Larry,

Thanks, this is a big help to me. I have to repeat the question though,
even now that you've narrowed the field down to LPI masks - your use of the
plural suggests more than one type of LPI mask. Are the different types for
different substrate materials, different degrees of environmental
protection, different dielectric requirements, etc., etc.? Or do they all
perform more or less the same and any minor differences are due only to
different manufacturers having their own recipes?

TIA again and Best regards

Pete Duncan





                    "Larry J.
                    Fisher"              To:     [log in to unmask]
                    <PWBMaskGuy@A        cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
                    OL.COM>              Subject:     Re: [TN] Which Solder Mask
                    Sent by:
                    TechNet
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                    06/08/01
                    09:12 AM
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                    PWBMaskGuy






Peter,

In answer to your inquiry about solder mask types and thickness
requirements:

1. Liquid, photoimageable (LPI) solder masks have become the standard in
the
PWB industry for two main reasons:

  A. Screen defined thermal or UV cure solder masks bleed mask onto the
pads
and cannot hold tight tolerances.

  B. Dry film solder masks are too thick and too expensive

The LPI masks are applied across the entire active surface of the board (by

blank screening, spraying or curtain coating) and then dried. Next, the
solder mask is photo defined with artwork and then unexposed mask is
developed off. Finally, the mask is cured (typically for 1 hour at 150 deg
C). The end result is a sharply defined solder mask that can hold tight
tolerances and solder mask dams between surface mount pads.

2. The thickness requirements for solder masks as defined by the
IPC-SM-840C
is loosely defined as "If a specific thickness or breakdown voltage is
required, is shall be specified by the end user on the procurement
document".
In reality, if you can get .4 mils (about 10 microns) of solder mask
thickness at the knee of the circuit traces, you should be pretty safe most

of the time. Much less than that can result in cracking of the mask,
scratching off the mask, etc. which can result in solder shorts. Requiring
much more than .4 mils as a minimum can lead to excessive solder mask
thickness in other areas and can result in solder shadowing during wave
soldering.

I hope this helps some.

Regards,

Larry Fisher

Allen Woods & Associates

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