Tom & Gloria
      Tom's comments regarding hot air leveling (HASL) are the best method I
have found to date for oldlot date code (LDC) PWBs.  I would like to add the
following:
1. Depending on age and the original thickness and type of solder coating,
solder stripping may be in order.  Un-fused electroplated solder typically
allows for degradation of the base metal.
2. Perform careful inspections for delamination and measling with regard to
your internal and customer requirements.
3. Also be careful to observe dimensional changes in warp and twist.
4. Depending on the design of your PWB, you may not have an edge suitable for
clamping. Clamping may and probably will cause some minor damage during
processing.  If you have areas where such damage is preferable, give your
service supplier CLEAR instructions.  Also be sure to understand the risk of
this process once the PWB is fully depaneled.  Consider dedicated tooling
which will clamp to mounting holes if all the PWBs will share a common
mounting hole feature.  
5. Also consider having a removeable breakaway strip incorporated to your
design as delivered.  This can be used as your clamping point and permit easy
removal by a shear.  Such strips can be used for a number of things, tooling
holes for assembly of ultra-high density designs, SIR patterns, TDR runs for
controlled impedence designs, bar codes.......

   Experience has proven that PWBs of 3-7 years of age can be successfully
soldered to military standards using RMA fluxes.  Our practice was to HASL
all old LDC PWBs for the current month by sending them to a local fabricator.
 PWBs were immediately placed in kits and not returned to stock.

Steve Mikell           Process Engineer, SCI Systems Plant 13
[log in to unmask]   Huntsville, AL