ESD control on the SMT line: where do charges build up beyond the workbench?
On an SMT line, charge builds up mainly where material is separated, moved or picked up by vacuum: peeling cover tape, conveyors, pick-up nozzles and blowing off with compressed air. Effective measures are grounded machines, dissipative contact surfaces, ionization at unavoidable insulators and regular field measurement per IEC 61340-5-1.
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Why does an SMT line generate different charges than a workbench?
At a workbench the operator is usually the main source of charge; on an SMT line it is machines and material. Tapes, films, boards and components are constantly separated, slide along conveyors and are picked up by vacuum. Each movement creates charge through friction and separation, without anyone touching the component.
The dominant risk is therefore not a person discharging into the part (HBM) but the Charged Device Model (CDM): the component or board becomes charged and discharges abruptly as soon as a pin touches grounded metal such as a nozzle or test probe. The event lasts nanoseconds and can damage gate oxides without the functional test revealing it.
- Process-essential insulators: board substrate, carrier and cover tape, trays. They cannot be grounded.
- Isolated conductors: components on the board and metal parts with no ground path. They hold charge until a contact drains it.
Which stations on an SMT line generate the most charge?
The critical points are all transfers where material is separated or moved: unpacking at goods-in, peeling cover tape in the feeder, releasing the board from the stencil, vacuum pick-up, transport on conveyors, depaneling and packing. The table maps each station to its typical charge source and countermeasure.
| Station | Typical charge source | Measure |
|---|---|---|
| Goods-in and storage | Opening cartons, pulling tape, repacking outside the EPA | Open only inside the EPA, check packaging and marking |
| Feeders and reels | Peeling cover tape, unwinding the reel | Grounded feeders and carts, ionization at the peel point |
| Stencil printing | Separating stencil and board | Grounded stencil, dissipative supports |
| Pick-and-place | Vacuum pick-up and placement by the nozzle | Dissipative nozzles, machine bonded to the ground point |
| Reflow and conveyor | Friction on belts and edge guides | Dissipative belts, grounded mechanics |
| AOI and test | Fixtures, blowing off with compressed air, manual handling | Grounded fixtures, ionized air, operator grounding |
| Depaneling | Routing or breaking, extraction through plastic hoses | Dissipative extraction, ionization at the cutter |
| Packing and shipping | Film bags, cushioning, labels | ESD packaging per IEC 61340-5-3 |
Charge often arrives with the goods: reels repacked outside the EPA charge the line before the first machine runs. See ESD incoming inspection: checking packaging and marking and, for the machine itself, Choosing a pick-and-place machine.
How are machines and contact surfaces grounded on the line?
Every machine is bonded to protective earth and the common ground point of the EPA, including its housing and every conductive part that touches components or boards. The mains connection alone is not enough: what matters is a continuous path to the contact surface, meaning nozzle, gripper, conveyor belt and stop.
The chain often breaks at inconspicuous points: anodizing, plastic bearings, rubber buffers and toothed belts separate moving parts from the frame. A gripper on an insulated bearing is an isolated conductor, even if the frame is grounded.
- Make contact surfaces dissipative so that charge drains in a controlled way.
- Bond moving parts with sliding contacts, grounding brushes or braided straps.
- Ground feeder carts and reel holders via dissipative castors or a ground cord.
- Ground operators with heel straps and a dissipative floor, at fixed stations with a wrist strap (person to ground < 3.5 x 107 Ω).
A ground tester checks not only resistance but also AC and DC voltage on the protective conductor and high-frequency noise, which reach the component through the ground.
Grounding systemsHeel straps, wrist straps and grounding components for operators and equipment.
View productsWhen does an SMT line need ionization instead of grounding?
Ionization is used where grounding cannot work: on the board substrate, on carrier and cover tape and on isolated components in transport. The ionizer produces positive and negative air ions that neutralize charge on these surfaces. It complements the grounding of machines and people but does not replace it.
Machines use fan ionizers in enclosures, ionizing bars above conveyors and ionizing air nozzles for spot points. DC units with closed-loop control keep a steadier balance and need little airflow, which matters in the placement machine, where draughts shift small parts.
A unit with drifted balance charges parts itself. Offset voltage (typically < ± 35 V in the EPA) and decay time are therefore checked regularly per IEC 61340-4-7 and emitter points cleaned.
How do you check that the measures on the line are working?
Effectiveness is measured with an electrostatic field meter directly at the process: boards and tapes are checked at transfer points after the critical step, readings are recorded and compared with the limit in the ESD control program. Grounding and ionizers are also tested periodically, and critical points are monitored continuously.
- Define measuring points: feeder exit, after printing and placement, reflow exit, after depaneling.
- Always measure at the same distance and ground the field meter itself.
- Measure suspicious spots with and without the ionizer.
- Test operator grounding daily, worksurfaces per test plan (to ground < 109 Ω).
- File results in the control program per IEC 61340-5-1.
Continuous monitoring pays off at wrist-strap stations and on machines whose ground is disconnected during maintenance: the monitor reports a break immediately. For instrument choice see Choosing an electrostatic field meter.
Frequently asked questions
Is it enough for the machine to be grounded via the mains?
No. The protective conductor grounds the housing, but not automatically nozzles, grippers or belts. Moving parts on plastic bearings or belts can be isolated. The path must be continuous to the contact surface, and it is measured rather than assumed.
Why is CDM more dangerous than HBM on the line?
In automated processes hardly anyone touches the component, but it becomes charged through movement and separation. On contact with grounded metal it discharges within nanoseconds. Operator grounding does not prevent this; only low charge on the component does.
Can boards be blown off with compressed air on the line?
Only with ionized compressed air. Ordinary dry air charges the board and components through the fast flow. Ionizing nozzles or guns release ions of both polarities at once and neutralize the charge right at the surface.
How often should measurements be taken on the line?
Operator grounding every working day; machine grounding, worksurfaces and ionizers per test plan. Field measurements at transfer points make sense after line changes, new materials and falling humidity. The company sets the intervals based on risk.
What equipment does your SMT line need?
Ionizers for machines and transfer points, grounding systems for people and equipment, and instruments for field and ground testing.
Standards-based
Measures per IEC 61340-5-1.
Measurable
Field and ground testing on the line.
Process-oriented
Each station with source and countermeasure.
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We help you choose the right products; technical details are clarified with the manufacturer.