product throughput or yield, increasing reliability in the field which improves customer satisfaction leading to increased future business.
One test equipment manufacturer noted that GMR heads were being damaged during or after testing. These heads are extremely sensitive to ESD, and require additional handling precautions.
It is very important when designing and implementing an ESD Control program to know the ESD susceptibility of the ESD Sensitive (ESDS) devices you are trying to protect. Classification of these devices should include all simulation models human body model (HBM), Machine Mode (MM), and Charged-device Model (CDM) that will properly characterize the devices’ sensitivity when handled at various locations within the facility . This will allow for the most economical program design.
Gene Chase, an ESD Consultant with ETS Inc., is quoted as saying “Millions of dollars are lost every year due to ESD . Many of these incidents occur within the computer and communications industry.” Examples of losses from ESD may be any of the following:
- Lost Time
- Loss of Connection
- Loss of Data
- Shocks to Personnel
- Upset to A System Requiring A Re-Boot
- Damage to Equipment
- Equipment Hardware Failure
To properly determine the return on investment (ROI) from your ESD Control program, you must collect return, repair and scrap cost data before and after implementation.
To continue reading ESD Control, Return on Investment Click Here.