Matthew:

     From my perspective, the question that needs to be addressed FIRST is
     where in the process is the ENTEK applied to the board.

     1) ENTEK BEFORE or AFTER Bare Board Test

        By itself, ENTEK is an insulator. Therefore, if it is applied PRIOR
        to bare board testing, the bare board test probes can create "false
        opens" due to residue buildup on the tips of the probes.  This
        might indicate that ENTEK AFTER bare board test would be better.

        On the other hand, a mild microetch is used to prepare the surface
        prior to ENTEK application. If the dip process is used and the
        microetch is fresh (i.e. strong solution), excessive etching can
        result.  This can result in circumferential barrel voids at the
        knee of the holes, where the copper is the thinnest.  If ENTEK is
        applied AFTER TEST, this type of defect will not be discovered
        until the board has been populated with expensive components.

        Some people subscribe to the policy "NO chemical operation shall be
        performed AFTER bare board testing".  Each vendor's process and
        controls must be evaluated to assess the likelihood for this type
        of failure.

     2) In-line vs. Batch

        In my opinion, in-line would be better and more repeatable, if the
        total volume of boards that the vendor processes warrant the
        equipment expense.  Properly managed, the dip process will work.

     3) "Spotted" Boards:

        Another "problem" to look out for is moisture spotting during
        shipment.  ENTEK uses a DI rinse in the process and, if vias are
        small (0.013" dia.), moisture can be trapped inside the barrel of
        those vias.  Placing the "drip-dried" board in a plastic bag for
        shipment, can result in residual moisture STILL residing in the
        bag.  Shipment of the boards to the final destination (shipping
        vibration, change in temperature, etc.) causes the moisture to
        migrate to the inside surface of the bag and, in turn, back onto
        the board.  The thickness of the ENTEK on the board will be reduced
        where there is intimate moisture contact. The appearance looks like
        "water spots" on a drinking glass.  They COULD result in localized
        ENTEK "voids" and potentially cause solderability problems, if the
        boards are left on the shelf for an extended period of time.

        One test for "dryness" of a board is a deceptably simple one:

          a)  Take a piece of colored construction paper (or the cardboard
          stiffener on the back of a pad of paper) that will darken when
          exposed to water and place it on a flat surface (e.g. table).

          b)  Take a "dry" board and firmly slap the face of the board onto
          the cardboard.  If it shows dark spots on the cardboard when the
          fab is removed, the fab is NOT dry and could caule spotting
          during shipment.

        My suggestion is to have the boards BAKED after ENTEK to eliminate
        this potential problem.

     I advocate the use of ENTEK for fine pitch QFP designs, because of the
     resultant pad flatness, as compared with HASL.  However, it does have
     some "side effects" to watch out for.  It is NOT the "panacea for all
     ills", but it is better than some of the alternatives in certain
     designs.

     The above opinions are my own, etc.,etc., etc.

     Good Luck!

     Bill Fabry
     Plantronics, Inc.
     (408) 458-7555

     e-mail:    [log in to unmask]


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