First of all the pictures you show look pretty bad and
I don't believe I would ever ship something like
these. However, I want to point out that these may not
be 'rejectable'.

After coating LPI there is a hold time to allow the
mask to debubble.  This debubble time varies on many
things but primarily on the thickness of the copper
trace.

Think about the geometry of the trace.  If the trace
was 1oz (.0014") the mask would cover fairly evenly
and there would be little or no mask build up at the
trace.  At the trace thickness increases the trace
acts as a dam forcing the mask to build up along the
trace.  This is exactly what you are showing in the
pictures. The fact that the mask is very thin (most
pigment removed) at the top of the trace is a good
indicator that there is high copper here.)

The debubbling hold time is designed to allow the
volatiles (solvents) to escape the mask.  What it
looks like in your picture is that the boards were not
debubbled long enough before they were tack cured.
What you have left are small dots where the solvent
was forced out of the mask leaving behind a puddle of
mask.
I have done a considerable amount of cross sectioning
on this anomaly and have yet to find a void in the
mask.

I glanced over several of the emails responding to
your  original question and suggest that
contamination, curing, moisture etc. are based on
peoples inexperience with LPI's. This is a classic
debubbling issue.

I said in my opening statement that these boards may
not be rejectable. They are rejectable for the simple
reason that they are ugly but probably not rejectable
for performance issues.  If you cross section these
puddles you will see that there is more mask present
(rather than less which would be the case with  a
void).  If you have more mask you have more insulation
and in fact your insulation performance would be
increased.  Ok, this is pretty weak but...

The area I would concentrated on with your supplier
are: a) control of the plating process and
     b) if there is anything you could do to help them
control the plating (robber buss/thief area)

pete menuez
[log in to unmask]





--- Brian Ellis <[log in to unmask]> wrote:
> Steve
>
> This is incredible. Change your supplier as top
> priority :-)  Seriously, as the others
> say, the "bubbles" are probably due to solvent/water
> entrapment in the track corners.
> However, I see another problem which may/may not be
> related. Am I correct in thinking
> that it has been hot and humid where you are,
> lately? If so, it would seem that there
> has been hygroscopic contamination left on the
> board. On your first photo, see the
> myriad white points between the tracks? This looks
> like vesication to me. The most
> common cause is hygroscopic contamination (ionic or
> non-ionic) which pumps atmospheric
> humidty across the mask layer (all polymers can
> allow humidity through). The osmotic
> pressure rises until it actually lifts the mask off
> the surface, locally, forming a
> vesicle or, if you like, a microblister (see my book
> for detailed explanation). This
> could possibly be the cause of your bubbles along
> the track edges, as well, although
> these seem rather gross: the photo I use to
> illustrate vesication also has bubbles
> along some of the track edges, but the vesication
> between the tracks is on a similar
> scale. Whatever, it is a totally unacceptable fault.
>
> Brian
>
> "Stephen R. Gregory" wrote:
>
> > Hi all!
> >
> > We just got some boards in at receiving inspection
> that have thousands of
> > little "bubbles" in the soldermask all over the
> boards...they seem to
> > concentrate all along the edges of the features of
> the board. You can see
> > pictures of it at:
> >
> >
>
http://www.driveway.com/share?sid=e25a88c4.8e904&name=Pictures
> >
> > One picture is bubblemask.jpg, the other is
> bubblemask2.jpg...what would
> > cause that? It's a LPI...don't know the brand
> name. I'm thinking that either
> > the boards weren't dry or cleaned well prior to
> soldermasking them. I'm
> > rejecting the boards of course...
> >
> > My freedrive is filling up fast with all the
> pictures of the "pretty boards"
> > that we get from our vendors!!!
> >
> > -Steve Gregory-
> >
> >
>
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