At 13:07 24/06/98 -0700, you wrote:
>Gabriela Bogdan wrote:
>>
>> Hi, TechNetters!
>> Could you please share with me your experience about pro and con's
>> regarding solder mask
>> between fine pitch-16mil- pads in relation to the finish of the PCB pads
>> ?
>> Not being greedy, I'll save the next question for after some answers.
>> Thank you,
>> Gaby
>>
 Gaby ,
Untill now you got answers from assembly peoples and not from manufacturing
area .
Limitations on solder mask between fine pitches are coming mainly from image
transfer :
1 ) resolution of photoimagible solder mask : depends on solder mask itself
, exposure system and artwork . Generally ,( depends on type of soldermask
used )  , soldermask is lower than pads , so there is always gape between
soldermask and the artwork ( contrary to circuitry photoresist , that is in
contact with artwork ) .
2 ) registration problems : circuitry registration is according to drilled
holes , soldermask according to circuitry . Add to this type of artwork :
first generation or diazo and coresponding dimensional stability of the
artwork .
3 ) adhesion of " soldermask dam " to dielectic base depends also on width
of the " dam " . Neglecticing of this point will lead to " washing out " of
soldermask in developing stage ,during solder ( HASL ) application or even
during assembly .

These points should be known to each PCB manufacturer , otherwise he will
supply PCB without " dams " or with " dams " on SMT pads .
Regards
Edward
Edward Szpruch
Eltek Ltd - Israel
Tel  972 3 9395050
Fax 972 3 9309581
E-mail :  [log in to unmask]

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