Printed Circuit Design & Fab Online Magazine - Current Issue

Designer’s Notebook

John Burkhert

Buried and blind vias solve most HDI routing studies.

A popular answer to a high density interconnect (HDI) problem is to start with a simple printed circuit board and then proceed to add on layer after layer. This is known as a sequential lamination process. For the sake of balance, the layers are always added to the top and bottom in pairs. A notation we use describes the sequence.

A typical example is a board that starts with N number of layers in the initial pressing and has three additional lamination steps after that. Each additional pressing adds two layers: one above and one below the previous step. The shorthand for that type of construction is 3+N+3 or simply a 3N3 stack-up.

We could get more detailed and substitute the actual number of layers in the first pressing for the N and call it, for instance, a 3+4+3 board for an even 10 layers. The fact is the fabricator is more concerned about how many layers are added afterward than how many are used in the first step (FIGURE 1).

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Read more: Regarding the Use of Core Vias in a PCB Design

John Burkhert

Flexible printed circuits have unique requirements for footprints owing to the nature of their application.

Here is another lesson I learned the hard way: taping out an FPC (flex printed circuit) using the usual components and finding it doesn’t really work that way. Several things separate a rigid board from a flex. One of the main tenets behind the different design rules is reducing the risk of the circuit peeling up when it gets flexed. Even without continuous flexing, a flex circuit can be under tension where it is folded, twisted, spindled or mutilated.

Ah, but the flexible section is generally not where we install components. Normally, a stiffener covers part of the flex, and components are on the other side. Therefore, it is rigid, right? Not really. Most stiffeners used on flex circuits have a degree of flex to them. Flex stack-ups are intended to be as thin as possible; it’s one of their advantages. Even stainless-steel versions have some give. Many are made of FR-4 or another layer of polyimide, not all that stout.

In short, this means we want something more like a Class 3 footprint in that the maximum size pad is preferred. More area gives it more bite on the surface. A typical rule for flex is to use a fillet to taper to the line width of the traces. Any abrupt angles are stress-risers and need to be avoided. Round things off rather than squaring them.

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Read more: Component Footprint Differences between Rigid and Flex Circuits

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