<<   1. Our contractor is using WS-609 solder paste from alphametals for 
   double-sided process. The board is cleaned after each side process before 
   going to the next side. My question is: is it necessary to clean after
each 
   side process? can it be cleaned only once after the assembly?  >>

Yuan,
If they are doing this operation now, I would not recommend stopping it.
 Flux residues (of almost any kind) become progressively harder to removed
with each successive exposure to reflow temperatures.  The WS609 is a pretty
good water soluble paste, but don't know what it's cleanability would be if
you baked it on.  Why subject the board to an additional cleaning challenge?
 As to whether or not such interim cleaning is "necessary", only a designed
test could tell for certain.

Doug Pauls
CSL
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