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January 2007

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Subject:
From:
Joyce Koo <[log in to unmask]>
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Date:
Mon, 29 Jan 2007 08:58:49 -0500
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Dave,
Are you telling me the LPI is so thin it has tunneling effect? (no longer an
insulation! under pressure).  Thanks.
"As an aside, I regularly, and 100% successfully, probe thru soldermask with
blunt scope probes and little pressure. Not really a
functional insulator ,but happily the ATEX certification people think it is"
Why ATEX certification people think it is?  What they relay on the
insulation properties for the performance of the c.c.t?
Many thanks.
                     jk

abcd

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of David Greig
Sent: Monday, January 29, 2007 8:45 AM
To: [log in to unmask]
Subject: Re: [TN] Soldermask Types implied in IPC-D-325A


Some other SM considerations:

Suitability to coat insides of via's without cracking - some of the newer
LPI materials are actually good for this.
Via tenting - small <0.25mm drill via's - again there are LPI materials that
can achieve a fill or coat depending on hole size,
both without voiding and cracking if the design has enough gap between drill
sizes.
Method of application, typically curtain coat or spray. Fabricator specific,
and limits what can be achieved.
Matt or gloss, lower solder balling or higher surface resistance?
Thermal, flux and cleaning withstand, aging, moisture absorption, Dk and Df
(more so Df) etc etc.

There are so many factors, some more relevant than others for a specific
design. Unfortunately the generality of the
constraining implied by SM-840 is far too wide to properly specify materials
for all but "consumer" applications. A bit like
slash sheets, but worse.

As an aside, I regularly, and 100% successfully, probe thru soldermask with
blunt scope probes and little pressure. Not really a
functional insulator ,but happily the ATEX certification people think it is


Best Regards

David Greig
______________________________
GigaDyne Ltd
5 Albany Business Centre
Gardeners Street
Dunfermline KY12 0RN
United Kingdom
t: +44 (0)1383 624 975
www.gigadyne.co.uk
______________________________

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jeffrey Bush
Sent: 29 January 2007 12:45
To: [log in to unmask]
Subject: Re: [TN] Soldermask Types implied in IPC-D-325A

FYI - .0015" is not an accurate callout.  I suggest using the IPC-SM-840
specification in lieu of this.  To obtain .0015"
minimum on all feature geometries you may need more than 1 coat.  The "wet"
weight of most LPI coatings target .0012", which
after curing result in thickness less than this.  Many OEM specifications of
old required a minimum of .0006", which was based
on epoxy, deposited masks.  Minimum thickness needs to be based on that
material's DWV, rather than a arbitrary thickness value.


Jeffrey Bush
Director, Quality Assurance and Technical Support VERMONT CIRCUITS
INCORPORATED
           76 Technology Drive - POB 1890
              Brattleboro, Vermont 05302
                Voice - 802.257.4571 ext 37
                    Fax - 802.257.0011
                       <http://www.vtcircuits.com/>

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Saturday, January 27, 2007 2:11 PM
To: [log in to unmask]
Subject: Re: [TN] Soldermask Types implied in IPC-D-325A

Hi Andrew,
That FAB note has a number of shortcomings.
"Construction to be SMOBD, using liquid photoimageable (LPI) solder mask
material, type XXXX. Apply to both external layers in
accordance with IPC, (Type B, Class 2). Use appropriate solder mask artworks
for each side of board."
I would thing SMOBD should be SMOBC.
I have never heard of a solder mask material Type "anything;" why put
anything like this "down" if it does not have an accepted
technical meaning?
"...in accordance with IPC" is meaningless without giving an IPC document
number-IPC-D-325 is a 'documentation' document; for specification purposes
you should be using IPC-SM-840.
(Type B, Class 2) also do not correspond to any accepted way of specifying
solder mask.

I would recommend a FAB Note like this:
"SOLDER MASK OVER BARE COPPER ACCORDING TO IPC-SM-840, CLASS H, TYPE LIQUID
PHOTO IMAGEABLE (LPI), 0.0015 MAX THICKNESS, COLOR:
GREEN, BOTH SIDES OF THE
BOARD.   ALL FIDUCIALS, LANDS AND HOLES, EXCEPT VIAS, SHALL BE FREE OF
SOLDER
MASK MATERIAL."

If you are in the process of wring a specification document for your PCBs,
my White Paper in the subject may be helpful-I am
sending the details in a separate e-mail.



Werner

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