Dear Technetters,

Thank you for all valuable inputs.  The problem is now resolved. The reason
is that they are slightly temperature zone inside the baking oven, when the
ventilation is fixed and proved for no temperature zone after the thermal
couple calibration, the problem is fixed.  In addition, the problem boards
come from low temperature zone.

_____________________________________________________________


At 06:45 AM 1999/4/15 -0500, David D Hillman wrote:
>Hi Albert! I suspect that you are driving off/volatilizing some type of
>ionic contamination with your baking procedure. One method investigating
>this is to bake a set of boards at 90 C for 65 minutes, run the omegameter
>test and compare results. A temperature of 150 C is high enough to
>volatilize a number of different ionic contaminates species. Doesn't such a
>high bake temperature result in solderability problems for your pwbs? Good
>Luck.
>
>Dave Hillman
>Rockwell Collins
>[log in to unmask]
>
>
>
>
>Albert Mok <[log in to unmask]> on 04/14/99 07:48:44 PM
>
>Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
>      to Albert Mok <[log in to unmask]>
>
>To:   [log in to unmask]
>cc:
>Subject:  [TN] ionic contamination
>
>
>
>
>Good morning, technetters,
>
>We aer using Alpha 600SMD for detecting the ionic contamination of bare
>PCB.  All boards are D.I. rinse and dried prior for the testing.  The
>reading come out is about 10ug NaCl/sq.in.  However, when the boards sent
>to baking at 110 C for four hours and the boards are tested again, the
>reading drop to 3ug NaCl/sq. in.   We do not suspect inadequate baking at
>the first time.  We bake the boards at 150 C for 65 mins.  The solder mask
>is photoimagable solder mask.
>
>This is not a single case.  Everytime when the contamination is failed,
>then bake the boards again, the result turn to positive.  However, if we
>prolong the baking time at the first baking, it does have this result
>apparently.
>
>Do you have this experience?
>
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