Protecting LEDs from static electricity during production

A static discharge too small to feel can damage an LED chip outright or leave a hidden flaw. Here is how production lines guard against it.

The small shock you get from a door handle after walking across a carpet in winter is harmless to a person. To a semiconductor crystal the same event can be serious. On top of that, people only feel discharges above a certain strength; weaker ones pass unnoticed yet can still be dangerous for electronics. That is why protection against electrostatic discharge (ESD) is set up as a system of its own in LED lamp production.

What a static discharge does to an LED

The blue chips at the heart of white LEDs are built from extremely thin semiconductor layers. A sudden high-voltage pulse can puncture those layers on a microscopic scale. There are two possible outcomes:

• Obvious damage. The chip fails straight away: it does not light, or lights very faintly. This is normally caught during factory testing.
• Latent damage. The chip keeps working and passes the first check, but carries a weak spot. Over time leakage current rises, the chip runs hotter, light output drops and the lamp fails sooner than it should.

The second case is worse, because the fault shows up in the customer's home. Because the LEDs in a typical household bulb form one series chain, a single damaged chip leaves the whole bulb dark.

Chips are not the only sensitive parts. The driver's control IC and transistors can also be harmed by a static pulse. Even if such a driver keeps working, it may deliver an unstable current and cause the lamp to flicker. For that reason, protection covers every area where boards are handled.

Where the charge comes from

Static charge builds up through friction and separation, and a production floor has plenty of sources:

• synthetic clothing and shoe soles;
• plastic trays, foam and ordinary polythene bags;
• conveyor belts and rotating parts;
• tape being pulled off a roll;
• dry air, since low humidity lets charge build up more easily and linger longer.

An operator can pick up a charge simply by walking to a bench, standing up or opening a bag. The moment they touch a chip or a board, that charge discharges.

How a protected area is set up

The zone where sensitive parts are handled is clearly marked and runs under one set of rules. The basis for these rules is described internationally in standards such as IEC 61340-5-1. Typical measures include:

1. Grounded wrist straps. Connected to ground through a resistor, they bleed charge off the operator's body gradually.

2. Static-dissipative bench mats. The work surface drains charge slowly to ground.

3. ESD footwear and flooring. Charge also drains through the feet, which matters for staff who stand and move around.

4. Clothing. Anti-static coats are worn over ordinary clothes.

5. Ionisers. Units that release ions into the air to neutralise charge on insulating materials that cannot be grounded.

6. Humidity control. The air is not allowed to become too dry.

Handling and transport inside the plant

Protection does not end at the edge of the bench. Chips arrive on reels and in anti-static containers, and part-finished boards move around the plant in dedicated boxes or shielding bags. An ordinary plastic tub is a poor choice for a sensitive board, since it can collect a charge itself.

Machines are part of the system too. The frames of pick-and-place machines, conveyors and test fixtures are grounded, because moving belts and plastic parts build up charge as readily as a person does. A grounding lead that is disconnected during maintenance and never reconnected is a typical and easily missed mistake.

Keeping the rules alive

The measures only work if they are followed every day. Production sites usually build in:

• a check of wrist straps and footwear on a tester at the start of each shift;
• periodic measurement of bench mats and grounding points;
• training for new staff;
• a rule that nobody enters the protected area without a strap.

A small lapse, such as taking off a strap to carry a few boards, can create a batch with hidden damage that is almost impossible to separate out later.

What matters for the user

A finished LED bulb in a user's hands generally has nothing to fear from static: the chips sit inside the cover and housing, and the driver is designed for disturbances from the mains side. The risk appears when working with a bare LED board or strip, for instance when replacing the LED module in a light fitting. In that case, avoid touching the chips, avoid working in synthetic clothing in very dry air, and touch a grounded metal object first. These are simple habits, but useful ones.

Zaferoğlu Elektrik's own assembly takes place in Ganja, where Zəfər İşığı production began in December 2021. Trade buyers can put questions about production conditions directly to the company.