Printed Circuit Design & Fab Online Magazine - Current Issue

Designer’s Notebook

A good library is built with an understanding of the manufacturing limits.

One of the primary factors in the quality of a printed circuit board design system is the makeup of the component footprints. The board can only be as good as the foundational pieces. Making it up as you go along is not a process for the long term. Errors or inconsistencies in the library account for a fair share of the feedback we receive from the fabricator. That is the wrong time to consider the fundamental building blocks of our collective occupation.

The source of the component footprints should be considered. A good cross-section of the supply chain provides the customer with schematic and layout symbols. This is, of course, to make it easier for us to implement their chips and other parts. CAD tools often come preloaded with a number of device examples to get you started.

Take those "freebies" with a grain of salt. One of the ways this kind of help can get in the way is in terms of traceability. One of the more important aspects of a good library is to have one and only one instance of a particular part. Naming conventions come into play here. Many, but not all, of the baseline libraries use naming conventions outlined by IPC-7351.

Read more: Wrong Steps: How Your PCB Footprints Could Be Holding You Back

Design constraints often morph with lessons learned from the prototype.

Can we just assume that every board design is going to be a nonlinear effort? While we know that everything is subject to change except the tape-out date, there are a few ways we can taxi toward the runway of product launch.

Today's supply chain is tighter than the one in the textbooks on product management. The printed circuit boards are often in the critical path, whether they are test jigs for prototypes or the final mass-production units. A schedule slip on P1 cascades to P2 and everything gets compressed. Execution is essential. Otherwise, we won't have time to learn the lessons of the first iteration before committing to the next.

This creates a dynamic where there are likely to be updates to the schematic at different points in the layout cycle. When we're designing something that is new from the ground up, we make educated guesses and assumptions about almost everything. Chips don't lie and can't fix themselves after a layout that doesn't "let them eat."

Keep 'everyone' in the loop for drastic revisions. Fundamental lessons learned on the initial design are inevitable. Thus, the designer must be able to create a second revision where it doesn't appear to have bug-fixes slapped on top of the first try. An example of this is when an analog circuit needs to have a series element added.

Read more: Surviving the Rush to Tape Out the Printed Circuit Board

Components drive circuit layout, but PCB designers are here to stay.

Like almost everything these days, the art of PCB design has its underpinnings in data. We're data driven – and swamped in a sea of information. Some of my favorite info is found on the literature offered up by the printed circuit fabricators. We get what's called a technology roadmap. While maps usually refer to geography, these roadmaps show the way into the future.

Open one up and it looks like a spreadsheet with physical attributes on the y-axis and calendar years filling up the rows. The numbers are sorted into columns with the values decreasing over time. The left column of data provides today's mainstream values for important characteristics of the circuit board.

We focus on minimum air gaps and line widths for inner and outer layers. Machining tolerances, dielectric thickness numbers, and so on are at their highest in the first column. Mass production boards should have technology that is aligned with standard processes.

When Google launched the first Chromecast dongle, Flextronics was the ODM. It owned Multek back then, which had five fabrication plants. Each site had different equipment and cost structures. To use its bottom-tier factory, we couldn't stack microvias, so I had to spin the board to stair step the 1-2 and 2-3 vias. The more price-conscious you are, the more you go for older factories.

Read more: Where Are We Going with PCB Design?

TECs can be a countermeasure to high current density.

Thermoelectric cooling, or TEC, is seen as a breakthrough in small refrigerators that do not consume a lot of power. Sporting goods stores carry micro-fridges that plug into a car's weird circular power plug. Some of us – not me, of course – can remember when those plugs were used to create a glowing hot element to light cigarettes. Getting cold out of the same socket took a little more technology than creating a short circuit.

Where would you use a TEC device? Aside from keeping a six-pack (or a transplant organ) on ice, electronics can be kept at a reasonable operating temperature with the addition of a component or a cold plate like the one in FIGURE 1. It is 40mm square and 3.2mm thick. The basic function is that it gets cold on one side while getting warm on the other. Put it in the other way to warm up the contents. This can be placed below the board or above the high load component if it has a flat top. More remote placement is possible by incorporating a heat spreader. We'll circle back to that shortly.

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