Saving my Mac SE/30. My first foray into SMD work
tothwolf at concentric.net
Mon Apr 27 22:17:08 CDT 2015
On Mon, 27 Apr 2015, Ian Finder wrote:
> On Apr 26, 2015, at 23:22, Tothwolf <tothwolf at concentric.net> wrote:
>> Still...I see a lot of people using SMD tantalum parts to replace SMD
>> aluminum electrolytics, and after seeing so many of these SMD tantalums
>> burned to a crisp after they developed internal shorts, I certainly
>> wouldn't use them for general purpose bypass work. SMD solid polymers
>> which do not exhibit that type of failure mode are readily available in
>> the same case sizes (round cans) as the original aluminum electrolytics
>> and are also available in the same case sizes as SMD tantalums. Not
>> only that, but solid polymers are less expensive than tantalum parts.
> The trick to replacing the aluminum cans with Tantalums is to
> over-provision substantially on voltage. A tantalum will eventually
> react much more violently to transient spikes than an aluminum can, as
> such anomalies slowly degrade the oxide layer in the cap, eventually
> leading to thermal runaway.
> A popular example are the 47uf 16v SMT cans that are common in Apple
> gear. People replace them with 16v tants but they should really be going
> up to about 25v.
> If you leave adequate headroom in the voltage spec, you will typically
> not suffer this failure. Still agree that if you wanna be truly safe
> polymer is the way to go.
According to the product engineers at Kemet and AVX whom I contacted when
I was first attempting to understand the mass failures I was seeing on the
boards I have (very old, low volume production, commercial product), solid
tantalum capacitors should generally only be used at half their rated
voltage, and there should be some sort of current limiting to prevent them
from going up in flames and instead "self-heal" when they do eventually
develop tiny holes in their oxide layer. The current limiting requirement
makes them less than suitable in these sort of retrofit applications where
they are being used to replace aluminum electrolytics in general purpose
bypass applications on a DC power bus (on my boards, they were used for
/everything/). Having seen first hand what solid tantalum SMD capacitors
can do when they fail short, there is no way I would use them in this
application. Solid polymers or better made modern SMD aluminum
electrolytics are just too easy to obtain...
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