Processes and pitfalls of multilayer fabrication every designer needs to know

Multilayer PCB’s are layers of copper area’s etched to their patters to form a single circuit that all connects together, just like in the design. Easy eh?

We wish!

When we scratch away at the process behind the design we lean that maybe all is not what we might think. Defining the stackup before routing even starts is essential. This is because the stackup will define where the designer can route, where ground and power planes are located and where any controlled impedances will be routed too.

  • Time spent at this stage isn’t wasted, it’s well spent as this can save time later in tearing up chunks of design work and starting again.
  • Being realistic about the number of layers will also be beneficial at this stage, this means balancing (as always) cost against technical performance.
  • Discussing the requirements with the fabricator at this stage means that this stage of the build is designed and we don’t have to mess with it later.

Copper balance is a factor to be considered that few outside PCB design and fabrication know about – yet with multilayer boards it can have a profound effect. The effect of bad copper balance is  a warped board and is caused if a board is designed with some areas of highly dense copper features where on other parts it’s less dense. Planes which over lap across the layers is often enough to cure this, either that or filling in between routed areas with ground pour. If doing this, make sure that no creepage distances, or other circuit features are compromised.

Drill holes and vias seem like a simple affair – on the whole they are. But when a plated hole is defined in your design – is this the size of the hole drilled or after plating? Normally this isn’t an issue, but if using custom assemblies with fine wires, problems can arise where the wire doesn’t fit in the hole as expected.

Pouring ground shapes – the design system needs to keep the features it creates large enough to be successfully etched. Tiny copper features can come adrift and cause circuit havoc. So as usual check what the fabricator can make and set the pour tool and DRC to filter any tiny features out – or a re-design is likely to follow sooner than expected.

Layer order. Every fabricator has their own default definition of layer orders.

Don’t assume that they’re all the same – make sure that each of the layers in the design are well labelled and the order of the layers is clear.

The effect of incorrect layer order may be a semi functioning circuit, but with controlled impedances referenced to power planes rather than ground, there may be some unusual and hard to find problems occurring.

As with everything in the PCB world, every part of the process has rules, processes and limitations. Knowing what they are and working with these processes is going to lead to better made designs.

Circuit Mechanix © 2016



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