Joyce is correct.
What works best:
A well-defined CAD library of the finished PWB component pad designs, based on the final gerber files from the board fabricator (NOT the designer's CAD file of layer-one SMT artwork, which should never be used for creating the aperture library or for ordering stencils.)
A well-defined gerber library of the stencil aperture designs (based on component part number) to match.
No separate, process-engineer-controlled manipulation of the aperture designs. Any changes in the stencil aperture design should be done in the CAD aperture library (stencil) file, so when a new stencil is ordered, the stencil aperture modifications are always re-captured in the new stencil. There needs to be no communication between the process engineer and the stencil fabricator, except the confirmation file just prior to stencil fab.
This puts the control in the designer's hands, but the process engineer works directly with the designer to get what he/she needs.
In real life, the designer's layer-one SMT artwork and the actual stencil aperture design are almost never the same. Some modifications are always required.
In real life, the layer-one artwork from the finished design database (the designer's layer-one SMT artwork), and the layer-one gerber file from the board fabricator are almost never the same. There are always some very minor changes due to the autorouting during fabrication layout.
So, the final layer-one gerber file (SMT file) FROM THE BOARD FABRICATOR should be used to create the stencil design file, but again they are seldom ever 1-to-1. Items such as crosshatched solder deposits for square belly pads, general aperture reductions, horseshoe-shaped apertures for MELFs, etc. are done to the stencil file. This is the only file or information given to the stencil fabricator. There should be no other modifications given to the stencil fabricator, whether through a separate email, phone calls, bar napkins, or Christmas cards.
So the print process engineer gets solder paste stenciled the way he/she needs it to be, and the designer gets board pads designed appropriately for current-carrying capacity, heat-carrying capacity, etc. The board design file is separate and different from the stencil design file.
And never the twain shall meet.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Wednesday, December 14, 2011 9:00 AM
To: [log in to unmask]
Subject: Re: [TN] TI Paste Recommendations
In addition, just by change the surface finishing (hole finishing) of the stencil, you would be able to increase/decrease amount of the paste. The opening should really linked to a specific finishing if you really really want to lock down the amount of paste. Being said that, I always like design in control of EVERYTHING (they should own everything). Put in to CAD may not be a bad idea if design know what they are doing in DETAILs ("devil" is in the details). My 1.92 cents.
Joyce Koo
Materials Researcher - Materials Interconnect Lab
Research In Motion Limited
Office: (519) 888-7465 79945
Mobile: (226) 220-4760
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Wednesday, December 14, 2011 9:54 AM
To: [log in to unmask]
Subject: Re: [TN] TI Paste Recommendations
Pete,
I would suggest that paste apertures should be part of a PCB CAD system though such is seldom, if ever, the case. Most CAD system do not support paste apertures well.
Yes, there are a lot of paste aperture modifications made to paste gerbers and that "Hand Tweaking" is a source for errors and inconsistencies.
As an example here is a link for something I worked up for parts reduction patterns, it is something I would like to see implemented in a paste aperture conversion code. I would also like to see the same kind of thing done for home plate aperture designs though that requires some knowledge of the components sitting at the aperture locations. This patter provides good support at thermal pad corners yet reduces allows for better outgassing than window pane patterns.
See: www.kondner.com/files/Stencil_5_Dot_Math.pdf
Dot Diameter = Sqrt(Rectangle) / 2
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pete
Sent: Wednesday, December 14, 2011 9:13 AM
To: [log in to unmask]
Subject: Re: [TN] TI Paste Recommendations
William, there's been some great information on QFNs posted in this thread, they are different than most SMT and it's good to know as much as you can about the assembly issues.
But from a CAD library standpoint, do you really need to define the paste footprint? Do you build home plate apertures into your 0603 footprints? There's a very good chance that manufacturing engineering is making quite a few modifications to your paste mask gerber file anyway. Check with them, they might prefer you just give them a gerber file that matches the pad shapes, and they will apply their expertise from there.
Now, via placement in and around the QFN, sodlermask apertures - those you need to be concerned with in your design. The Desginers Council Listserver had a discussion about some of this in the October archive.
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