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January 2007

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Subject:
From:
Jeffrey Bush <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Jeffrey Bush <[log in to unmask]>
Date:
Mon, 29 Jan 2007 07:45:03 -0500
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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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