The Importance of Being Grounded
Vladimir Kraz, Director of Instrumentation, Electronic Solutions Division, 3M Company
The proper grounding of all conductive items in the production workplace is an essential element of ESD (electrostatic discharge) management. ESD damage to components and assemblies in manufacturing is getting more and more attention. Smaller geometry and faster speeds of semiconductors have resulted in devices with increasingly higher ESD sensitivity. A typical sensitivity of integrated circuits and other devices is now in a 100V CDM range and many of the devices are already in 30 to 50V CDM sensitivity range. CDM stands for charged device model, and is defined as when a small device suspended by either pick-and-place vacuum picker or tweezers is charged and then is placed on a printed circuit board (PCB) making electrical contacts to the traces on the board and generating rapid discharge that can be quite harmful. Even if the device itself is not charged, a PCB or any other metal surface on which the device is placed, such as the shuttle in the IC handler, can be.
Conductors (i.e. any metal part) in the manufacturing process present ESD danger to the devices even if the voltage on them is low. This is because metal parts of the tools have high electric capacity and can contain plenty of charge. In a simple example, imagine a large bucket filled with an inch of water. Even though the water level is fairly low, the total amount of water in this bucket can be substantial. Because of that, it is important to realize that the damage to semiconductor devices in a real life situation can occur at lower levels than specified for standard device models.
A typical manufacturing tool, such as pick-and-place SMT machine, has many metal parts, which are supposed to be electrically connected to each other and then to ground. In reality, there is little assurance that such connections are good and that every part of the tool is properly grounded at all times. There are several typical reasons for that:
Grounding via Ball Bearings. This is quite a common situation in many tools. Ball bearings are, unfortunately, several pieces of metal separated by insulating lubricant. Indeed, when the tool is not moving, the ball bearings offer reasonable conductivity because in this state, the lubricant is squeezed out and the balls offer electrical connection between the rings. However, when the tool is moving, i.e. when it handles the electronic parts, the electrical connection is far from being guaranteed due to the lubricant, which insulates balls from the rings as it should for reducing friction.
Assumed Connection. Quite often it is assumed that, when two pieces of metal are pressed against each other, there is good electrical connection. This assumption is very far from reality. Many parts of today’s tools are made from anodized aluminum, which is, on its surface, an insulator. There is often no electrical connection between parts of the tool frame constructed from aluminum extrusion due to the nature of how the different parts are connected together. On occasion, one can find grounding wires trying to make connection to a painted surface which, of course, won’t work, unless it is a conductive paint.
Broken Grounding Wires. In a brand-new tool, grounding connections may be good; but after some time, the wires bend, stretch and break as shown in Figure 1 (as presented by Jos van de Giesen of NXP (former Philips Semiconducor) at the 2006 ESD Forum in the Philippines.
This article is an excerpt from the IPC Printed Circuits Expo, APEX, and
the Designers Summits, March 31-April 2, 2009 Las Vegas, Nev. To view
the complete article, including all graphics and tables, please download
the .PDF version.
June 2008