Book Excerpt: 'The Printed Circuit Designer’s Guide to... Designing for Reality,' Chapter 4

Estimated reading time: 2 minutes

This is an excerpt from Sunstone Circuit's book: The Printed Circuit Designer's Guide to... Designing for Reality by Matt Stevenson

Chapter 4: What CAM Can and Cannot Do
With an understanding of the overall manufacturing process, we can dive into the most important design-for-reality areas. Most front-end CAM tooling departments can do a great deal to help turn a marginal design into a manufacturable product, but even these superheroes have limitations to their powers. Some of the most common edits that a CAM tooler makes will not be visible to most people but can drastically improve the yield and reliability of the product in manufacturing and beyond.

Why Did CAM Change My Design?
If CAM changed your design, it was probably related to solder mask. CAM departments spend a great deal of time making edits to the solder mask layers. Conceptually, the solder mask layers seem like one of the simplest outputs for a design, but that is not always the case in reality. Simple, right? Flood the entire board and add clearances for all of the solderable features, like SMT pads, through-hole pads, and test points. Ensure the openings are wide enough to provide enough pad for an effective solder. Minimize the size to optimize the dam between pads in tight pitch areas, and not expose adjacent copper features with the solder mask clearance. (This could cause solder bridge shorts.)

Sure sounds simple. In reality, solder mask clearances often come defined in the electronic footprint of each component and, depending on the source of the electronic footprints, create the condition where there are different rules for every component, some good and some, well, not so good. Designs can often come into CAM with no openings in the solder mask file for a given component, which can create its own set of challenges if not identified prior to manufacturing. It is, after all, difficult to solder parts onto a pad covered in solder mask (Figure 4.1).

An effective approach would be to use a standard solder mask swell to all of the parts. A good practice is to use a global setting in the CAD tool while designing and/or saving components to your parts library in order to account for the swell. Depending on the CAD tool, one approach might be more appropriate than the other. CAM toolers with a thorough understanding of the manufacturing process and its variations will make logical edits to the original file. These alterations allow for process variations while maintaining the functionality of the parts. They also prevent unwanted violations of the manufacturing specifications, such as IPC-A-600.

Another approach, recommended by IPC design classes, is to provide zero swell (net land or pad size). This allows each supplier to clear the solder resist to the minimum of their process capability, keeping the design rules consistent, and allowing the flexibility to vary clearance (which they already must do anyway.) CAM toolers might suggest other design modifications before sending the order into manufacturing.

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