Differences between Cadence Allegro Native STEP output and Artwork's STEP Output

May 2016

In Allegro v16.6 Cadence added the ability to export a STEP 3D model for the etch classes. While this seemed a very good option for FEA simulation engineers, Allegro board designers have reported that the resulting output is not really useful for FEA purposes.

Let's run through a small example - we'll output STEP directly from Allegro and import it into SolidWorks; We will then output Artwork's 3Di from the same .brd file and convert it into STEP using Artwork 3Di2STEP (optimized for FEA.)

Exporting STEP Natively from Allegro

We'll start with a multi-layer PCB layout (we've removed a lot of the geometries to keep things from getting too big)

We'll then use the File | Export | Step menu pick to open the Step Export dialog:

We are interested in getting out the conductor layers (etch/pin/via) and the vias connecting each layer.

Clicking on Export produces a STEP file very quickly.

Exporting 3Di to Get to STEP

In order to export 3Di we have installed Artwork's AWROut plug-in. We start the program and see the dialog.

The main dialog needs no user input. We'll go to the Settings dialog and adjust our parameters.

AWROut Settings

• We are not clipping out any region of the board (we could if we wanted to)

• We are unionizing all data on conductor layers - this avoids collisions/interference in 3D between shapes/pads, shapes/clines and clines/pads that might cause the mesher to fail.

• Since we're not interested in components we don't check: Render Component Outlines

• We are interested in our substrate dielectrics so we check: Render dielectric board outlines

• We are using 16 segments per circle for our arc fracturing on our conductor layers.

Next, we'll use 3DVu to open the 3Di file and export STEP.

Start by opening the file and selecting File | Export | STEP for FEA

Our STEP Output Settings We'll produce AP214 (standard STEP with color attributes) We'll organize/sort the output by layer. We'll remove any duplicate pin objects (not necessary in this case but a good check) We'll expand our dielectric body by 0.1 mm (insures a safe Boolean for the next step) We'll generate negative dielectric to fill in the air gaps on each conductor layer. We'll merge our via stacked cylinders into a single long cylinder We'll force our vias to have 6 sides in the crossection to reduce mesh count Wherever a via passes through a conductor or dielectric body we'll "drill" a corresponding hole. We're not going to create hollow vias (though we could).

File Size Differences?

When we look into our directory we see that the two STEP files produced vary greatly in size! The Artwork file is more than 100X larger than the STEP file produced by Allegro.

Can this be correct? Let's open both files in SolidWorks and find out.

Importing the Allegro Native STEP Output into SolidWorks

We'll start by importing the Allegro STEP into SolidWorks. It imports very quickly and this is what we see initially:

Looks good. But upon closer examination, not so much.

First, the board is one solid body (there are "holes" drilled into it where the vias pass through.) However there's no internal spaces to allow for the conductors. For FEA you can't have conductor and dielectric occupying the same physical space.

Let's hide the board and check out the conductors.

Holy Cow! What happened to our internal layers? They're gone!

And our vias. Where are the vias? Gone.

Let's have a closer look at the top and bottom conductor layers and see what's there:

Our conductors have no thickness. Zero. They are not really solid bodies. Just surfaces. This won't do. You can't do a 3D simulation with zero thickness! Well at least we have nice contours. We can just extrude everything ourselves.

Well, apparently not. SolidWorks won't let you extrude two surfaces that touch or interfere. And it appears that our traces interfere with our pads and our pads interfere with our shapes.

Well, the STEP from Allegro imported but we'll not be able to do any 3D FEA simulations unless we spend quite a few hours redrawing and cleaning things up. Not what we hoped for.

Importing the STEP from Artwork

OK, now let's do the same with the STEP file from Artwork. I think I'm beginning to understand why it is 100X bigger than the Allegro STEP file.

After waiting quite a while for SolidWorks to digest this large STEP file we see:

Looks pretty similar to what we initially saw with the Allegro STEP. So let's dig in a little deeper by turning off all the dielectric geometries and just examine the conductors.

This is quite different than what we saw coming in from Allegro. We have all the inner layers and we have all the vias as solid bodies. Now it makes sense why the STEP file was so much larger.

Let's get a better look at the via geometries. I've turned off all the conductor layers except bottom and all the vias except those that run from bottom to top.

Notice that the vias are a single long cylinder with six sides. Also notice that where a via passes through a conductor layer there is a corresponding hole to remove any interference that would trip up the mesher.

I've turned on the top layer and one of the inner layers just to give another look.

If I turn off the via geometries you can see the holes in the conductor geometries where the vias pass through more clearly.

In order to eliminate air gaps internal to the PCB stackup, we build the dielectric geometries from two separate sources. The board between conductors is pretty straightfoward. However on each conductor layer we have to generate the "fill in" dielectric by performing a Boolean operation between the conductor geometries for each layer and the board outline.

Summary

If your intent is to do some sort of 3D FEA simulation or analysis on your PCB then the direct STEP output from Cadence Allegro is not usable - you are better off using Artwork's 3Di extractor and converting to STEP to get a model that has been optimized for FEA simulators.