I agree working with the stencil vendor is a good approach, yet you need to have control over the stencil design. The only way to do that is to create a stencil file, rather than depend on that "good relationship".
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Post, Scott E
Sent: Friday, December 16, 2011 6:37 AM
To: [log in to unmask]
Subject: Re: [TN] BGA SMD vs NSMD
Since we do most of our assembly in-house we're pretty confident in our stencil aperture designs and build them into the pad stacks in our ECAD tool. Even if we were using contract manufacturers, I don't see how their particular processes or pastes have any influence on what the apertures should be. A good pad stack design is a good design, period. The last thing I'd want when troubleshooting a problem from across the globe is to be wondering what the paste aperture really is.
I do know that when I was searching for ECAD software for laying out test vehicles none of the lower end tools had provision for custom paste apertures and most of them gave very little control over soldermask. That's a case where 1:1 artwork and a good relationship with your stencil vendor(s) is a sensible approach.
Scott Post
Staff Manufacturing Engineer
P.O.U. 0000-001E-0CTC
2151 E. Lincoln Road
Kokomo, Indiana 46904-9005
765-451-2983 (Phone)
765-451-0287 (FAX)
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Thursday, December 15, 2011 4:16 PM
To: [log in to unmask]
Subject: Re: [TN] BGA SMD vs NSMD
Pete,
I have found the paste aperture adjustment logic in PCB design software rather poor. Such tools will under and over size well but when it comes to QFN thermal processing or home plate designs for passives these tools are totally lacking. I use PADs and Altium here.
I would suggest folks generate 1:1 paste to pad apertures from the CAD tools and then provide rules to the stencil processor. The rules defined need to be coordinated with the stencil fabricator capabilities and hopefully the data exchange can be done electronically.
I would be curious what PCB design tools you are using and how you adjust paste aperture using that tool. I am sure there are new tricks and capabilities I am not aware of.
Thanks,
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pete
Sent: Thursday, December 15, 2011 10:40 AM
To: [log in to unmask]
Subject: Re: [TN] BGA SMD vs NSMD
Most CAD systems allow you to adjust the soldermask aperture over (or under) size at CAM output. This lets you change based on the design. Some designs force you to use a smaller oversize, meaning you need to pay more for a fabricator that can do that for you. Other designs have more room, you can use a larger opening, and a less expensive fabricator.
Again, I wouldn't recomend adding soldermask apertures to the CAD library. There's a reason the CAD systems allow you to define them on the fly.
IPC-7351 really is a great "best case scenario" document. But we rarely get to work in a "best case" world.
And don't let your fabricator change your artwork! They have to make some process adjustments, but the final result in copper should match the gerbers you gave them. Give them an inch........
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