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July 2000

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 28 Jul 2000 20:37:29 +0300
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Jim

If any vendor delivered me circuits like those and then said they were acceptable, I
would blacklist him illico prestissimo.

Brian

Jim Gilbert wrote:

> Pete,
>
> I agree with your evaluation 100%.  I also believe this is a debubbling
> issue because I've seen it many times first hand.  Some inks are more apt
> to bubble than others.  To resolve the issue with bubbles, hold times prior
> to tack bake are a standard in the fabrication business (for those of us
> who apply the ink with a screen).  If hold times are creating production
> problems then I suggest looking at other inks.  Some inks will debubble
> fast enough that virtually no hold time is required.
>
> Outside of the aesthetics, if the soldermask isn't voided then these boards
> are still acceptable.
>
> Jim Gilbert
>
> -----Original Message-----
> From:   Peter Menuez [SMTP:[log in to unmask]]
> Sent:   Thursday, July 27, 2000 7:48 PM
> To:     [log in to unmask]
> Subject:        Re: [TN] "Bubbly Soldermask..."
>
> 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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