nichrome

October 1, 2020

What you oughta know about Electrostatic Discharge

Introduction

Static (or stationary”) electricity is, simply, electricity that is not moving. And it is harmless as long as it remains static. Unfortunately, an electrical charge is unstable and is always looking for an opposite charge to unite with. When a charge moves, it becomes a current-often a greater current than a microcircuit can handle.

You may already know that touching a chip lightly with your fingertip can degrade a circuit so that it never again performs to specifications. But did you know that brushing certain kinds of clothes (like your polyester shirt) over a printed circuit board can do the same thing? In fact, unprotected electronic devices can be zapped” just by waving a charged object nearby.

The smallest charge you can feel is 3,000 volts; the smallest charge you can see is 5,000 volts; and the smallest charge you can hear is 10,000 volts. But some of the newest semiconductor devices available in the mid 1980s and early 1990s were susceptible to as little as 10 volts, or one three-hundredths as much as you can feel.

Electrostatic discharge (ESD) has therefore become more and more of a hazard as microcircuits have become smaller and more sensitive. This section contains rules for preventing ESD damage to equipment you are working on. It also contains instructions on setting up an ESD-safe workstation. These rules and instructions hold true whether you are working at your shop or at a customer’s site.

Here are the rules you need to learn and follow to prevent ESD damage.

Video

ESD Prevention Rules

Grounds

Before working on any device containing a printed circuit, ground yourself and your equipment to an earth or building ground Use a grounded conductive workbench mat and a grounding wriststrap, and ground your equipment to the mat.

WARNING:
Make sure you are not grounded when:

Bodies

Don’t touch anybody who is working on integrated circuits.

If that person is properly grounded, your zap” may not cause any damage, but just to be on the safe side, do not touch or brush against other technicians.

Bags

Use static-shielding bags for boards and chips during storage, transportation, and handling.

When you are ready to leave your bench and take a board to a storage place, first put the board in a static-shielding bag. Leave all Apple service exchange components in their ESD-safe packaging until you need them.

Leads

Handle all ICs by the body, not the leads.

Also, do not touch edge connectors on boards or cards, exposed circuitry, or printed circuits. Handle ICs, boards, and cards by the edges, or use extractors.

Synthetics

Do not wear polyester clothing or bring plastic, vinyl, or styrofoam into the work environment.

The electrostatic field around these nonconductors cannot be removed.

Metals

Never place components on any metal surface.

Use antistatic, conductive, or foam rubber mats.

Atmosphere

If possible, keep the humidity in the service area between 70% and 90%, and use an ion generator.

Charge levels are reduced (but not eliminated) in high humidity environments and in areas where an ion generator is used routinely.

Setting Up an ESD-Safe Workstation

Materials Required

Conductive workbench mat, with ground cord Wriststrap, with 1 megohm resistor and ground cord Equipment ground cord, with alligator clips Ground/polarity tester

Setup and Procedure

After you gather the materials above, remove all ESD hazards from the area:

These nonconductive materials cannot be grounded and will retain a charge for hours and even days. Since the static field surrounding them can easily damage sensitive components, it’s best to keep these materials completely out of your work area.

After you remove the ESD hazards, proceed as follows:

  1. Use a ground/polarity tester to verify proper grounding of the power outlet.

    Ground/polarity testers vary slightly in design, but all are very simple to use. Insert the three prongs of the tester into the three-prong outlet. If the outlet is wired incorrectly, most testers show a light pattern that matches a code given on the tester.

    If the tester does not verify proper grounding, move to another outlet that is safe-whether you are at the customer’s site or at your shop.

  2. Connect the ground cord of the workbench mat to ground

  3. Use a wriststrap ground cord. Fasten it to the workbench mat and to the wriststrap. The wriststrap should touch your skin.

    All objects in the service area should be grounded to the same potential. Touching the chassis of a machine will bring you to the same potential as the machine, which is better than nothing. However, since just the act of shifting your weight from one foot to the other can generate static, momentary touch” grounding is not enough. That is why you need the continuous grounding provided by a grounded wriststrap.

  4. Finally, ground the equipment you are working on.

    Use alligator clips and a grounding cord to attach any metal part of the device you are working on to the grounded workbench mat.

    If you are working on a product that has a three- prong power cord, you can attach the ground pin of the power cord to the workbench mat using your alligator clips and ground cord. (Of course, the unit will not be plugged into the wall outlet.)

WARNING:
When you discharge a CRT or work with a powered-on CRT, do not wear a wriststrap, and do not work on a grounded pad.