Hi,

Looking for some expertise regarding bare board fabrication and the solder
mask process. We have a supplier (China) that decided to add two layers of
mask versus one layer (not called out in our specifications).  The reason
for doing this is explained by them below.  Our end customer does not like
the two-layer mask and thought it was due to a rework process.  I am not
familiar with a two-layer solder mask process, is this something new?  Is
this OK?  Their solution is to not do it anymore, but I want to understand
is this something others have seen and approved.  If so, why?

In addition, during this two layer process, they say they will have air
bubbles (note 2 below), which we found in a cross section.  They wrote that
going to one-layer would eliminate the air bubbles? Not sure if that is true
or if their process is questionable.

Lastly, we also found thin solder mask stringers stretched across some
pads.  They claim dirty worktable (note 3 below) I wonder if this was caused
by the extra solder mask layer application?

I appreciate any comments.

Thanks,
Jim
*
COMMENTS FROM SUPPLIER
A1: (1) Supplier adopted twice solder mask coating process to produce this
PCB. They are NOT reworked PCB. As the copper thickness of this PCB is 2OZ ,
Supplier adopted twice solder mask coating process based on below
conception.
a: Considering the working power will be huge when PCBA’s working, to
perform twice solder mask coating process will enhance PCB quality and
prolong PCB cycle life, preventing solder mask being broken down if the
solder mask aging after long time usage
b: As usage frequency of this PCB is higher than general PCB, adopting twice
solder mask coating process will prevent outside signal interferer for PCB
when the solder mask’s aging.
(2) As adopt twice solder mask coating process, solder mask  is too
thick(within 60~70um), the air bubbles in the solder mask is difficult to
get away
 (3) The stringers on pad is caused by adhering foreign stuff from worktable
before OSP process.”*

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