Designer’s Notebook

John Burkhert

Shortening and folding traces takes creativity and persistence, as long as the timing budget is met.

Printed circuit boards are becoming more complex, with high-speed interfaces more common. Whether it is a PCIe, Ethernet, USB or memory of some kind, clock nets proliferate across the board. Those clocks have kindred spirits in nets that want to hit the receiver in conjunction with the ticking clock.

Crucial parameters of a group of traces include the target length or maximum. Less is more. Most other signals on the board will switch periodically. Meanwhile, the clock switches all the time. The clock uses the same voltage, but the constant stream of “10101010101…” creates more energy fields than a seemingly random sequence of ones and zeros. These constantly shifting reactive clock net fields are the reason we shield the clock, giving it space to do its thing.

Shorter traces equal lower electromagnetic emissions. Shorter clocks have comparatively lower emissions and are less lossy. This gives rise to the use of available length matching tolerance to minimize the length of the clock, starting with finding the longest member of the group. Look at that net; locate any extra bends or places where it can be shortened.

Read more: Length Matching Routing for PCB Busses

John Burkhert

The primary purpose of surface finishes is to prevent oxidation of the copper prior to soldering components.

Back when I held a soldering iron, we used a mixture of tin (63%) and lead (37%) for the solder (Sn63). The boards had the same coating on the plated holes and surface-mount pads. The application for surface mount is referred to as hot air solder leveling (HASL) and applies to any of the available solder types. The beauty of Sn63 is it has a lower melting point and is eutectic. “Eutectic” means the metal solidifies rapidly over a short temperature range. The benefit is fewer disturbed solder joints and good “wetting,” where the surface finish and the solder form a cohesive bond for a reliable connection. You can still buy Sn63 off the shelf at the local electronics store.

On the other hand, lead is a dangerous metal that can cause birth defects and other health issues. The Europeans took the vanguard with the RoHS initiative. If you want to sell electronics products to consumers, the lead content must be the minimum possible – not eliminated entirely but found primarily as a trace element within chips.

SAC (Sn-Ag-Cu): a heroic alloy. Metallurgists all over the world looked for replacement formulas. Tin is still viable and is generally mixed with small amounts of silver and other elements such as antimony, copper or bismuth. Tin makes up the bulk of the alloy, typically around 95% to 99.3%. If pure tin was used, the results could be problematic. Tin whiskers from dendritic growth present a shorting risk.

Read more: PCB Surface Finishes: When to Change It Up

John Burkhert

Does changing the clock make any difference to the PCB layout?

Every six months, it becomes apparent not enough of my household items are part of the “Internet of Things.” (Cue ominous violin squeals.) Can we talk about my spouse’s wall-clock fetish? At least the one in the bedroom doesn’t tick! And, of course, if a battery is near its end, an adjustment will probably put it out of business for good. It’s always something getting me back up on that step ladder.

Setting and resetting clocks is also a thing in PCB design.



Read more: External Oscillators: Placement and Routing Tradeoffs

John Burkhert

Shake, rattle and roll: Your devices often experience it all.

The stark choices of organisms are to adapt, move or die. Our electronics sometimes tough it out so we can do our jobs or simply have a good romp on our favorite ride. No matter the purpose, extreme weather puts an electrical system to the test.

Whether the element is sand, saltwater, sunshine or perhaps a lack of thereof, many dangers age a system prematurely. Most faults caused by the environment are single-component failures. Okay, a part failed. Why? What is the root cause, and what can we do to prevent it from becoming part of a larger trend? Answering that two-part question is the gist of reliability engineering.

What broke is not always evident. Cosmetic damage or a burn scar may point the way if you’re lucky. In most cases, diagnosis is not that easy. Check connectors first, while the board-level investigation usually centers around the FETs that bring power to the device that is out of spec or failing altogether. Somewhere in there a tiny junction has burned up. The repair and return unit or perhaps field service technicians are a good source of reliability anecdotes.

Read more: Preparing a PCBA for Harsh Environments

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