Beyond Design: High-speed Rules of Thumb
Global PCB Connections: A Technical Overview of Long-flex Printed Circuit Boards
The Shaughnessy Report: A Handy Look at Rules of Thumb
Simplifying Your Design
November 9, 2023 | I-Connect007 Editorial TeamEstimated reading time: 1 minute
It’s safe to say that millions of dollars, not to mention man-hours, are wasted each year because of over-constrained, overly complicated PCB designs. Much of this is due to the increase in signal speeds and rise times, even in “mature” PCBs, and the extra cost is already part of the budget.
For this issue on simplifying PCB designs, the I-Connect007 Editorial Team spoke with IPC instructor Kris Moyer about ways that designers can avoid overconstraining their designs and making them needlessly complex. As Kris says, streamlining your design comes down to having a solid understanding of fab and assembly processes and the silicon tradeoffs that can simplify or overcomplicate your design, as well as the need to start working with fabricators early in the cycle.
Andy Shaughnessy: What are some typical snafus and missteps that you see designers make to overcomplicate their designs?
Kris Moyer: Here’s what often happens: Let’s say you have one connector on your board that needs tight tolerance. But rather than dimensioning to just that connector, locally, designers will do a tight tolerance to the data from the global dimensioning system, which now constrains the entire board.
Or, if they need perfect coplanarity on a BGA part for good BGA mounting, they’ll put co-planarity back over the entire board where they don't need it, because regular chips, gull-wings, and so on don't need the same amount of coplanarity as a BGA—or they'll try to hold layer tolerances: “I need a 2-mil layer plus or minus 10%,” because they know that 10% is normal for tolerance, but they missed the part of the spec that says 10% or 1 mil, whichever is greater. Fabricators can't hold that tight a layer-to-layer tolerance when it's below a certain layer thickness.
Below about a 10-mil thickness, the best fabricators can do layer-to-layer is 1 mil for nominal processing; If you want to hold a tighter tolerance, you're paying for 100 to get five boards. That’s just a couple of examples. Another is overly tight hole tolerances: “I want to have 150-mil diameter hole ±1 mil.” Again, it's unreasonable, right?
To read the rest of this interview, which appeared in the November 2023 issue of Design007 Magazine, click here.
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