ESD Audit for PCB Assembly: What to Measure and Where
An esd audit is a measurement exercise, and it fails when it turns into a paperwork exercise. The written program, the signs and the training records are necessary, but they do not protect a board. What protects the board is a continuous low impedance path from every charged object, including the operator, to a known ground, and the audit exists to prove that the path is intact at each point where a component is touched.
The audit therefore follows the product rather than the floor plan. It starts where parts are received, follows them through storage, printing, placement, reflow, handling, test and packing, and at each station it asks the same question: what can charge, what can discharge, and through what. A station that passes its own test but sits next to an ungrounded cart is still a risk.
Scope: What an Audit Covers
A complete audit covers four groups of items: personnel grounding, workstation grounding, materials and equipment, and packaging and transport. Within each group the audit checks both the hardware and the practice, because a wrist strap that is worn over a sleeve or a heel grounder on a shoe with a rubber sole measures correctly on the bench and fails in use.
The reference document is usually the industrial standard adopted by the site, such as IEC 61340-5-1 or the equivalent national program, and the audit should quote the clause that each limit comes from. Where a customer imposes a stricter limit, that limit goes on the audit form. Without a written set of limits the audit becomes a matter of opinion.
Wrist Straps and Footwear
A wrist strap is checked as a system, with the strap on the wrist and the cord connected to the ground point. The measured resistance of the complete path, including the skin contact and the one megohm series resistor, is typically required to stay below 35 megohms and above 1 megohm, and the range is what makes the strap safe as well as effective. A strap that measures open is a static generator rather than a protection device.
Footwear and heel grounders follow the same logic with a different range, because the path runs through a floor that is itself part of the circuit. A heel grounder is measured on the operator on the floor where the work is done, not on a bench, and the floor resistance at that point is recorded with it. Where seats or trolleys are used, the same test is applied to their conductive elements.
Workstation Grounding and the Common Point
Every conductive element at a bench should terminate at one common point, and that point should connect to the equipment ground. Daisy chaining several benches onto a single wire creates a path whose resistance depends on every connector in the chain, and a single loose screw then disables the whole run. The audit checks continuity from each element to the common point and the bond from the common point to ground.
Frequency matters as much as the reading. Wrist straps are tested at the start of every shift with a test station, and the audit repeats the measurement with an independent instrument. Workstation bonds are checked on a schedule that reflects the movement in the area: a bench that is rebuilt for a new product is re-verified, while a bench that never moves can be checked less often.
Surface Resistance of Materials
Bench tops, floor mats, tote boxes and fixtures are all classified by their surface resistance, and the classification determines what they may be used for. A conductive material drains charge quickly and can damage a device if it touches a lead, while a dissipative material drains it slowly and is the usual choice for a work surface. The measured value depends on the electrode geometry and on the humidity, so the standard test method has to be named on the record.
Humidity is the variable that a single measurement misses. A dissipative surface that measures comfortably inside its range at 50 percent relative humidity can measure an order of magnitude higher in a dry winter room. Where the site controls humidity, the audit should check the reading of the monitor against its own instrument, and the topic of moisture control for parts is handled separately in the notes on floor life control.
Ionizers and Airflow
An ionizer is used where an insulator cannot be grounded, such as a plastic housing or a moving belt. It is audited for balance and for decay time, because an unbalanced ionizer delivers a net charge instead of neutralising one. A balance within about 35 V of zero and a decay time from 1000 V to 100 V within a defined limit are typical acceptance criteria, and both are measured with a charged plate monitor at the point where the product sits.
Airflow matters because it determines where the ions go. A nozzle that is aimed above the product, or a fan that is blocked by a stack of magazines, produces a reading at the monitor that does not describe the product. Ionizers also need their emitters cleaned, since a contaminated emitter loses its output while still drawing the same air.
Sensitive Steps in Assembly
The steps that most often generate a discharge are the ones with the fastest movement and the least grounding: separating a board from a carrier, peeling cover tape from a reel, unwinding a protective film, and sliding a board across a surface. Each of these is a triboelectric generator, and the charge that is separated has to go somewhere. The audit should stand at each of these steps and watch how the board is handled.
Packaging figures in the same analysis. A board that is removed from an antistatic bag while the operator holds the bag on the outside is protected, while one that is slid out over a bench is not. The standards behind the tape and reel side of component handling are described in the notes on tape and reel packaging.
Packaging, Transport and Storage
Storage is where a program is most often weakened, because the boards are out of sight. A shelf that holds bare boards should be treated as a work surface and grounded, and the bags that carry them have to be closed rather than folded over. Reels and trays that are returned to stock should be re-sealed, and the conductive properties of a reused tote should be verified rather than assumed.
Transport inside the plant is part of the same path. A cart with conductive castors is grounded through the floor, while a plastic cart is an isolated island. Where carts are used, the audit checks the castors, the contact with the floor and the bond from the shelf to the frame. Data on component moisture sensitivity is handled in the notes on moisture sensitivity levels, which is a separate control with its own records.
Instruments, Calibration and Records
The audit depends on its instruments. Resistance meters that are used for wrist straps and surfaces are calibrated on a schedule, and the calibration certificate is kept with the audit records. Continuity testers should be verified against a known resistor, and the charged plate monitor used for ionizers has its own calibration because its reading depends on the plate capacitance.
Records should be organised so that a trend is visible: the date, the station, the item tested, the reading, the limit and the pass or fail. A file of pass or fail entries with no numbers cannot show a bench that is drifting towards its limit, which is exactly the condition the audit exists to catch.
Findings, Corrective Actions and Re-audit
A finding is worth recording only if it has an owner and a date. The useful categories are a failed measurement, a missing item and a practice that contradicts the procedure, because each has a different corrective action. A failed wrist strap is replaced and the worn ones in the drawer are inspected. A missing ground bond is restored and the benches nearby are checked. A practice issue is retraining with a follow-up measurement.
Every finding should be re-measured at the same point within a short interval, and the re-audit should be carried out by someone other than the person who did the repair. The result is a program that is measured rather than declared, and that is the only kind that survives contact with a production schedule.
FAQ
How often should an ESD audit be performed? Personnel grounding is checked daily, the audit of stations and materials is usually monthly or quarterly, and the frequency rises with staff turnover or a layout change.
Can the audit be done with a multimeter? A multimeter can show an open circuit but not a resistance in the megohm range reliably at the correct test voltage. A dedicated meter with the standard electrode is required.
Do conductive materials always protect a board? No. A conductive surface that contacts component leads can discharge a device quickly. Dissipative materials with a controlled resistance are the normal choice at the bench.
