Peter
I disagree with you, sorry! These boards are definitely rejectable. A surface defect
which crosses between two tracks is just cause for reject, IMHO, no matter what the
cause. Although, on the photo, there is no crossing (even though one comes precious
close to it), some of the bubble sizes are over half the spacing. It is therefore
conceivable to have two bubbles from opposite sides joining. Any trapped contamination
could be a leakage path.
A bubble is always highly suspect and the top skin may break away during soldering,
leaving craters which would be contaminant traps.
If I remember correctly, my mind going back 30 or 40 years, there was a spec which
stated something like "an isolated unspecified fault between two conductors will be
deemed acceptable provided that it does not reduce the insulation path by more than
1/3. Two or more such faults within any inch square will be cause for rejection."
Brian
Peter Menuez wrote:
> 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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