Designer’s Notebook

John Burkhert

What Charlie Brown can teach us about board design.

Not all boards are alike. In fact, no two are exactly the same. That’s kind of the point. We always do something that hasn’t been done before, or we wouldn’t be doing it. The best we can say is many boards share similarities. Just the same, someone in every organization wants to know when the one-off job will be completed.

Often, there is a predetermined schedule in which someone who has never drawn a trace decides when the PCB layout needs to be finished. Such a schedule is usually the result of market forces. It could be back-to-school, CES, or even (especially) a rocket launch date that drives the deadline. Still, it’s not unusual for stakeholders to ask your opinion about the estimated tape-out date.

Life in a service bureau or as an outside contractor. Service bureaus live or die by the accuracy of their bids. If their bid is too many hours, the customer will shop around. If the bid is too few, the designers end up eating that cost with overtime or, worse, missing the date and facing customer dissatisfaction. The one thing that saves them is that the service bureau is working from a baseline plan that does not usually include co-development.

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Read more: Estimating the PCB Design Cycle with Limited Information (and Then Making It Happen)

John Burkhert

What’s all this noise? Look beyond the wires and connectors to think inside the box.

You can’t always hear it, feel it or see it, but every active electronic device radiates some kind of energy as it operates. For the most part, that’s the point. We want to hear the music; we want to feel the air conditioning or see the light. Those are the good things.

Meanwhile, we don’t want side effects: static on the radio, compressor noise from the AC or that annoying 60Hz hum from the light fixture bleeding over to my new bass amp. It’s these things we try our best to design out of the products we build.

We can adjust the tuner on the radio, and we can install the central AC unit away from the windows. I ordered a noise suppressor and plugged it into a socket where there’s no dimmer switches or high current motors plugged in. Then the amplifier and pedal board power cords were routed into the special apparatus, and I no longer get a wave of white noise when this MacBook Pro searches for a WiFi signal.

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Read more: Taming Electromagnetic Interference on a PCB

John Burkhert

Why a 3-N-3 stackup is the sweet spot.

The leading cause of HDI requirements comes from the chip vendors. The original ball grid array packages supported regular vias. Little by little, the pins got cozier. The 1.27mm pitch became 1mm, then 0.8 down to 0.65mm center-to-center. This was the final node where plated through-hole (PTH) vias was an option.

The next step down is 0.5mm class BGAs. We can still use a through-via embedded in the solder pad, but there are two issues. One, the via must be filled and capped to produce a flat surface that doesn’t permit solder to drain away during reflow (FIGURE 1). The other is that the typical “8/18” via has a finished hole size of 0.2mm and a capture pad of 0.45mm. On a 0.5mm pitch device, that leaves 50µm for a trace and an airgap on either side of the trace. That’s not practical.

 

 

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Read more: High Density Interconnect Printed Circuit Boards: How to HDI

John Burkhert Jr.

Anticipate the assembler’s needs in placement and routing.

We already design for fabrication, assembly and test. DFx can be extended to thinking about future uses of an assembly. Sometimes a printed circuit board needs to be revised right away. There are things we can do to facilitate rework. Clearly marking all the components is a good start. A robust design will lend itself to touch-up and rework scenarios. Let’s dive into some techniques.

Breadboarding for “science projects.” Ever seen a breadboard? In PCB design terminology, a breadboard is a rectangle with a grid of plated through-holes set on the same pitch as a DIP package (FIGURE 1). The holes will accept axial-leaded components as well as the odd transistor package. Notice the rows of pins are tied together but can be cut as required by the mad scientist in the lab. Jumper wires on the leads create the rest of the circuit. Development boards can usually afford a slimmed down version of this.

 

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Read more: Design for Rework: Extending the Usefulness of a PCB

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