to prevent IMC growth on leads: (1) get gold nickel finishing on solderable leads. You will need to dip solder tinned leads prior to use after long storage. (Solderability test)
(2) Nitrogen pack or vac pack - depend upon how many years you intend to use.  With dry pack.
(3) Low temp storage - depend upon what package material used in the components.  Choose temperature wisely.
(4) Pack the qty based on your logistic forecast usage.  If it is very long term, hermetic pack. Small pack only the qty you need within X month. 
(5) Assess cost benefit vs redesign of whole system plus full qualification.  If other parts on the system might facing the same notice soon. Redesign might be the cheaper option. But if you can not get COTS parts in your environmental req range, you don't have choice but life time buy.
(6) The decision should be carried out at program level, not engineer level (at least, project engineer level).  Make sure program is aware of your decision (no surprise when the true cost of life time buy showed up).
(7) Make sure fully documented where you stock your parts (especially for something that fall into "need to know" bases. People retire, things get lost in transition) and test-check frequency - make sure you have 6 month lead time of good parts to use just in cast anything go wrong. So you have time to correct action.
Etc.etc. 
Best of luck.   My 1.92 cents. 
--------------------------
Sent using BlackBerry


----- Original Message -----
From: Don Vischulis [mailto:[log in to unmask]]
Sent: Monday, October 14, 2013 09:16 PM
To: [log in to unmask] <[log in to unmask]>
Subject: [TN] Resistor Last Time Buy

My company was informed that the only QPL supplier of a wire wound resistor was terminating production. We are faced with placing a last time buy to last the life of the program. Shelf life /solderability preservation is a concern. Can anyone offer experience, references to specifications, or other advice on how to extend shelf life of tin/lead coated leads for many years?  Any other considerations to address?

Don Vischulis

Sent from my iPad

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