Better explanation than mine.
Eric Smith <space...@gmail.com> wrote:
>Some people seem to think that "reforming" an aluminum electrolytic
>capacitor is some kind of cheat, akin to zapping NiCd cells or
>rejuvenating CRTs. Actually reforming is the same electrochemical
>process that the manufacturer uses to "form" the capacitor in the
>first place, building up the aluminum oxide layer, before the sheet is
>rolled into cylindrical form. The manufacturer typically uses a
>forming voltage higher than the rated voltage, from 135% to 200%, to
>provide margin for shelf life.
>
>When the capacitor goes unused for an extended time (shelf life), the
>oxide layer gradually breaks down, increasing the capacitor's leakage
>current and reducing the effective usable voltage of the capacitor,
>which is proportional to the minimum oxide thickness. If the oxide has
>developed spots that are too thin for the applied voltage, it may be
>damaged ("punch-through") when that voltage is applied. Punch-through
>tends to be a runaway process, so even a small amount of punch-through
>usually completely ruins the capacitor. Reforming the capacitor by
>applying current-limited power rebuilds the oxide layer to prevent
>this type of damage, and to reduce the leakage current back to within
>the specifications. The current limiting is what prevents the
>reformation process from causing punch-through and damaging the
>capacitor. Many of the capacitor vendors actually publish
>recommendations for reforming their capacitors.
>
>See for example information on manufacture on pages 13-14 and a brief
>recommendation of reforming procedure on page of 17 of Kemet
>publication F3304 dated June 2009:
> http://www.kemet.com/ProductCatalog%20Documents/F3304.pdf
>Also pages 2-4 on manufacture and page 16 on "recondition" (reform) of
>"CDE Aluminum Electrolytic Capacitor Application Guide":
> http://www.cde.com/resources/catalogs/AEappGUIDE.pdf
>Also pages 1-5 of Nichicon "General Description of Aluminum
>Electrolytic Capacitors":
> http://www.nichicon.co.jp/english/products/pdf/aluminum.pdf
>
>In at least some aluminum electrolytic capacitor manufacturing
>processes, there is actually a reforming step done after assembly, in
>addition to the initial forming. See page 9 of the Panasonic
>"Aluminum Electrolytic Capacitors Technical Guide", dated April 2013:
> http://industrial.panasonic.com/lecs/jp/i/29880/TAL_E/TAL_E.pdf
>
>The reforming process WILL NOT fix other things that may go wrong with
>the capacitor, such as failed seals allowing the electrolyte to dry
>out, or corrosion, or punch through which can result if the oxide
>layer is degraded and voltage is applied without current-limiting.
>
>The US DoD published a technical handbook detailing their policies and
>procedures for reforming aluminum electrolytic capacitors that sit in
>inventory for years, MIL-HDBK-1131. As of 1999 this is "for guidance
>only and should not be cited as a requirement, but the information in
>the handbook may be useful in determining or evaluating requirements."
>For non-mil-spec capacitors, it recommends inspection and possible
>reformation every 3-6 years of shelf storage. It recommends disposal
>after 12 years of shelf storage, but AFAICT they're just being
>conservative, possibly due in part to not having enough practical
>experience with reforming very old capacitors.
>
>Shelf storage is of course equivalent to having the capacitor
>in-circuit but unpowered. Having the capacitor powered in circuit for
>any significant length of time will reform the oxide to some extent
>based on the applied voltage, though not up to original factory spec.
>
>When I reform capacitors myself, I use a reforming voltage of 135% of
>the rated voltage. Since I use a suitably low current limit, this has
>no significant probability of damaging the capacitor, but as with the
>initial factory forming, provides some margin for further shelf life.
>In my experience, aluminum electrolytic capacitors in equipment that
>has been unpowered for 30 years or more almost always need
>reformation, but they almost always meet factory specs (capacitance,
>ESR, and leakage at rated voltage) after reformation. Since I don't
>tend to restore equipment newer than that, I don't have any empirical
>data on how much shelf life they can have without needing reformation.
>
>I'm not particularly advocating for or against reformation, as
>compared to replacement. Anyone restoring equipment with electrolytic
>capacitors is advised to to read the references and decide for
>themselves.
>
>Eric
