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SMT Trends & Technologies: You Are Discharged
February 25, 2013 |Estimated reading time: 4 minutes
Editor's Note: This article originally appeared in the January 2013 issue of SMT Magazine.
It can be difficult to determine the exact costs related to electrostatic discharge (ESD) for any particular industry or how much impact the issue may have on a brand image. ESD, however, is still an important reliability problem concerning electronic circuits. Articles found on the Web suggest that one-third to one-half of all field failures are related to ESD, with some reports suggesting it costs some industries millions of Euros per year. With newer technologies leading to smaller circuits and smaller components, ESD failures have become an even greater concern. If you are producing high-quality products, or if warranty costs and recalls simply cost too much, it is important that you check whether or not your components are packaged in ESD-safe packaging.
Many manufacturers take precautions when handling electronic components: Protective clothing, ESD wrist-wraps, special carpets, shoes, etc. Not so obvious is how the components are handled and transported and how they behave right up to the moment they are picked by equipment before being placed on the PCB. At this point they might already be defective, something your pick-and-place equipment simply cannot determine, and you end up with a defective end-product. Even worse, the product will become defective when in use (shortened lifetime) and come back as a warranty claim.
In non-conductive packaging, components shifting (shaking) within their chambers will become more charged than those that are stationary. Components in larger pockets are more sensitive to static discharge damage than those in narrow pockets, as are components in sticks that slide back and forth. Using conductive (ESD-safe) material will solve many of the problems, but you have to consider all packing items.
A conductive carrier is generally used with tape and reel to protect ESD-sensitive microelectronic devices. It is often not clear to users which kind of cover tape should be used to protect against ESD. The same is true for stick components, where the tube may be ESD-safe, but not the stoppers. Most believe that just using ESD-safe carriers will protect components sufficiently, but insulating cover tape can have electrostatic potentials of over 10,000 volts.
Figure 1: Charge and discharge of components as a result of exposure to high electrostatic fields.
Using non-conductive covers can generate high electrostatic fields. No charge will occur on the microelectronic devices as long as their terminals stay in contact with the conductive carrier tape. However, the moment the device terminals no longer contact the conductive carrier they are subjected to the electrostatic field. The moment the device contacts the carrier again, a sudden discharge can occur. This all happens during transportation and handling of the tape and reel, long before it ends up on pick-and-place equipment. Even when on the equipment (Figure 1), it is rotated as the reel unwinds and gravity ensures that components are randomly positioned in their chamber (pocket).
Figure 2: ESD damage to SMD 0603 package. (Source: www.incompliancemag.com)
A large voltage peak also occurs when the cover tape is separated from the carrier tape, which may not only damage components, but also attract them to the cover tape. That causes the component to “stick” to the cover tape, resulting in the equipment being unable to pick the component. On the machine, you will see a drop in your pick rate. The faster the peel-off speed (e.g., for turret equipment which have an index time of 40 to 60 minutes and, thus, very fast peel speeds), the higher the voltage peak, increasing the chance of charging components which are likely to discharge elsewhere. So, while the pick rate is of great concern, the issue of ESD should be a top priority.
Components flipping out of their pockets, following the cover tape, rotating in their pockets, or turning upside down are visible symptoms detected and known by many. Suppressing the phenomena (solving it in hardware) will allow a component to remain in the pocket correctly and be mounted reliably--increasing the pick rate. It is not, however, enough to prevent your component from being exposed to all the negative effects of static fields.
To eliminate static field exposure facilities must ensure that all handling is according to ESD protection guidelines, especially when dealing with safety-critical electronics. Without ensuring that components are in ESD-safe packaging your end-product may already be defective before it leaves the facility.
Eric Klaver was born in Rosemere, Quebec, and, via Oklahoma, he moved with his family to Holland at the age of 12. After finishing school, he became a radio officer for the merchant navy. After studying computer science where he learned how to program and design ICs Klaver enjoyed a career at Philips. This was followed by a move to Assembléon in 1998. As commercial product manager he translates high-level technical developments into easy-to-understand commercial material. Klaver specializes in vision technology and feeding and is currently the chairman of IEC work group TC40WG36, which specializes in component packaging.