_______________________________________________________________________________ From: DouglasScott on Mon, Jun 9, 1997 8:09 AM Subject: RE: ESD - Membrane switches and LCD Displays To: [email protected]
Jim, I had the same problem (design issue). The key here is to have the longest possible path length between the ESD source and the potential circuit trouble point. Several things will help reduce or eliminate the problem. 1. Add a guard band, in etch, inside the switch layer. This guard band should cover the entire perimiter of the layer and should be connected to chassis ground (the metal panel). This can be done with a tail that connects the band to a mounting screw for the panel. It needs to be adequately spaced from the active circuit. Your membrane switch vendor should be able to help with this point. 2. The window in the label will satisfy your safety problem with LCD's. That is, the user cannot come into contact with the leaking liquid from a broken LCD. 3. You don't necessarily need an air space between the window and the LCD, but it helps. In one case, I added a 1/8" spacer between to keep the discharge from getting coupled through the LCD to the driver circuit. 4. Many LCD's come with a metal band around them tpo hold them to the circuit board. This metal band may or may not (usually not) be grounded to chassis. In this case, I designed a beryllium copper spring that attached to the mounting screw and made spring contact with the metal band. With the above changes, our design now takes 16 kV and won't fail. Occaisionally we see a momentary darkening of the LCD, but there are no operational failures or damaged parts. Hope this helps some, Scott Douglas Principal Compliance Engineer ECRM Incorporated Telephone: 1-508-851-0207 Facsimilie: 1-508-851-7016 e-mail: [email protected] _______________________________________________________________________________ From: [email protected] on Fri, Jun 6, 1997 7:59 PM Subject: ESD - Membrane switches and LCD Displays To: [email protected] 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 ------------------ RFC822 Header Follows ------------------ Received: by macgtwy.ecrm.com with SMTP;6 Jun 1997 19:59:22 -0400 Received: by highlight.ecrm.com (AA11197); Fri, 6 Jun 97 19:40:14 EDT Received: from ruebert.ieee.org by maildrop.ecrm.com (TAA01439); Fri, 6 Jun 1997 19:37:39 -0400 (EDT) Received: (from daemon@localhost) by ruebert.ieee.org (8.7.5/8.7.3) id NAA23709 for emc-pstc-list; Fri, 6 Jun 1997 13:41:27 -0400 (EDT) From: [email protected] Message-Id: <[email protected]> List-Post: [email protected] Date: Fri, 6 Jun 1997 10:21:00 -0800 Organization: EMC Distribution Service To: [email protected] Subject: ESD - Membrane switches and LCD Displays Importance: Normal Mime-Version: 1.0 Content-Type: text/plain; charset="IBM-8202" Content-Disposition: inline Content-Transfer-Encoding: 7bit X-Mailer: Connect2-SMTP 4.30A MHS/SMF to SMTP Gateway Sender: [email protected] Precedence: bulk Reply-To: [email protected] X-Resent-To: Multiple Recipients <[email protected]> X-Listname: emc-pstc X-List-Description: Product Safety Tech. Committee, EMC Society X-Info: Help requests to [email protected] X-Info: [Un]Subscribe requests to [email protected] X-Moderator-Address: [email protected]
