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May 2010

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From:
"Thayer, Wayne - IIW" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Thayer, Wayne - IIW
Date:
Thu, 20 May 2010 08:45:55 -0400
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You can check with other vendors.  Generally we don't bother with silks for our designs if the parts are small--they can just get in the way.  But some vendors offer (for a bit more money) photo-defined legend, which is basically a very highly pigmented LPI soldermask product.  Not only can you image 0402 outlines, you can make legible (for those under 40) reference designators for them!

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Wednesday, May 19, 2010 6:59 PM
To: [log in to unmask]
Subject: Re: [TN] Component legend marking on PWB with 0402 components

Hi,

  Yes, I have the same issues. Marking for 0402 and smaller is "Very
Difficult". Having looked at a lot of boards for assembly I also find 0402
marks result in "Less Than Dense" packing. Folks that use and mark 0402
could usually have used 0603.

  Here is an interesting side note:

        If you look at what these new silk screen printers can do from a
marking standpoint they are amazing. The problem is the interface media to
the fab house is typically gerber format and in house design rules trash
thin lines or lines close to pads. There almost needs to be a complete
re-think about silk screen rules as these new printers are a light year
ahead of old silk screen technology.

Bob Kondner



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Temkin, Gregg
Sent: Wednesday, May 19, 2010 6:49 PM
To: [log in to unmask]
Subject: [TN] Component legend marking on PWB with 0402 components

On a first shipment from a new PWB supplier, we found all the 0402 component
outlines deleted from our legend layers.  The PWB vendor said they
automatically deleted the marking during DFM because it exceeded their
manufacturing allowances and could have resulted in ink covering a portion
of the pad.

Our in house PWB designer uses a program to develop his pad and marking
dimensions.  The program would not generate a marking outline for this
device, so he designed the outline to be the courtyard plus some
manufacturing allowance.

Other vendors have supplied with PWBs with the same outline marking
configuration; seemingly without any problem.

I've reviewed IPC-7351A, but couldn't find mention of PWB marking
recommendations.  Is there guidance that I missed of is there another
document I should be looking at?  Is it generally assumed that outline
marking of 0402 and smaller devices is impractical?  Is placing a remotely
located legend the best way of showing 0402 placement and orientation?

Thanks for your help.

Gregg




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