Mark Ross said:

>I have a question regarding what we can expect with "old" soldermask
>technology. This is our quandry.  We would like to pour solid copper close
>to pads and traces on our PCB designs.  However we have started doing this
>and we are anything but happy with the results.  This leads me to the
>following list of questions.  First a little background.  The pcb's are all
>single sided FR-4 1 ounce starting copper.  The soldermasking technology
>used in Lonco or Enthone and is screened on.  With that said....

I'm going to assume you mean a screened PC401 type mask. You do not mention
whether you use SMOBC boards, but I will assume so (if not, then perhaps
you should move that direction).

>1.  What is the minimum acceptable comfortable tolerance for registration of
>    screenable masks?  Is it still +/- 0.010.  I don't want to know a
>tolerance
>    that will cause a premium, just what board houses are comfortable with.

The registration tolerance to an absolute position should be better than
0.010" for even a screened mask. Typically, I's say your design should
allow for 0.008" min. clearance on screened mask and 0.004" for LPI to meet
most DFM rules. Our suppliers can put a web of LPI mask between 0.010" wide
pads on 0.020" centers.

>2.  What is the minimum accpetable comfortable tolerance for swells on pads?
>    Is it still 0.010 per side for a total of 0.020.  Again, as above, what
>    are board houses comfortable with?

Sorry "swell on pad" is not in my dictionary. If you mean the buildup of
tin lead on HASL finished copper pads, I'm used to seeing about 0.002" max,
but the exact manner of meaurement must be agreed, and there is usually a
permissible deviation and average height used.

>3.  If I was to use a zero swell on the pads and allow "some" soldermask
>on pads
>    why or why not is this such a bad practice if the solder joint is good?
>    I may end up doing this to our designs to help with short problems we are
>    experiencing where the lead is longer than the pad, the mask is
>    misregistered but still withing tolerance and causes the lead to short to
>    a nearby track.

You could of course permit this in your own criteria. In general it is not
acceptable, and you must worry about not only what can be seen but thin
bleed of mask onto the pads. This would be more pronounced for screened
epoxy.

>4.  Does LPI cause a board you pay $1.50 for to go to $15.00 or is this just
>    a board house trying to scare us because they can't or won't do LPI?

I'd say the latter. LPI is in general more efficient than screened epoxy.
For high volume boards, it is the norm today. But a $1.50 for a board big
enough for 250 components impiles you are using a _very_ low tech board.

>You must understand that the designs are shrinking by a factor of 2 every
>year, the component count is not going down (usually around 250 components
>minimum), and I have hit the point where I can't afford to keep traces 0.020
>to 0.030 away from nearby pads.

With apologies for sounding vain, most of us here passed that point a while
back, Mark. You ought to be able to go to 0.010" or less unless the boards
are being processed in relatively primitive manner.

>That is why I am asking these questions.
>BTW, Double sided boards are out of the question, I do need to keep this job.
>Thanks for any input you guys can offer!

Check other suppliers - but if you are describing a paper or phenolic board
with punched holes, then the comments I have made may be irrelevant. That
type of product is something I know almost nothing about. I thought most of
that pretty much moved to the third world.

If you continue very long with a doubling of density annually, you will not
be able to use punch and crunch single side boards for long, so maybe you
better send management a wake-up call.

I would suggest you order a copy of a book by Norman Einarson - "Bare Board
PWB Design Manual" 1995. Order from Printed Circuit Technology,
617-272-0938. This book is well written, but aimed at board technology in
the 90's.


cheers,


Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com


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