A board can pass every optical check and still fail six months into service because one gate was damaged by a discharge nobody saw. The damage is latent: a thin oxide breach that survives final test, then drifts out of specification in the field. Buyers usually learn this the expensive way, which is why ESD control has become a document question at audit rather than a line on a capability sheet.
Here is what an auditor asks for, what the numbers should read, and where a programme breaks first.
What an ESD control programme has to cover
| Element | What is measured | Typical limit | Evidence produced |
|---|---|---|---|
| Grounding and bonding | Resistance from carts, racks, benches to ground | Bonded metal under 1 ohm, per the control plan | Bonding check sheet at setup and after moves |
| Personnel grounding | Wrist strap system resistance | Below 3.5 x 10^7 ohm | Daily tester log per operator, per shift |
| Worksurfaces | Surface resistance to ground | 1 x 10^4 to below 1 x 10^9 ohm | Periodic verification record with test points |
| Flooring | Resistance to ground | Below 1 x 10^9 ohm | Verification record, per zone |
| Ionization | Offset voltage, discharge time | Offset within plus or minus 35 V | Ionizer test record per unit |
| Packaging | Surface resistance of bags, trays, totes | Dissipative or shielding range per ANSI/ESD S541 | Incoming check plus supplier certificate |
| Humidity | Relative humidity inside the protected area | Plan defined, commonly 30 to 60 percent | Continuous logger data |
A control plan that lists limits but no test frequency is the first thing an auditor rejects. ANSI/ESD S20.20 sets the requirements and leaves the frequency to the plan, so the plan has to state the interval and justify it against how the area is used. Daily for wrist straps, monthly for benches, quarterly for flooring is a common split, and it has to be backed by records that carry dates, tester serial numbers and names.
Device sensitivity decides how strict the area has to be
Not every board needs the same protection. A board carrying parts in HBM Class 0, meaning below 250 V, sees damage from voltages a person cannot feel. Thin oxide CMOS gates, unprotected GaN HEMT gates, laser diodes, high speed converter inputs and some MEMS sensors sit in that group. A board built from parts rated above 1000 V can be handled in a far simpler setup.
So the most fragile part on the assembly sets the rules for the whole lot, and its HBM rating belongs in the RFQ, not in a general statement that the product is static sensitive.
Where boards actually get charged
- Peeling cover tape. Pulling tape off a reel separates two surfaces and can generate several thousand volts on the carrier tape. Reels should be opened at a grounded station, not at a bench with a coffee cup on it.
- Foam and bubble bags. Ordinary foam is a charge generator and a charge holder. Bags and trays have to be dissipative or shielding, and the part number should be on the packaging specification.
- Sliding totes. A plastic bin pushed across a shelf charges both surfaces. Ungrounded carts do the same at every stop.
- Clothing. Synthetic smocks and vinyl aprons generate charge as the wearer moves. Cotton or a specified smock with grounding at the bench removes most of it.
- Dry air. Surface resistance climbs as humidity falls, so a floor that passes in summer can fail in a dry winter month. This is why humidity belongs in the plan and not in a facilities footnote.
- Vacuum tools and peel-off gloves. Any nozzle or glove that rubs a surface without a ground path becomes a charger.
The seven failures audits find most often
Wrist straps tested weekly instead of daily, or logged for a shift that did not run. Ionizers installed and never tested, with electrodes worn enough that offset drifts past 35 V. A rework bench added outside the protected area and never bonded. Consigned material arriving in retail packaging that enters the line without inspection. Test fixtures with bare plastic near the board. Boards shipped in bubble wrap because the shielding bags ran out. A control plan copied from another site, quoting zones that do not exist here.
None of these show up in a final inspection. All of them show up in a field return rate two quarters later.
What to request before you audit, or before you place the order
- The ESD control plan, with a map of the protected areas and the limits for each.
- Compliance verification records for the last three months: wrist straps, benches, flooring, ionizers.
- Training records with refresher dates for operators, inspectors and rework staff.
- The packaging specification, with bag and tray part numbers, and how moisture sensitive parts are packed.
- The list of processes that leave the protected area, such as coating, programming or test at another site, and how parts are protected in transit.
- The goods-in procedure for customer supplied parts, since that is where unverified material usually enters.
How SUPERB Assembly runs it
SUPERB Assembly works to ANSI/ESD S20.20 across the assembly floor: conductive flooring, continuous wrist strap monitoring at the benches, ionizers over manual assembly and rework, and ESD safe handling from goods-in to packing. The floor runs at 22 plus or minus 3 C and 45 to 65 percent relative humidity, with ISO 8 conditions in the SMT areas, which holds surface resistance in range all year, not only in humid months.
Incoming material is counted and checked on an X-ray counter, which confirms reel quantity and packaging integrity before the parts enter the protected area. Bare boards are verified against IPC-6012 on arrival, and every assembly is built and inspected to IPC-A-610 Class 2 or Class 3 against J-STD-001, so the workmanship standard and the handling standard are stated in the same quotation. Finished boards leave in shielding bags, with desiccant and a humidity indicator card where the assembly carries moisture sensitive parts.
- Labeling and packaging: bags, trays and cartons that keep a finished lot protected.
- PCBA workshop: the line, the equipment and the facility conditions behind these records.
- PCB testing guide: what final test can and cannot catch.
Frequently asked questions
Is an ESD certificate enough evidence?
A certificate says a programme was set up. Records say it ran last Tuesday. Ask for the log and check the dates against your production window.
Do conformal coated boards still need ESD packaging?
Yes. The coating protects against moisture, dust and contamination. It does not protect an exposed connector pin, a test pad or a card edge from a discharge during handling.
How do I check how my consigned parts were handled?
Ask for the goods-in record for that lot: quantity counted, packaging condition, date code recorded, moisture level read, and the area the parts were stored in. A supplier who cannot produce that record for a specific lot has a procedure, not a practice.
Next step
Send the BOM, the assembly drawing and the workmanship class you need, plus the HBM rating of the most sensitive part on the board. The reply confirms the handling requirements, the packaging, the records that ship with the lot, and quotes with the inspection scope written in.
