Designer’s Notebook

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

John Burkhert

Why you should take the time to use the dimensioning tool.

One of the many hats worn by a board designer is that of draftsperson. Before settling into a position doing layout, I had a contract to create drawings on vellum – with a pencil! Before they turned me loose, the manager asked me to write my name and phone number on a piece of paper. That was all it took; get to work. Ever since seventh grade drafting class, my normal handwriting has been in all caps. We no longer need legible handwriting to land a job, but some of the things I learned at Cavro Scientific have stuck with me all these years.

Dimensions and tolerances. The electronic data alone are enough to fabricate a board most of the time. Whether it’s an omnibus file like IPC-2581 or a collection of Gerber and drill data, the hole size and locations are provided with the circuit pattern. Why take the time to use the dimensioning tool? Here’s why. Someone must inspect the PCB before it leaves the fabricator. Someone else inspects it on the way into the assembly factory. All the fab and assembly drawings are inspection documents.

Read more: I’m a PCB Designer. What Must I Know about Drafting?

John Burkhert

And why your library naming convention should be memorable.

As a designer who has put himself into the public eye, a lot of questions come my way. Several beginners have approached me with basic questions I can usually answer by sharing something I’ve already written. Sometimes I’ll end up writing a few paragraphs that eventually expand into a column to share with everyone. One gentleman, whom I will call Aakash, has leaned on me so many times I think he might be compiling a book. The messages pile up, as do the answers. Let’s get to one of those types of answers.

More than one person has asked about starting their first job going from the ground up. “How do I make all the parts I need for my first board?” The short answer is to build them one at a time. This is the internet age. This is the 21st Century. There must be a better way! It’s not that we’re impatient;  we just don’t have that kind of time. Software vendors know that, and the internet does too. The software installation comes with a bevy of common parts, but most boards go well beyond the generic SMD capacitors and resistors found in the default library.

Read more: Generating PCB Footprints for Your First Layout

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