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An assessment of PCB layout tools

Posted: 01 Feb 2013 ?? ?Print Version ?Bookmark and Share

Keywords:PCB layout tools? design rule checkers? schematic?

As logic devices' edge rates become faster and PCB designs become smaller and more advanced, a number of issues and pitfalls can arise at the layout stage if you do not have appropriate tools at your disposal to manage your requirements.

In-depth experience using the various PCB layout tools available today is the best indicator of the direction to take regardless of densities, application, or speed requirements. Some design tool parameters that require some experience involve design speeds ranging from a few megahertz (MHz) to over 15GHz (GHz) with board layer counts going from single layer to 50 layers, sometimes more.

An experienced designer always works diligently with tighter constraints, and uses the more advanced manufacturing technologies paired the right layout tools.

From the perspective of the experienced PCB designer, compared to the ideal PCB layout tool (table), most current commercial tools lack key features and attributes described in the chart.

Table: My ideal PCB layout tool.

Today's PCB layout tools have a combination of some of them, but no vendor has all of those capabilities embodied in one tool. Thus, no tool can be perfect. One can only choose the best tool that is most suitable for his/her needs. Since we're not at the ideal design scenario yet, we have to make do with today's available tool technologies.

Among the most popular PCB layout tools today are Cadence Allegro, Mentor Graphics Pads, and Altium designer. Each has its own unique capabilities, advantages, caveats and limitations. Although you can use any of these three tools to design virtually every kind of board, it doesn't mean you should. Choosing the right tool for the layout should be at the forefront of PCB layout planning and must never be ignored.

As an example, using Cadence Allegro to layout a single sided board with a few components will be counter productive when you have tools like PADs and Altium Designer also at your disposal. Similarly it is not advisable to design a high speed board with edge rates in excess of 10GHz on any tool other than Cadence Allegro.

The three tools offer some sort of portability. Altium allows import of Orcad schematic, Allegro board files, as well as Pads PCB database. Pads allows import of Altium board file. Allegro allows import of Pads Ascii file.

Having said that, when one applies any sort of import, it is never a 100% clean transition from one CAD tool to another without tweaking the data to a certain extent and running design rule checkers (DRCs) and netlist checks on the schematic afterwards. Therefore the only time the designer would port the data is when the company is switching platforms.

Cadence Allegro
Allegro is one of the oldest and the most diversified tools in the industry. No one can question the power it gives to the designer to tweak every aspect of the layout. It also has one of the best constraints managers that give better control on signal and power integrity of the board. Figure 1 shows how detailed and comprehensive its constraint manager is.

Figure 1: Allegro's Constraint manager at a glance.

It is also the best tool to handle a large number of board layers. Ever since Cadence bought OrCad, their developers have tried to integrate the two. Therefore, one will find that Cadence OrCad for schematic capture is integrated to a certain extent with Allegro for layout. Figure 2 shows cross-Probing being done between Cadence OrCad and Allegro.

Figure 2: Cross-Probing done between Cadence OrCad & Allegro.

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