ESD Prevention

On-Chip Networks

On-Chip ESD protection mechanisms generally work in two ways. First, to dissipate the large ESD current transient safely using a low-impedance discharging channel to prevent thermal damages in the silicon and/or metal interconnects. Second, to clamp any huge ESD induced voltage pulse to a safe level to avoid dielectric degradation or rupture. The complete ESD protection solution should be realized at the chip level, where the main emphasis is to create an effective discharging channel from any pin to every other pin on a chip.

Devices that are used as ESD protection elements are diodes, BJTs, and MOSFETs. The most commonly used circuits on Goddard projects are based on CMOS technologies and the ESD protection element that is used is silicon-controlled rectifiers (SCRs), see Figure 3. The main purpose of the SCR is to function as a switch that can turn on and off small or large amounts of power. As a positive ESD transient appears at the I/O pad with respect to ground, the SCR is pushed into a regeneration mode to form a low impedance discharge channel to shunt the ESD current safely. As the SCR moves into the "ON" state, I/O pad voltage will be clamped to a safe holding voltage. If a negative ESD pulse comes to an I/O pad with respect to ground, a parasitic device, like a diode or transistor, will forward turn on and take the charge. With the advantages of a parasitic SCR in CMOS devices to handle large ESD pulses, a drawback to note is its ability to latch up.  Latch-up occurs when a voltage spike activates one of a pair of parasitic diodes or transistors, which combine a circuit with large positive feedback. The result is that the circuit turns fully on and causes a short across the device.

Figure 3. Cross-section of a CMOS inverter showing parasitic SCR device inside.

Program Control Plan

Any project at Goddard and under Code 560 should evaluate their ESD sensitivity levels for their parts. If the project deems that the ESDS parts are critical or necessary to the success of the mission requirements then an ESD Control Plan (ESDCP) must be written to ensure that ESDS parts are not exposed to ESD pulses from handling to installation. An ESDCP requires the coordinated efforts of all levels of engineering, quality assurance, and project management to be effective and successful.

ESD Standard

Any ESDCP at Goddard should refer to a standard of acceptable ESD practices and procedures. The two standards that are used at Goddard are:

• NASA-STD-8739.7, for projects established before 1999
• ANSI/ESD S20.20-1999, for projects established after 2000

The ANSI standard S20.20-1999 outlines the requirements for an ESD control program with administrative and technical requirements. The standard advises, "When handling devices susceptible to less than 100 volts HBM (Human Body Model), more stringent ESD Control Program Technical Requirements may be required, including adjustment of program Technical Element Recommended Ranges." In other words, it is up to the project to define the level of ESD protection required and develops acceptable minimum standards for control devices. Using the NASA-STD-8739.7 would be a good template in defining ESD protection requirements for a project.

Training Plan

A project that has identified that they will be using ESDS parts should invest in developing a training plan. Any personnel who will be working on ESDS parts or doing maintenance work in facilities that are designated as ESD protected areas, must be ESD certified. ESD certification classes are offered through the NASA ManufacturingTechnology Transfer Center (NMTTC) or another facility, which is NASA certified to instruct in ESD protocols.

Protected Areas

The primary function of an ESD protected area is to reduce static withstand voltage levels of ESDS parts. Caution signs must clearly denote to alert personnel that the area is ESD controlled. The below items are what a typical ESD protected area should have and their resistance or voltage recommended limits:

• Work Surface/Mats: <1 x 10⁹ Ohm
• Wrist Strap: 0.8 x 10⁶ to 1.2 x 10⁶ Ohm
• Foot Strap: 0.8 x 10⁶ to 1.2 x 10⁶ Ohm
• Conductive Flooring: 1 x 10⁵ to 1 x 10¹¹ Ohm.
• Seating: <1 x 10⁷ Ohm.
• Bags: <1 X 10⁸ Ohm
• Ionization (other than room systems): <± 50 V offset.
• Ionization (room system): <± 150 V offset.
• Protected Garments: 1 x 10⁵ to 1 x 10¹¹ Ohm.
• AC Power Tools: < 1 Ohm
• Battery Power Tools: < 1 x 10¹² Ohm

Organizational Responsibilities

In developing any ESDCP the following organizations within Goddard are essential in preventing ESD:

1. Project Quality Assurance (Code 400 & 300)

• Oversee ESD awareness and certification training for all personnel working or have access to an ESD sensitive area.
• Maintain work instructions, drawings, and other documentation for ESD cautions, markings, and precautionary procedures.
• Audit and certify ESD-protected areas on a regular basis. Frequency should be stated by application of project (refer to NASA-STD-8739.7 Table 7-1).
• Ensure that access to protected areas be limited to persons who have completed ESD training.
• Communication between personnel, ESD monitors, and project managers is essential in ensuring that parts are functional and not damage from ESD.

2. Parts Engineering (Code 562)

• Parts Engineer identifies and classifies ESD sensitivity levels of parts. If sensitivity level is unknown, Parts Analysis Laboratory has ESD sensitivity tester, which can determine sensitivity level using HBM, MM, CDM.
• Parts Engineer selects parts with lowest ESD sensitivity, if possible. If not, verify that ESD protection circuitry is in place.
• Perform and assist in failure analysis of ESD sensitive items.
• Collect and maintain part ESD sensitivity data from supply vendors and manufacturers.
• New technologies and correction of discovered deficiencies database be maintained.
• Document ESD procedures in Work Instructions (WIs) and maintain Procedure Guidelines (PG) for Code 562.
• Certify that technicians, engineers, and authorized personnel in test labs are ESD trained.

3. Packaging and Shipping (Code 239)

• Parts that are received, stored, kitted, or ship for Goddard projects will be handled by Code 239
• Implement ESD precautionary handling and packaging procedures during receiving, processing, inspection, and packaging. If there is an issue with packaging an ESDS part, inform Parts Engineer for assistance.
• Package ESD sensitive items in ESD/moisture protective material for handling and shipment.
• ESDS assemblies, parts, and equipment must be marked with an ESD caution symbol in a readily visible position. ESD protective packaging should be marked.

Humidity Levels

Humidity is an important factor in the generation of static electricity. As humidity increases, the surface resistivity decreases. This condition means that insulator materials rubbed together or pulled apart in a humid environment generate lower static charges than the same materials rubbed together or pulled apart in a dry environment. It is recommended per ANSI/ESD S20.20-1999 that relative humidity be maintained between 30% and 70%. NASA-STD-8739.7 has a recommended humidity range level of between 40% and 60%, which is within the ANSI limits. Humidity above 60% is uncomfortable for people and below 40% increases the risks of static generation for insulators. Projects should equip their ESDS areas with active humidity monitoring equipment. A way to minimize ESD in a protected area is to have a contingency plan in place should the humidity levels fall below the recommended lower limit.

ESD Control Matrix

To be effective, an ESDCP must be comprehensive and adaptive to fit the needs of the project requirements of their ESDS parts. Older, less sensitive parts require minimum precautions where as parts that are susceptible to few volts require extensive precautions. The requirements are based on an area sensitivity classification system, which lists five classes of sensitivity:

• Class I areas contain parts with ESD withstand voltages ranging up to 199 volts. 
• Class II areas range from 200 to 499 volts.
• Class III areas range from 500 to 1,999 volts.
• Class IV areas range from 2000 volts and up.
• Class V areas do not contain devices that are sensitive to ESD damage or for non-project Research and Development.

Based on these classifications, each ESD protected area should be classified according to the most sensitive device handled. For example, a device with an ESD withstand threshold of 100 volts would be handled in a Class 1 area, and all other devices in that area would be handled the same way.

To visualize what control measures are needed for each sensitivity class an ESD requirements matrix was developed. The matrix is separated into two categories:

1. Research & Development (Non-Spaceflight) - Table 2
2. Spaceflight/Potential Spaceflight - Table 3

The Research & Development matrix shows the minimum requirements that are needed for parts or assemblies that are being tested for research or self-training purposes. The Spaceflight/Potential Spaceflight matrix, however, requires more stringent requirements to ensure that flight parts are kept safe from accidental discharges while being stored, handled, and packaged. Replacement of damaged flight parts usually cost 10 times more than their equivalent commercial parts. Also, with the increased use of Commercial-Off-The-Shelf (COTS) parts for spaceflight applications, extra requirements are needed to ensure that these parts are not damaged or destroyed while being inspected or assembled.

R = Required     OP = Optional (Consult Project)     NR = Not Required

Table 2. ESD Requirements Matrix for Research & Development Applications

R = Required     OP = Optional (Consult Project)     NR = Not Required

Table 3. ESD Requirements Matrix for Spaceflight/Potential Spaceflight Applications

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