4 ms·

There are many reasons to not use tantalum caps (inability to handle voltage transients requiring derating being the primary that comes to mind), but I've never

by 2bluesc 10y ago

There are many reasons to not use tantalum caps (inability to handle voltage transients requiring derating being the primary that comes to mind), but I've never heard of a company instituting such a policy because their assembly process was so poor. Did they also blanket ban diodes?

VLM 10y ago
Might be a retcon. There are sometimes good reasons not to use tantalum aside from the explosion and fire inevitability. At really high temperatures (engine computers etc) tantalum loses voltage rating down to less than 50% in some cases at the limit whereas ceramics lose capacitance with high temp (weird but true for X7R dielectric). This is a general rule of thumb and I'm sure you can find mfgrs whitepapers explaining how their expensive series is immune to this general class effect. In the bad old days tantalum volume was like 100 times smaller for a given capacitance than a ceramic but its not the bad old days anymore and its like 3:1 ratio for some MLCC and because of voltage derating a tantalum derated to 25V (aka a 50V+ cap) might not be all that much smaller than a 25V rated MLCC. Size of cap is no longer a relevant criteria, but customers are more excited than ever about burning exploding personal electronics, so ... Another fun one is leakage current for tantalum is about three orders of magnitude (whoa!) higher than MLCC. Its possible to build little ultra low power IoT things that sleep most of the time and a tantalum leaks more mAH than the processor but a MLCC would waste less. High frequency response is fun because neither is inherently better but in some applications one or the other is better. Generally the MLCC is going to have a higher Q resonance at a higher frequency. If that freq is irrelevantly high to the app and EMI issues, thats a great cap, whereas maybe for some apps sometimes tantalums could be resonant, admittedly at a lower Q, in the region of freqs important to the app or EMI/EMC issues. Not that ceramics are problem free. Piezoelectric effects are hilarious. Aging after heating drops ceramic capacitance, its possible to solder a SMD cap and watch its value drop a couple percent over the next day or so. As with all effects you can acquire manufacturers whitepapers explaining how their extremely expensive exotic series cap can be soldered in place of another of the same family if there's a design mistake and you need to correct it in production, so sure you can pay a lot of money for tantalums that work at 230C or temperature stable ceramics. Something I don't understand is ceramics being ceramics mean they should be good up to lava like temps and alloys being alloys mean you should be able to find something with a matching coeff of expansion thats good up to liquid metal temps, so why can't you buy ceramic caps rated to orange-hot temps? I haven't looked into it deeply enough to find out. There are practical reasons wet tantalums can't run at cutting torch temperatures but ceramics superficially don't seem to have those limits.

by 2bluesc 10y ago

VLM 10y ago

Might be a retcon. There are sometimes good reasons not to use tantalum aside from the explosion and fire inevitability. At really high temperatures (engine computers etc) tantalum loses voltage rating down to less than 50% in some cases at the limit whereas ceramics lose capacitance with high temp (weird but true for X7R dielectric). This is a general rule of thumb and I'm sure you can find mfgrs whitepapers explaining how their expensive series is immune to this general class effect. In the bad old days tantalum volume was like 100 times smaller for a given capacitance than a ceramic but its not the bad old days anymore and its like 3:1 ratio for some MLCC and because of voltage derating a tantalum derated to 25V (aka a 50V+ cap) might not be all that much smaller than a 25V rated MLCC. Size of cap is no longer a relevant criteria, but customers are more excited than ever about burning exploding personal electronics, so ... Another fun one is leakage current for tantalum is about three orders of magnitude (whoa!) higher than MLCC. Its possible to build little ultra low power IoT things that sleep most of the time and a tantalum leaks more mAH than the processor but a MLCC would waste less. High frequency response is fun because neither is inherently better but in some applications one or the other is better. Generally the MLCC is going to have a higher Q resonance at a higher frequency. If that freq is irrelevantly high to the app and EMI issues, thats a great cap, whereas maybe for some apps sometimes tantalums could be resonant, admittedly at a lower Q, in the region of freqs important to the app or EMI/EMC issues. Not that ceramics are problem free. Piezoelectric effects are hilarious. Aging after heating drops ceramic capacitance, its possible to solder a SMD cap and watch its value drop a couple percent over the next day or so. As with all effects you can acquire manufacturers whitepapers explaining how their extremely expensive exotic series cap can be soldered in place of another of the same family if there's a design mistake and you need to correct it in production, so sure you can pay a lot of money for tantalums that work at 230C or temperature stable ceramics. Something I don't understand is ceramics being ceramics mean they should be good up to lava like temps and alloys being alloys mean you should be able to find something with a matching coeff of expansion thats good up to liquid metal temps, so why can't you buy ceramic caps rated to orange-hot temps? I haven't looked into it deeply enough to find out. There are practical reasons wet tantalums can't run at cutting torch temperatures but ceramics superficially don't seem to have those limits.