The pure Difunctional has basically been replaced with a
          tetrafunctional additive. The reasoning for this is to
          provide a UV blocker for the LPISM (liquid photoimable
          solder mask) process. Without the UV blocker, the potential
          of ghosting, or image transfer thru the laminate from one
          side to the other is highly possible and highly detrimental
          to the soldermask image integrity. Especially since SMT is
          hear amongst us! It sounds like difunctional by choice would
          be past experience or strictly a cosmetic issue. The Thermal
          properties of the tetra or multi functional are somewhat
          superior to the difunctional. There still are a few
          suppliers building straight difunctional, but from a
          fabrication stand point, most select the the raw stock with
          the tetra or pigment additive.

          Dave Hoover
          Senior Process Engineer/Hadco Corp. Tech Center Two
          Watsonville, CA
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