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What are the differences between SMD and TH Aluminium Electrolytic Capacitors? Why are TH variants available with such greater lifetimes? (For example, Digi-key has SMD aluminium caps up to 10k hours, but TH up to 37k hours.) It would appear that the differences extend beyond the way they mount and look.

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    \$\begingroup\$ TH are generally bigger and able to take expansion/contraction more readily than SMD which are built down to a size? \$\endgroup\$ – Majenko Nov 4 '14 at 1:14
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Because they were engineered this way. There is no reason why you can't put on of the 37K hour parts on an SMD pad, but not many people would buy it. This is mainly because large parts/pads are significantly harder to reflow solder, can break easily, and take up more PCB area.

The primary reasons you would need a big value cap is 1.) you need it for a filter, and 2.) you need it for high current applications. If you didn't need high current flow on a big cap, than you would be able to surface mount it with a small pad, but if its top heavy, it might break easily and rip the copper pad on the PCB off. You have to use HUGE pads and you have to put thermal/structural relief vias under them. All of the above make running the PCB through a wave solder line worth the extra money.

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