I used to catch grief all the time for refusing to mess with someone else's design until I could find sufficient documentation (like a schematic! and some notes would have been nice) to get some idea of why the original engineer picked the parts he did!
Good point, Nancy. Today too many "prestige" manufacturers are no longer taking care of their good name: take for example Panasonic. It was considered that the very best TV screens available were the famous "Viera" plasma displays by that company. But, inexplicably, they decided to design their circuitry to intentionally diminish the contrast in several steps, so that the marvelous deep blacks and amazing contrast just went down the drain at "X" hours of use thanks to the on-purpose firmware. This intentional, "By design" move was probably made in order to make the panels last a given number of hours, but the buyer was actually cheated! As in your case with your Sony camera, the Panasonic plasmas were pretended to be free from defects, and operating "as designed"... but the owners kept noticing that the rich, deep blacks were turning a dull dark grey instead. I guess the Class Action Suite is still in the court, and the company still denies anything wrong, and keeps sending defective-by-design products to many unsuspecting customers. On the theme of counterfeit electronic components, I can attest to the seriousness of the problem: I've seen examples of almost perfect falsifications of large electrolytic capacitors dressed like Hitachi ones, but the initial performance is marginal and with many units open-circuited on delivery. the only way to identify these fakes is by checking the numbers in the official Hitachi catalog, where you can see the "FA" series does not exist in the Hitachi line of capacitors.
But the problem is much more serious when you start finding other components like TO-3 cased power transistors: I've seen and tested several examples of faked Motorola MJ15001 and 15002 power transistors that were in reality repackaged 2N3055 or similar!!! The fake factory got or implemented a way to imitate the exact appearance of the expensive, heavy duty 15001 by placing inside the can a much lesser, garden variety 2N3055. At first sight, the transistor looks and tests "good", but will fail miserably at higher currents or breakdown voltage, well under the authentic one specifications. The same is happening in IC's and many kinds of electronic components, so beware!. Amclaussen.
Umm, it's not a cost thing; it's a performance thing. The destructive electrolyte produces a much lower than normal ESR, and if you wait a bit, it also produces a much higher than normal capacitance for the size of the part (the already mentioned photo of a small capacitor in a larger capacitor -- physically and electrically -- shell comes to mind). Why would a design engineer choose a known-good part with higher ESR and larger form factor? Especially when accelerated testing (acceleration of the NORMAL failure mechanism) shows that the lifetime of the part is still many times the required mission life. It was only testing to failure (frequently done by consumers) that demonstrated that there was a new failure mechanism in play, and the standard accelerated testing is terminated way before failure.
No, the electrolyte does not age; it attacks the dielectric and the underlying metal. It starts doing so as soon as the capacitor is assembled (which accounts for the negative time parameter in the Weibull plots -- most people start the clock when a voltage is first applied across the part). There are now some nice photos of the effects of the chemistry in the Wikipedia "Capacitor Plague" article.
Again, this "stolen formula" story is poular (and widespread -- multiple places and multiple times), but unintended consequences are way more likely.
The stolen formula story makes for a good movie, but it's not likely true. In 2004, I met the chemist who had DELIBERATELY altered the electrolyte formula to reduce the ESR so as to produce a marketing point for cutting into the Japanese market share. The Wikipedia article "Capacitor Plague" now has some nice (if you're in the failure business) photos of the results.
Voltage derating is for the normal failure mechanism. The bulging capacitor failure mechanism is a different mechanism. It is totally chemical and starts as soon as the capacitor is assembled, continuing even if there is no voltage across the capacitor (of course, as with all chemistry, heat accelerates the reaction).
Another thing nobody mentioned is internal heating of the capacitor. Caps have a parameter called equivalent series resistance (ESR), which varies with frequency. High ripple current applications (e.g. output caps on a switching power converter) require special attention to ESR. Ripple current applied through the ESR causes heating...specifically, P= i*i*ESR.
I can see a bright but inexperienced engineer noticing two different 100uF caps on a BOM...five pricey and 20 cheap. Logical thing to do is make all 25 the cheap version, saving a few cents in component cost and removing a "duplicate" line item from the BOM. Seems like a great solution until the manufacturer gets a flood of returns 2 years later, realizes ESR was overlooked and the costlier low-ESR caps were on the BOM for a reason....
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