Are you sure it forms a bead ? I used to use tantalum electrodes on copper vapour lasers because if I remember correctly tantalum had a melting point of over 3000C (that's C not F!) ----- Original Message ----- From: "Morgan Olsson" To: "Microcontroller discussion list - Public." Sent: Sunday, September 26, 2004 4:26 PM Subject: Re: [EE] capacitor types > Russell McMahon 10:11 2004-09-26: > >- The failure mode is often (usually?) a very hard short circuit. A bead of metallic tantalum forms between the leads. This clamps the power supply to ground and can be quite hard to find. On a board with many tantalums used for power supply decoupling with one only shorted to ground,troubleshooting without suitable tools can be difficult. > > To detect trace- component shorts I use to set a power supply to a couple hundred millivolts, one amp current limit, then inject it using hook probes at strategic places, and detect which way the curent goes by measuring voltage drop separately using microvoltmeter at two points along the traces. > > When measuring delicate cirquits, place a decent schottky to violtage limit the power supply. I remember once repairing an old HP calculator, and when i was finished i shut of the soldering station, and then the calculator died permanently. Researching it I found that the bench power supply (an old Mascot 7xx IIRC) was sensitive to the glitch that comes when shutting off the soldering transformer, outputting a spike of up to 15V, and the calculator was designed for 3V. That evening I ripped out the power supply card and exchanged for for a couple 78xx! > > >- As well as short circuiting on failure the capacitor may do some or all of smell REALLY bad, smoke, shriek, emit a jet of fire or explode. I have seen a single tantalum capacitor do all 6! :-) > > Seen that on a mosfet that was supposed to shunt regulate 400VDC large capacitor ;) Then that conducting plasma fed high voltage through the air to the controller board above. More popping... Grand strike... > > >If you want a product that generates service calls out of warranty, use tantalum decoupling caps and set the warranty period as short as possible :-). > > I only have had returns on semiconductor failures, and on several years old designs the power supply electrolytics. Never a tantalum - but att design time i am as careful about their rated voltage as I am with the semiconductors, and never allow a design that might have spikes. > > Anyway, if affordable and there is room, the dry aluminium caps can often be an reliability upgrade from both smala aluminium caps and tantalums. But also sometimes large ceramic caps can do the same. > > /Morgan > -- > Morgan Olsson, Kivik, Sweden > > _______________________________________________ > http://www.piclist.com > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist _______________________________________________ http://www.piclist.com View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist