PCB Design: Tips and Tricks for Rigid-Flex Designs – OnTrack

PCB Design: Tips and Tricks for Rigid-Flex Designs – OnTrack


Hello everyone, my name is John Magyar,
senior application engineer with Altium, and I’d like to talk about rigid
flex design. Now rigid flex is a mature technology that’s been around for a
couple of decades and it originated in the military and aerospace industry to
meet their requirements for weight space and reliability. With today’s shrinking
form factors and increasing design complexity, rigid flex is starting to
become more common to achieve these complex design requirements. If you’re
not using rigid flex you may be limiting your ability to achieve an optimal space
weight and reliability requirement. If you’re interested in trying rigid flex
for the first time I’ll cover a few basic guidelines here. And as always,
speak with your fabricator for more details. First we’ll take a look at cost.
Cost for rigid flex design is more than a traditional PCB fabrication. If we put
a relative data point for traditional PCB, rigid flex can be very expensive,
perhaps double or more. But working with your fabricator you can manage cost by
making decisions wisely about the layer stack up and constraints that you follow
to ensure success. So that should bring the cost of rigid flex somewhere in the
middle, and that’s reasonable given the benefits it will offer you. So from a
technical perspective the rigid flex design is all one monolithic outline.
You’re designing one PCB that consists of multiple rigid sections and flex
sections. So it’s traditional PCB linked
together by flexible PCB, and it’s designed as one complete outline. You can
verify the outline in the PCB tool using 3D animation, or at a minimum you can
create paper or mylar models to dial in the mechanics just right. One of the key
concerns when designing rigid flex is to think about the bending radius. Where the
bending points are, you want to make sure that that bending radius is about 10x
the thickness of the ridge- the flexible section. And now I’ll talk about the
stack up in general. So we have this one board that consists of rigid and flex so
on the left I have the stack up applicable for the rigid section and
then going through the middle continuously is the flexible section, so
I end up with two stack ups. And the fabricator will use this stack up for
all rigid portions and this stack up on the right for all flex sections. And
that’s how the construction will be, the flex layers actually cover into the
rigid section so they’re combined. And there are many more configurations you
can use, this is just a simple four layer example, but for how many layers you have
you always want to try to keep the flexible section symmetrically in the
center as a common guideline. Another thing to think about for routing when we
route traces on flexible sections, we want to first make sure that we’re
running perpendicular to any bending line. That will ensure that the copper
doesn’t fatigue and we’re out early. Another thing to consider when we have
flexible connectors, use curved traces to get around corners instead of the
traditional 45 or 90 degree straight traces that you would normally use on a
rigid PCB. So, flex you have to bend the copper with an arc, again so that the
copper doesn’t fatigue. Another consideration for routing is when we
have a two-layer flex section, you want to stagger your traces so that they
don’t end up one on top of the other. So here, this lower one, if they’re parallel
they’re going to be not on top of each other, so again that ensures that we
don’t get a thickness build up here where copper from one layer is directly
on top of another for traces. Some other things to take into account would be for
vias. So you want to use vias sparingly using vias in the flexible section, your
flexible material, is not as stable as the rigid section, therefore minimizing
use of vias is advised. For a hole size within a flex section, you probably don’t
want to use anything less than 10 and then for the overall diameter, you want
to add 10 to that, so that you have a nice wide via for anchoring it to the
polyamide surface. This ensures that the via doesn’t peel or fatigue during
repeated flexing. Another consideration is that when we route traces to the via
that we use tear drops. Tear drops expand the copper so that we end up with a more
structural connection to the via itself. You’re reinforcing that copper area,
again to minimize fatigue and potential cracking over time. Some other things to
keep in mind is, would be, when placing a via in the rigid section, keep it a
minimum of 50 mils away from the boundary point of the flex. The reason is
you can get instability in the materials right at that very edge where
there’s a transition, so for drilling purposes keep things back a minimum of 50
mils to ensure good results in the rigid section. Another common question is how
to handle power and ground polygons. Power and ground typically require
nice, wide conductive areas. You want to keep these as hatched polygons. Solid
polygons will not flex so it’s recommended to use hatch. If you must use
solid traces, you have to minimize their width and make sure that it’s not a
problem for bending. So, these are just some of the basics to get started, but
using rigid flex will help you achieve the space weight and reliability
requirements for today’s more complex products. If you have any questions or
comments please drop them in the box below, and thanks for watching. you

4 thoughts on “PCB Design: Tips and Tricks for Rigid-Flex Designs – OnTrack

  1. For traces on the flex PCB, is there a minimum and a maximum allowed trace width before proceeding with the hatching? Also, is it possible to have more that 2 copper layers on a flex PCB?

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