ESD protection in electronics assembly: gear and soldering

ESD protection exists because an electrostatic discharge can damage an electronic component without leaving any visible trace. A board passes the test on the line and only fails at the customer's site. That is why electronics assembly relies on EPA zones, grounding wrist straps, antistatic clothing and footwear, and why THT and SMD soldering each have their own techniques and their own inspection.

What ESD is and why the damage cannot be seen

ESD (electrostatic discharge) is the release of charge built up on a body, clothing or a tool. A person often does not feel it, yet for a sensitive integrated circuit it is enough to weaken its structure. The component does not always stop working at once: it may run correctly during the test and fail a few weeks later in the user's hands. Such a fault is hard to detect, so plants focus on making sure charge never builds up, or drains to ground immediately. These rules apply at every workstation in electronics assembly work, from kitting components to inspecting finished boards.

The EPA zone: ESD protection rules on the shop floor

An EPA is an area protected against static electricity. It has marked boundaries and signs, plus grounded tables, mats and flooring. The zone is supervised and regularly checked with resistance measurements according to IEC 61340-5-1 or ANSI/ESD S20.20, depending on which one the plant uses.

When entering the zone, the usual order is:

You put on antistatic clothing (a coat or jacket fastened all the way) and ESD footwear.
You check your footwear on the tester at the entrance, if the plant uses one.
At your workstation you put the wrist strap on your bare skin.
You test the strap with a tester before starting work.
You take components out of ESD packaging only on the grounded mat.

Ordinary plastic film, plastic cups and private bags are not brought into the zone, because they build up charge easily. Detailed rules are set by the workstation instruction of the given plant.

How the antistatic wrist strap works and what to do if it fails the test

The strap connects your body to ground. Its cord contains a current-limiting resistor, usually 1 MΩ: it protects the person from shock while still letting charge drain away. The strap is checked before every use, while worn on the skin.

The check measures resistance and body voltage. Permissible values differ between standards, so the one adopted by the plant applies. If the tester shows an error, you do not start work: you adjust the fit of the strap, check the cord and report the problem to the shift supervisor. A worn strap lets through a higher voltage, so it is replaced.

Antistatic clothing and footwear

ESD clothing and footwear are mandatory equipment at such a workstation, and their use follows from the instruction. Footwear works together with the floor: each type of footwear is qualified separately with a given type of floor, because a shoe and a floor that each pass the test alone can fail together. That is why you do not swap ESD footwear for your own, even if it looks similar. Requirements for antielectrostatic clothing and footwear also appear outside electronics, for example in paint booths. The markings and types of equipment are described in the article on work clothing and protective equipment.

THT and SMD soldering: how they differ

Feature	THT	SMD
Mounting	leads pass through plated holes	the component sits on the surface of the board
Example components	axial resistor, electrolytic capacitor, connector	tiny resistors, capacitors, integrated circuits
Soldering	wave, selective or hand soldering, as a separate pass	mostly on an automatic line, by hand for rework
Typical defect	incomplete hole fill with solder	tombstoning (the component stands on one end)

In THT, hand soldering requires heating the lead and the pad so that solder flows into the hole. A popular claim that half hole fill is enough in class 2 is often wrong: the requirement is higher for classes 2 and 3, and the exception applies to holes connected to internal layers that draw heat away. With SMD the components are so small that you work with a magnifier or a microscope.

How solder quality is assessed

Solder joints are assessed according to IPC-A-610 in three classes:

class 1: general electronics,
class 2: products with a defined purpose,
class 3: high-reliability applications, e.g. medical.

A defect includes, among others, a component that is misaligned or does not match in form or function. Visual inspection complements automated optical inspection (AOI) and X-ray, and does not replace them. The typical defects an inspector looks for are a cold joint (dull, uneven), a bridge between leads, tombstoning and incomplete hole fill. The same approach to quality applies in other departments, as described in the article on work in production.

Frequently asked questions

Do you need an IPC certificate to solder?

In job ads, knowledge of IPC-A-610 and J-STD-001 is usually welcome and sometimes required. The J-STD-001 course for operators is training, not a certification, and it requires a prior course in electronics assembly basics.

Do you need very good eyesight for SMD?

There is no official eyesight requirement, but job ads often mention good eyesight and work with a magnifier or microscope. A manual dexterity test is sometimes part of recruitment.

Can you enter the EPA zone without a wrist strap for a moment?

No, if you work with components. The strap is put on and tested before you touch a board, and the plant checks workstations with measurements.

What is tombstoning?

It is an SMD defect in which a tiny component lifts during soldering and stands on one end, so it connects to only one solder pad.

This text describes the general rules. What you are entitled to depends on your type of contract: an employment contract (umowa o pracę) or a contract of mandate (umowa zlecenie). Your own contract and payslip are what count.