Since you had observed the oxidationin SMT Component you have to check the Moisture sensitive level of the component.

The JDEC standard deeply explain the self life of the SMT components as well as the storage condition. If the component is to old (i.e the component stored for long time more than the self life), I would suggest not to use the component.

The JDEC standard will help you more in this issue.

The JDEC standard also explain how to increase the self life of the component.

This kind of depends on the termination style - leaded components can be pre-tinned using more active flux if necessary to break down the oxide layer. Chip components are trickier and replacement with new components should be considered.

Prevention, of course, is the preferred route and if long term storage is likely then vacuum packing is a good option to prolong solderability shelf life.

I would suggest you contact your solder supplier. They can evaluate the solderability of your components and recommend a suitable flux.

To answer a question like this you need to know what the original finish was on the leads. If this is only a few components perhaps a simple flux operation may remove the oxide.

You may also want to consider flux and solder dip, to tin the leads.

The answer is to retin the leads to remove the oxidation and prepare the leads for the SMT operation. A wetting balance test can be performed to insure the leads meet solderability.

The leads can be dipped in either lead free or leaded flowing solder pot.

Re-tinning by dipping into fairly aggressive flux then into molten solder is the positive method of removing oxide (and unwanted platings).

If these are fine pitch QFPs (less than 1mm centers) this becomes a challenge.

I know that Fancort offers a service to do this. Check out their website.