Designer’s Notebook

John Burkhert

Pros and cons – and costs.

It’s almost inevitable that a component that works well and lasts a long time will eventually be put on a list of parts not to be specified for mass production. Newer, better parts are on the way. The thinking goes that the microcontrollers and other devices on a board are already fine-pitch, so another one can be accommodated. That’s how we end up with those five-pin regulators with a tiny diamond-shaped pin trapped between four beveled rectangles.

Advantage: Component-to-component spacing. The via-in-pad trick enables high component density by enabling routing that is 100% internal to the board, with no exposed traces. The space normally set aside for the fan-out via can be used for the next component with the following stipulations:

  • Test access is maintained
  • Rework clearance (for desoldering)
  • Electrical isolation (shielding)
  • Thermal considerations (heat sink, heat pipe)
  • Mechanical interference (headroom)
  • Pick-and-place accuracy.

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Read more: Microvias: An Answer to the High-Density Blues

John Burkhert

The best designs use the least amount of material possible.

Why evenly distribute copper on a PCB? Look at the material stackup as it alternates between conductor and dielectric material. The goal is to build a mirror image of copper weights as you work outward from the centerline.

Going beyond specifying alternating shape and route layers, the “greenest” PCB involves a minimum of etching. It’s intuitive that removing less material requires less time in the solvent tanks. Time is money, so that should be reason enough to have all layers biased toward copper fill.

Besides being easier on the equipment, copper-biased design will help maintain an even thickness across the entire board. While fabricators generally offer a +/-10% thickness tolerance, we often want a tighter distribution when it comes to the actual PCB thickness.

Basically, we must permit the 10% thickness tolerance, while aiming for a 5% variance by providing artwork that makes the most of the raw materials. The more evenly we design the board, the more consistent the outcome.

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Read more: 2 Approaches to Ensuring Even Copper Distribution

John Burkhert

Make every chip a layout unto itself.

Being a printed circuit board designer is not easy. Parts we used to take for granted have become really hard to come by. Geopolitical trade wars and a pandemic were serious triggers for the undersupply. We really didn’t need a Japanese chip factory to burn down to make things worse. A giant cargo hauler clogging up a vital shipping artery for a week was no help either.

The fear, uncertainty and doubt sown into the supply chain put the squeeze on purchasing managers who, in turn, did their best to secure as much material as possible. Ordering more inventory than their forecasted requirements is a typical kneejerk reaction for the big players. Some purchase orders may be defensive measures, an effort to block competitors that are caught shorthanded themselves.

Automakers are a vital sector of the US, German and Japanese economies. They have been busy lobbying their respective governments to pressure chipmakers, with the goal to create a sufficient supply of devices for the vehicles they want to build. Propping up that industry with their ruggedized devices leaves even less bandwidth for other industries.

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Read more: Is it Time to Design Printed Circuit Boards around the Bill of Materials?

John Burkhert

Getting all the parts and processes aimed in the same direction.

Printed circuit board technology never sleeps. At this very moment, engineering teams are working out ways to increase circuit density with finer-pitch devices. When it comes to placing these components on a PCB, the margin of error shrinks along with the pin pitch. Let’s look at how we can enable these parts on the assembly line.

The first step in mass production of a PCB assembly is preparing the board to take components. The boards may be baked in an oven prior to starting the assembly process. Although they are packed in sealed containers with a little bag of desiccant, the sponge-like dielectric materials still absorb water one molecule at a time. Prebaking releases the steam that could interfere with reflow soldering.

Ideally, all parts on a board will use the same type of technology and will be roughly the same class of components in terms of pin-pitch and other physical aspects (FIGURE 1). Tall and heavy components plus small and light ones are not a good mix. Tall ones create so-called shadows where the surrounding area doesn’t get as hot during soldering.

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Read more: Component Placement is a Game of Compromises

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