What Jeff indicates is absolutely correct. However, please keep in mind
that IPC-SM-840 is a soldermask material specification with good
guidance for applications. The actual application specification invoked
on the fabricator is IPC-6012, which clearly states that if a minimum
thickness is required, it should be placed on the master drawing. All
three documents, IPC=2221, IPC-6012 and IPC-SM-840 are recommending the
same.
Gary Ferrari
FTG Circuits
Director, Technical Support
860-350-9300
Jeffrey Bush wrote:
> 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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