Methods of Protecting Printed Circuit Board From ESD Damage

Large steel buildings, cars, mountains, and even people can survive realistic atmospheric lightning. Humans can also create their own miniature lightning (sparks) and survive. However, when these sparks reach the IC, they can cause major failures. In this tutorial, we will discuss ways to protect PCB printed circuit boards from ESD damage. We will demonstrate that analog parts with larger geometries are best suited for protecting field-programmable gate arrays (FPGAs) with their smaller geometries. By taking these steps, the ICs in the FPGA maintain higher reliability and deliver consistent quality performance.

Equipment regardless of stature is bound to be affected by lightning, steel buildings, cars, mountains and even people. In this application note, we will explain the mechanisms that protect ICs from ESD damage. Protecting ICs and boards from ESD and EOS damage is a critical aspect of product reliability and performance.

Lightning can be fun and entertaining, or dangerous and destructive. Maybe all of these things at the same time - it just depends on where you are, what you're doing, and how tall you are. Thunderbolt is always bad for ICs.

A few years ago, we lived in a 10-story steel hotel building. The afternoon thunderstorm moved across a wide open field. We felt comfortable and safe due to the steel frame of the building. Not plugged into our computer, so nothing to worry about. It was a spectacular show that lasted about 10 minutes as the storm passed.

Where do sparks artificially generated sparks come from? They are caused by triboelectrification. This is a big word. This happens when two materials come into contact (friction helps) and then separate. Some electrons will transfer to one of them. How many electrons move and to which surface depends on the composition of the material. This is a common phenomenon because almost all materials, insulators and conductors, exhibit triboelectric properties. We are familiar with many common resources. Petting a cat's fur, rubbing a balloon against a person's hair, and walking across a carpet can all exhibit triboelectric effects.

No wonder it hurts when we walk across the carpet and touch doorknobs! A general rule is that 5,000 V can jump about 1 cm (0.4 in) in air at 50% RH. For someone who is five or six feet tall, this is the spark. It was painful, but we survived. Now, change your perspective. Could this spark cause damage microinches high to a transistor in an integrated circuit (IC)? In this case, a centimeter spark is a huge, frightening display of lightning.

Now, let's talk about integrated circuits. Microprocessors have long increased the density of digital semiconductors. Manufacturing techniques have resulted in transistors getting smaller and smaller. In 1971, the Intel® 4004 computer processing unit (CPU) was introduced with 10 µm geometry. In the 1980s and 1990s, the process made parts smaller than bacteria. In 2012, ICs are 1,000 times less dense than 1971 technology, and the functions on a chip are smaller than a virus. In 2012, one could buy an FPGA with 28 nm capabilities and a package with 6.8 billion transistors, and the future promises to double the density in the next few years. Small transistors are tightly packed together and need to operate at low voltages (typically 1 V and below) to keep the heat generation under control.

To put 28 nm into perspective, note zero: it's 2.8 billionths of a meter (0.000000028). Let the distance between San Francisco and New York City represent one meter (about 4000 kilometers or 2500 miles). Now 28 nautical miles (one of 36 million parts) is 0.11 meters or 4.4 inches. How much lightning must strike a device of such a small geometry to damage it? How to protect this necessary and useful FPGA?

The simple answer is to use I/O interface devices that bridge the digital and analog worlds. Analog mixed-signal ICs have relatively large geometries (10 to 100 times larger than digital ICs) and higher voltages (typically 20 V to 80 V or more), making them more robust than tiny digital transistors . Although today's analog mixed-signal devices are generally ESD tolerant, they do benefit from discrete ESD devices.

Semiconductor manufacturers take electrical over stress (EOS) and electrostatic discharge (ESD) very seriously. First, for obvious reasons, EOS and ESD can damage parts during manufacturing, package assembly, and testing. But more importantly, these negative forces will directly affect the quality and service life of the circuit in the hands of customers.

Initially, a part that is under electrical stress may appear to be functioning normally. It might even be operating in a slightly degraded manner and still pass the automatic test equipment (ATE) checks, only to fail later in the field. EOS and ESD failures are preventable and undoubtedly critical quality control issues.

EOS and ESD damage is most likely to occur in manufacturing where ICs are made. We might think of the IC as being protected by a series capacitor. This is not the case. The second chance for damage is when the customer mounts the IC on the PCB to manufacture the product. We can see that the capacitor has an operating voltage of 50 V, but the distance between the two metal end connections is only 0.28 inches (7 mm). Since the spark just jumped 0.4" (1 cm), the small gap around the capacitor is vulnerable to damage. The result could be that the IC paid with its life. Finally, EOS or ESD damage can occur when customers operate the product in their environment.

Of course, there are many opportunities for large losses. We can actually see the results of EOS and ESD destruction inside the IC. For this, the epoxy material of the packaging must be removed. This is usually done with hot acid in a double-gloved isolation box. This process is very dangerous. The fumes are deadly. One breath would result in a painful death. A single drop of acid on human skin can result in amputation of a hand or arm at best and death at worst.

Bond wires and pads marked REF are provided so we can orient and compare photos. A liquid crystal material was applied to the mold (pink), similar to the liquid crystals used in mood rings and forehead thermometers for children. With small changes in temperature, the color will also change. When power is applied to the IC, the area that draws too much current (marked here with a yellow box) heats up and changes color.