ESD Best Practices Print E-mail
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Written by Carl Newberg   
Monday, 06 December 2010 16:28

Essential features of robust ESD control programs per ANSI/ESD S20.20.

What does best practice mean? It boils down to a quality system that results in the highest yield and the highest quality product. In the case of ESD control, it not only means high yields, but also no hard or latent defects in parts shipped to customers.

An excellent place to start for protecting ESD-sensitive devices is to implement an ANSI/ESD S20.20 static control program. ANSI/ESD S20.20 (hereafter called S20.20) requires two main areas be specified: administrative requirements and technical requirements. A careful review of S20.20 reveals some overlap in the requirements; however, they support each other well. Administrative requirements define the program and many of its necessary “non-technical” elements (the first four items below); the rest is documentation of the technical requirements that make up the program.

The administrative requirements in S20.20 define an ESD control “plan.” The plan must document several items:

Device sensitivity determination. A first step is to understand how sensitive your products are to electrostatic discharges. It is important to have some idea of both Human Body Model (HBM) and Charged Device Model (CDM) sensitivities because, in most cases, these will be very different. While knowing the exact withstand voltage of every device is not necessary, know the general sensitivity of the parts handled. For instance, if manufacturing boards with standard CMOS-like components, the typical HBM withstand voltage will usually be in the 1000s of volts range, and the CDM values will usually be 500 - 1500V. This ESD control program would look very different than one in the hard disk drive industry, for example, where handling of parts with reportedly less than 1V CDM ESD thresholds is becoming common. The current HBM and CDM test methods loosely classify device sensitivities (Table 1).



Industry has recognized all devices have become more sensitive over the past 20 years, yet the classifications haven’t changed. Most ESD practitioners realize many of the most sensitive parts fit in a subset of the most sensitive classifications (HBM Class 0 and CDM Class C1). It is especially important to know if you have any of these devices. As far back as the mid 1980s, AT&T researchers realized they were manufacturing and handling parts well below the lower threshold of the most sensitive ESD classifications used at that time and adopted the phrase “Class 0,” which referred mostly to CDM values (even though CDM was a barely-heard-of ESD failure mechanism at the time). They did this to highlight that these devices needed extraordinary protection beyond the standard ESD control programs at the time. More recently, “Class 0” has been used in various areas of the industry. While some ESD control practitioners have objected to the use of the term, most who do use it realize it simply means “a very sensitive device.” While further discussion of the “Class 0” classification is beyond the scope of this article, suffice it to say that if you believe you are handling what the industry has been calling “Class 0” ESDS parts, at a minimum, develop a robust ESD control program or expect yield and reliability issues.

Assignment of an ESD program manager/coordinator. The organization must document the appointment of a coordinator or manager of the ESD control program. The purpose of this is to require management to take the ESD control program seriously enough to assign a person to this role. This is similar to the ISO 9001 requirement to assign a quality manager responsible for maintaining the quality system of an organization. The ESD coordinator is the focal point for the management of the ESD control program. This person is not required to have a certain level of training or certification; however, additional training in the technical details of ESD and ESD program management would be beneficial.

Training plan. The organization must have, and properly document, an ESD training program. The method of training, frequency of recurrent training and location of training records must be documented. In addition, a method of measuring the employee’s comprehension of the training material must be documented. This “test” can be a written test, on the job observation or another measurable method. The results of each test must be recorded and stored along with the training records. The training plan portion of S20.20 is the most common cause of formal assessment failures during an S20.20 assessment.

Compliance verification program. There also must be a documented compliance verification program for the organization. It is recognized that ESD control items and procedures lose their effectiveness if not continuously maintained. A defined and implemented compliance verification program ensures the ESD control program elements stay working during the life of the program. Some companies have chosen to have the compliance verification done by their own employees, while others have chosen outside vendors (such as a calibration test company). In any case, the more complicated the ESD control program, the more attention that needs to be paid to compliance verification.

For instance, a well-known disk drive manufacturer has four levels of compliance verification. The first lines of defense are the manufacturing operators. They are trained to do a visual check of their ESD controls at the beginning of each shift. They check not only their own wrist strap, footwear and garment systems, but they also do a visual check of their workstation, looking for ground wires, ionization discrepancies and non-ESD approved materials. The second level of compliance verification is a department technician that does a daily, weekly or monthly check of all ESD controls. An AQL-type of audit (statistical sampling) may be done on a frequent basis. However, enough items are inspected on a regular basis so that in a specified time frame all the items are checked. The third level is an audit performed by the ESD coordinator, ensuring inspections by the department technician are completed, and then spot-checking the ESD controls in each area. The fourth and final level is done by a third-party auditor on an annual basis. The compliance verification methods must be technically equivalent to those documented in ESD TR53, and the equipment used for the testing must be documented properly.

It is important to note that the most successful compliance verification programs are those that are regularly reviewed by management through reports that are made available to them, with subsequent follow-up to close corrective actions.

Grounding system. The system used to ground all conductive elements must also be defined. S20.20 describes three grounding systems: equipment (AC) ground, auxiliary ground or equipotential bonding system. One or more of these systems must be defined as the grounding system in the plan. This is followed by implementation of the grounding system defined. Many companies use more than one of the grounding systems. For instance, they may define the equipment (AC) ground as the primary method, but they may also define areas that use equipotential bonding for areas where the equipment ground is not available.

The second issue that must be documented as well is how personnel are to be grounded. Most companies simply state that all personnel wear a grounding wrist strap whenever working on ESDS items. Some allow the use of footwear/flooring if operators are standing. S20.20 requires seated operators to always be wearing a grounding wrist strap; any deviation requires a tailoring statement. (See below for “tailoring.”)

Documentation of the EPA (Electrostatic Protected Area) elements. This can be the largest section of the plan, as it defines the balance of the technical elements of the ESD control program. However, it can also simply refer to the S20.20 elements. Not all elements specified in S20.20 are required elements. However, if an element is documented in the plan, it then becomes an auditable requirement by an S20.20 assessor. An example of this is the use of garments. Many companies like to use garments in their factory for a variety of reasons. However, many don’t want to do compliance verification of the garments. If garments are formally documented in the ESD control plan, then compliance verification must be performed. An option is to require the use of garments somewhere besides the plan, which would avoid audit.

Another element typically addressed here is the flooring/footwear system. Table 3 in S20.20 specifies that the total resistance of an operator must be below 35MΩ whenever they are not using a wrist strap system. If the resistance is above 35MΩ, more testing must be done to verify that they do not generate more than 100V when walking. This is one of the most misunderstood sections of S20.20.

Packaging. The organization must specify packaging requirements for parts moving or stored inside and outside of the EPA. The safest way to do this is to specify packaging that meets the requirement in ANSI/ESD S541. It is important to address packaging in the compliance verification program as well.

Marking. Marking is another one of the misunderstood sections of S20.20. It is included mostly due to demands by the military that it be addressed. “Marking” includes both the signage used to identify the EPA, but it also is applied to the marking of packaging and actual devices that are ESD sensitive. Many commercial companies either make a simple statement that they mark packages containing ESDS components with a commercially available mark, or they state that no marking is required.

Tailoring. One of the strengths of S20.20 is the ability of the user to tailor the specification if necessary. Tailoring is not necessary if you choose to make requirements more stringent than those specified in S20.20. However, if you want to make a required element optional, or make a specified value less stringent, tailoring is required. For instance, to specify a lower resistance limit for work surfaces, no tailoring is necessary. However, allowing operators to not wear wrist straps when seated, or allowing work surfaces with a higher resistance-to-ground than 1 x 109Ω, requires a tailoring statement. The tailoring statement must have technical justification and data to support the assertion that using the specification as-is is not necessary or deleterious to the product.

Additional thoughts. Products that fall into that “supersensitive” range may require efforts beyond the standard S20.20 program. For instance, for devices sensitive to charged device model damage, you may need to specify a minimum surface resistance value for work surfaces or anywhere else ESDS parts may touch, or you may need to limit metal-to-metal contact (say from tweezers or other tools).

You also may need to determine if more critical ionization is necessary to remove charge from product before it is handled or touched. Some companies are finding they must beef up the compliance verification program with more frequent or stringent testing. You also might benefit from beefing up training, highlighting critical items needed for the most sensitive products. Many companies dealing with ultrasensitive devices find that standard measurement tools are not enough. These companies find necessary the use of more robust process assessment tools, such as an electrostatic voltmeter and ESD event detectors. Some are using constant monitors for wrist strap, grounding and ionization verification to ensure these elements work continuously. Some companies have even implemented computer-based factory monitoring of these elements.

The “best” ESD control program is one that prevents any ESD damage to the components manufactured or handled, without overkill, resulting in expensive controls that may not be necessary. Certainly, an excellent place to start is to have a well documented and implemented S20.20 ESD control program. Additional controls may be necessary, depending on the sensitivity of the components handled and the manufacturing process complexity.

Ed.: For more on ESD programs, contact the ESD Association (esda.org).

Carl Newberg is president of MicroStat Laboratories (microstatlabs.com) and director of S20.20 Manufacturing Programs for Dangelmayer Associates, LLC; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Monday, 06 December 2010 19:16
 

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