Designer’s Notebook

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.



Figure 1. A TEC device. (Amazon)

Read more: Thermoelectric Cooling on Printed Circuit Boards

Leave more metal behind.

Being cool used to be easy. Modern times call for a more comprehensive approach to keeping the lights on when it comes to our PCB layouts. The early days of electronics saw through-hole components bearing a single transistor that sat well above the board much like a water tower commands the skyline in a pastoral setting.

The device was free to blow off as much steam as required without scorching the stuff we call FR-4. Fun fact: Did you know that the FR in FR-4 stands for “fire retardant” and that 4 is the number of iterations that lead to the resin/glass combo that undergirds our industry’s history to this day? The material is rated by its ability to withstand high temperatures without breaking down.

One of the most important characteristics of FR-4 is the glass transition temperature, or Tg, where the material simply melts down and fails. It is measured in centigrade, and a working number is between 140° and 170°. That will not be sufficient for extreme environments and hard-working chips. There may be exceptions but you’re nominally looking at exotic dielectric materials that are meant to cater primarily to data centers and broadcast scenarios. The ancient art of ceramics plays a part in withstanding higher temperatures for devices and PCB laminates.

Read more: Thermal Considerations for Printed Circuit Board Designers

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