Except for the occasional pcb bonfire, they're great parts. ESLs are tiny, and their esr's are just right for damping a lot of linear and switching regulators. They work fine cold, too.
A polymer aluminum (or several big paralleled ceramics) in series with a 0.25 ohm resistor just annoys me somehow.
John
I spent a little time trying to do the same thing. This was about
1978, when the company I was working for was painfully switching over to partial SMT boards. Of the large value solid SMT tantalums, about 1 in 5000 caps would short and fill the burn-in room with toxic smog. Each radio has about 30 of these, so the failure rate was deeming unacceptable. I even pre-tested 100% of the tape and reel caps for capacitance, ESR, leakage, and changes to any of these with full working voltage applied. I tested about 50,000 caps, and never found a single reject (other than mechanical damage). We also tried juggling vendors, with little change in failure rate.Worse, the caps would sometimes fail in the field, in the same manner (fire and toxic smog). Also, the failures tended to continue throughout the life of the product. Attempts to perform an accelerated life test in the lab was a wasted effort. Nothing would fail, no matter how hot, cold, over voltage, or abusive I became.
After about 3 years of this nonsense, I started to notice a pattern in the field failures. We used polyurethane conformal coating to waterproof some of the boards. The boards without the coating would eventually blow a capacitor. Coated boards seem to last forever (there were other failures, but the tantalums survived). We also tried stuffing boards with hand soldered dipped tantalums, which experienced zero failures.
I began to suspect water incursion, caused by cracks in the Epoxy B package, or leakage around the leads. However, I never had time to investigate further. Management had other projects pending and deemed the problem mostly solved by the use of coatings. I did some work comparing the thermal expansion coefficients of Epoxy B and sold Tantalum, but forgot what I had speculated. I also had a rather bad experience with silicon epoxy semiconductor packaging, which would expand when hot, suck in corrosive flux, and shrink back to normal when cool, thus sealing in the corrosive flux.
Well, the alternatives also have problems. I like SMT tantalums.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
caps
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amics
I have had an LT1016 oscillate for lack of bypass. The folks at Linear weren't kidding about the need to bypass the positive supply rail to the part.
Yes, they are perhaps more accurately described as an incendiary device than an explosive device.
Years ago (before SMTs!) Tektronix all but banned tantalum capacitors from their products due to the incendiary problem. This was probably brought on by a lawsuit - a van full of electronic gear had gone up in flames, and the foresenic experts traced it to a 465's tantalums. This was despite significant voltage derating - though not obeying the 3 ohms/volt restriction later enforced at design review.
Tantalums of the time clearly had batch-related sensitivities. The 3 ohms/volt reduced the energy that a capacitor could receive during a "temporary" self- shorting micro-pore event, and the capacitors would self-heal. Of course, this often makes the capacitors pretty useless for most of their intended applications.
-f
Yep. In the late 1980's, one of the XT clone manufacturers apparently ran out of cheap junk electrolytics and temporarily shipped motherboards with tantalums. After about 2 years of operation, I get a call from the panic stricken customer announcing: "The computer is on fire. What do I do"? I thought about it for a few milliseconds and replied: "Turn it off". Over the phone, I heard the distinctive click of the oversized power supply switch found on XT power supplies. OK, so some customers are kinda slow thinkers.
When I arrived, I found a carbonized mess where once stood a tantalum dip cazapitor. I couldn't resist turning it on to see what happens. Bright red glow with attendant toxic fumes. If the office hadn't been well ventilated, I suspect it would have formed a mushroom cloud. I chopped off the remains, didn't bother replacing it with another, cleaned up the mess, checked out the machine, and left.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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