Tantalum after ReCap catches fire — why?

[jmb2011]

So I just recapped a Macintosh LC 1 and everything was going okay. First time for me recapping a vintage mac. Turned it on after recapping it got a startup chime, then next thing you know the cap pops then about an inch tall flame. Quickly blew it out, desoldered it, cleaned it up, and no damage done to the board. Soldered in a replacement and now it runs without issue.

question is, what caused it? Did I really just get ahold of a bad cap? I thought these tantalums are better than the electrolytic cans that they replaced. Sorta afraid of running the LC long if I’m not right there by it. C15 is where it occurred on the board. A 47mfd 16v cap.

It runs like new now with the exception that sometimes it doesn’t chime and won’t post. Maybe a loose connection somewhere?

 

[Byrd]

Just bad luck I'd say, check polarity and deep clean everything around where the old caps were noting your other niggles.

 

[mdeverhart]

You’ve found the main drawback of tantalums - when they fail, they fail catastrophically - blowing up, and often shorting their terminals. The main causes are reversing their polarity (installing them backward) and exceeding their voltage limit (which is why they are so excessively de-rated, 25-50%, so a 7.5V-10V cap for a 5V rail).

I’d guess you installed the one that blew up backward, though it’s possible you just got a bad cap.