Hi Joyce,
I do not know what the typical breakthru pressure range is, however something like a fastener against the PCB surface will
typically do it, as would a spring loaded test probe (like the tips of my LeCroy scope probes).
ATEX or intrinsic safety regs do regard SM as a CTI modifier (like 2221), but that's really all it's good for. It should not
really be regarded as functional insulation.
Over here in the UK we do not normally have the luxury of specifying manufacture X material Y, although I do occasionally spec
specific materials that I knew the fabricator could obtain. This is for hirel harsh environment, medical and some high speed
apps. Unfortunately these seems to become obsolete faster than most semi's.
It's with trepidation that ones tightly specifies, and with angst when not doing so...
Best Regards
David Greig
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: 29 January 2007 13:59
To: [log in to unmask]
Subject: Re: [TN] Soldermask Types implied in IPC-D-325A
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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