Mail*Link(r) SMTP FWD>DESIGN: Tantalum failure modes Gary, I believe you tantalum (Ta) electrolytic capacitors are being designed/used for the wrong thing (misapplication). The following are some of the general (albeit conservative) printed board design rules/guidelines should be used as "good design practices/recommendations" I use to help both circuit design and printed board design personnel: 1) A good low-impedance ceramic (or similar dielectric) bypass capacitor should be close coupled and used for each source of "current spiking", such as an analog/digital integrated circuit. This is not the job for Ta's or other electrolytic capacitors. 2) Tantalum electrolytic capacitors shall be "sprinkled" around the design (generally based on current mapping) for each group of 10-20 IC's. The purpose of the tantalum is not to supply the "surge" current for each active component, instead it is to add capacitively stored energy to the power and ground system to stabilize the voltage. 3) A low-impedance ceramic should be located as close to the input terminals/contacts as possible and an additional Ta shall be located "close-by". 4) Input power and ground terminals/contacts shall be located adjacent to each other (close coupled) and not located on opposite edges of the printed board. Ta Electrolytic Capacitor Design Guidelines/Requirements. * Voltage Derating - for all applications, Ta's shall be derated to ~66% of their full rated voltage. For "serious" Class 3 applications, Ta shall be used at no more than 50 % of their specified voltage rating, improves the reliability by about 8X * Dielectric Breakdown - Ta's in high impedance circuits will self-heal in about 3 ns. This "self-heal" may/will cause a false signal to be coupled into your functional circuitry with unplanned results. However, in low impedance circuits dielectric breakdown will cause permanent damage (due to the I sqr'd R heating). Therefore, a series resistance of ~3 Ohms/Volt is recommended in all applications (the series resistance may include the impedances of the power/ground distribution system. * Capacitor Impedance - like most electrolytics, Ta's have a resitive/capacitive impedance that lowers with increasing frequency to about 10 kHz, then there is a trough from about 10 kHz to ~1 MHz, then the impedance becomes inductive with increasing frequency. At ambient (~25 degrees C) for the same voltage-capacitance, Ta's have an impedance of about 0.07 Ohms over the frequency range of 10-150 kHz increasing to about 0.15 at 1 MHz. In contrast, aluminum electrolytics have a reasonably linear impedance of about 0.4 Ohms over the range of 10 kHz to 1 MNz. The "lower" ESR results in faster current dumps which may result in greater dielectric heating (I sqr'd R loss). * Ripple - ripple voltage/current and frequenct effect the reliability of the capacitor (all types). Read the capacitor manufacturers specification and application requirements/limitations. * Voltage Derating and Operating Temperature - failure rate is directly proportional to about the 3rd power of the ratio of applied voltage to rated voltage. Operating voltage should be about 50% of rated voltage. * Reverse Voltages - At ambient, Ta's will withstand a reversed voltage of about 15% of rated voltage. Aluminum electrolytics don't like reversed voltages. If you need some more info. contact me off net. Ralph Hersey Lawrence Livermore National Laboratory email: [log in to unmask] -------------------------------------- Date: 4/16/96 3:41 PM From: Gary Peterson Does anyone have an opinion, comments or data to support a rumor I heard? I'm told that tantalum caps can fail shorted if required to supply a current surge. To prevent this failure aluminum electrolytics are preferred on digital boards. I thought Aluminum electrolytics weren't very stable in capacitance over temp and were not preferred to tantalum for that reason as well as size (more Capacitance per volume in Ta than Al). Gary P. --- Gary D. Peterson _/_/_/ _/ _/ _/ SANDIA NATIONAL LABORATORIES _/_/_/ _/ _/_/ _/ _/ P.O. Box 5800, M/S 0503 _/_/ _/_/_/ _/ _/ _/ _/ Albuquerque, NM 87185-0503 _/_/_/_/_/_/ _/ _/ _/_/ _/ Phone: (505)844-6980 _/ _/_/ _/ _/_/_/ _/ _/ _/_/_/_/ FAX: (505)844-2925 _/ _/_/ _/ E-Mail: [log in to unmask] _/_/_/ ------------------ RFC822 Header Follows ------------------ Received: by quickmail.llnl.gov with SMTP;16 Apr 1996 15:39:11 -0700 Received: from ipc.org by simon.ipc.org via SMTP (940816.SGI.8.6.9/940406.SGI) id RAA00700; Tue, 16 Apr 1996 17:34:03 -0700 Resent-Date: Tue, 16 Apr 1996 17:34:03 -0700 Received: by ipc.org (Smail3.1.28.1 #2) id m0u9HtB-00008yC; Tue, 16 Apr 96 16:02 CDT Resent-Sender: [log in to unmask] Old-Return-Path: <[log in to unmask]> Date: Tue, 16 Apr 96 15:09:20 MDT From: [log in to unmask] (Gary Peterson) Message-Id: <[log in to unmask]> To: [log in to unmask] Subject: DESIGN: Tantalum failure modes Resent-Message-ID: <"ULFoe2.0.jzF.Fh0Tn"@ipc> Resent-From: [log in to unmask] X-Mailing-List: <[log in to unmask]> archive/latest/3469 X-Loop: [log in to unmask] Precedence: list Resent-Sender: [log in to unmask]