Jim I have quite a few products with the same basic user interface you describe and have tested extensively to ESD. To answer some of your questions:
If the metal plate on which the pushbutton overlay is applied is earth grounded it should attract the ESD discharge. I assume that your device is panel mounted and there fore the dischrage path would be to the panel. If for some reason you must float the panel, the overlay can be designed with a grounded internal shield ring around the perimeter of the overlay. This would be an internal trace just like those to the pushbuttons but positioned near the edge. ESD applied to the edge of the overlay will then be attracted to this sink path. Be careful about routing and the design of the drain lead. If you have a plastic bezzel that the plate floats on you could consider some conductive plastics. They act as a good sink path for ESD but will most likely give you some fits with safety engineers because it acts as both an insulator (@ power frequency) and a conductor (@ High frequency like ESD). The bezel usually is treated as a floating piece of metal in safety review. After all of this ESD concern you may find that you actually have a bigger problem with other common mode immunity tests - Fast transient and RF immunity (especially the RF conducted). You see, if the overlay panel is grounded you have a good common mode path through the overlay circuit. Common mode High frequency noise will find its way to ground through the push button circuits and can cause false push button action or actually thrash the data bus. This can also be an emission issue since the path allows radiated noise to escape too. A simple approach is to have hefty pull up resistors, but you most likely will need some form of filter. You may want to consider ferrite filter beads . On the display placement you have to way two issues. If the LCD is pushed up against the overlay membrane you may have a better looking display. The image will tend to be crisper and the viewing angle optimum. However, if you back the LCD off a quarter of inch or so then you have some level of impact resistance. Note that the LCD glass can be fragile in some designs. This separation would allow the overlay to absorb user impact. If the LCD is backlit then the image may be fine with the separation. I believe the best resolution is mock up a display and take a look and try the impact resistance as well. I believe our overlays are basically polly ethylene and the adhesives are a 3M safety recognized material (tested not to fall apart under temp or time). If you want to get into more detail let me know. Like I said I have been through this design ringer before. If you have any additional questions feel free to email me Regards Chris Wells Sr Design Eng. Cutler-Hammer [email protected] ------------- Original Text From: C=US/A=INTERNET/DDA=ID/jeichner(a)statpower.com, on 6/6/97 7:33 PM: I am looking for some guidance on the following construction: We have a remote control panel that contains an LCD display (approx. 2" x 1") mounted on a circuit board behind a metal panel. An opening in the metal panel allows the user to see the screen, and is only marginally larger than the display. Over the metal panel is a laminated label containing some membrane switches. All the circuit traces for the membrane switches are approx. 1/2" or more from the edges of the label, and are contained in the bottom layer of the label (ie one layer of plastic between them and the metal panel). The ribbon cable from the membrane switch to the pwb is routed through a hole in the metal panel. There is a clear plastic window in the label allowing viewing of the LCD and that window is more or less touching the glass face of the LCD display. We don't want to recess the LCD because of viewing angle problems. We are subject to 8kV air discharge and 6kV contact discharge. 1. Does this construction sound like an ESD test failure waiting to happen? 2. Does it make any difference whether or not there is an air space between the clear plastic window in the label and the glass face of the LCD display? Should we recess the LCD, and if so, by how much? 3. Are there preferred materials for any of the layers of the membrane switch label? 4. Any hints anyone? Any help will be greatly appreciated. Thanks, Jim Eichner Statpower Technologies Corporation [email protected] Any opinions expressed are those of my invisible friend
