Evaluating PCB layout tools: A board developer’s perspective

by Syed W. Ali , TechOnline India - December 01, 2011

No PCB layout 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.

As edge rates of logic devices become faster and PCB designs become more advanced and geared towards miniaturization, a number of issues and pitfalls can emerge at the layout stage if you do not have appropriate tools at your disposal to handle 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 15 gigahertz (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 (See Table 1 below), most current commercial tools lack key features and attributes described in the chart.

 

Table 1: 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 10 GHz 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 below 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 below shows cross-Probing being done between Cadence OrCad and Allegro.

 

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

Allegro PCB SI and OrCad Signal Explorer help to do a pre- and post-layout signal integrity analysis and are efficient at doing this for complex PCBs. Allegro also provides a much better integration to Allegro PCB router, formerly known as Cadence SPECCTRA, than any other layout tool. SPECCTRA is one of the most well known products used for Auto-routing.

However, Allegro is not without its drawbacks and limitations. Most importantly is its cost. A Cadence seat along with a suite that can handle almost all design needs can cost upwards of $90K. Spending this amount on a layout tool by a mid-sized company or start up OEMs does not seem plausible and would be the deciding factor for most.

Secondly, Cadence tools are sometimes too complicated and cryptic to understand. The GUI is not intuitive or user friendly. Many functions are hidden deep within the tool and many easy tasks are difficult to carry out. Moreover, Allegro part libraries are difficult to create and maintain.

Thirdly, Cadence has a tortuous file structure that is awkward to manage. Generating outputs for manufacturing can become an arduous task in Allegro. As an example, PCB designer has to go through numerous steps in specific order to setup and generate drill files and Gerbers.

Also, this tool is not effective at handling multiple copper fills for power and ground planes. Designers have to work diligently while dealing with static and dynamic shapes and suffice to say that handling numerous copper shapes in Allegro is not a walk in the park and can become too laborious for some.

Finally, Cadence Allegro is notorious for the use of its scripts and macros to perform fairly basic tasks that could be performed fairly automatically in other layout tools.

 

Mentor Graphics PADs:

The PADs platform, even though not as dated as Allegro, has been available for some time now. It is best suited for small to medium complexity boards. Hence, by 2005, PADs had become one of the most widely used platforms for PCB layout. It has its own schematic tools in DxDesigner and PADs logic, but can also work with OrCad capture with some limitations. PADs Logic and DxDesigner are fairly integrated into the tool and easy to work with.

PADs is known for its intuitive and easier to understand GUI. Figure 3 below shows the PADs design rules window showing constraints setup of a particular net class. Notice how the options are comprehensible and straight forward. Even though it is still a hassle to learn PADs, once you get the hang of it, performing tasks is fairly quick and easy. Its hot keys enable the designer to perform tasks quite promptly.

Figure 3: Design Rules window with physical constraints setup based on a particular net class

 

Library creation is much easier with PADs compared to Allegro. Part libraries are fairly easy to manage and maintain. One can find free part libraries online that are already built and time tested and vetted. The file structure is fairly straight forward. Project outputs are uncomplicated to set up.

HyperLynx from Mentor is also another resourceful tool that enables engineers to quickly and accurately analyze signal integrity at all stages of design life cycles and follows its intuitive GUI from PADs. Hyperlynx thermal is another tool that that allows designers to analyze board level thermal problems by performing thermal simulations. Both of these tools provide excellent integration with PADs.

PADs also comes with its own auto-router as an option. While it is not as powerful as its Cadence counterpart, it performs auto-routing of medium complexity with satisfactory results.

Like Cadence Allegro, PADs also has its disadvantages. First is its cost. Even though it is not as expensive as Cadence, one can end up paying tens of thousands of dollars for all its options. Secondly, it is not a very good tool in handling signal integrity issues.

Its design rules system is too simple and inflexible to carry out tasks required on high-speed boards. As an example, it is not an option to import the Chip pin-delay information into the tool as can be done in Allegro. This information can be quite important when dealing with timing analysis of high speed signals.

Thirdly, PADs is not effective at handling large number of layers. Having numerous copper layers in PADs can make the database size hefty and can become a burden on system’s resources.

 

Altium Designer:

Altium Designer was created based on the idea of a unified electronics design system. It uses a single data model that holds all the design data to create a product. FPGA design, PCB design and layout, simulation, CAM tools, and embedded software development are all housed in one software system.

Altium Designer is a decent PCB layout tool. The main advantage that it has over other tools is its cost. Due to its cost alone, it has taken over much of the PADs and Allegro market over the last few years. All the options are provided at one cost. Another plus that it has over other tools is its gentle learning curve. A designer who has used any of the other layout tools will not find it difficult to learn this relatively new tool.

Thirdly, Altium's tool gives the PCB designer an almost perfect balance between its intuitive GUI and its powerful options facilitating the designer to have more control over the layout. Figure 4 below shows how one can selectively modify characteristics of individual component pads. The schematic tool within the Altium designer is sync’ed perfectly with the layout tool which allows easier engineering change orders (ECOs) during the design process.

 

Figure 4: Changing characteristics of individual component pads in Altium Designer

Fourthly, creating a parts library for your project is easiest in Altium. The package includes thousands of already created and vetted parts from various manufacturers. Its library creating tools give the designer a comprehensive set of options that are fairly straight forward at the same time.

One of its library creation utilities also allows creation of package symbols based on the IPC-7351 standard. It also allows the designer to incorporate 3D bodies into the layout by importing step models to an extent that was not possible with any of the other layout tools.

The design community and mechanical engineers in particular favor this option. Detailed 3D drawings of the completed layout can be generated once the layout is finalized. Figure 5 below shows a 3D view of a completed board layout from within Altium designer.

Figure 5: 3D visualization engine of Altium Designer showing a completed board

 

Generating manufacturing outputs is very easy in Altium. Unlike other tools, output generation is not overly complicated and gives all the options that the designer would require. CAM tool is also provided with the Altium designer package, based on the CamCASTIC, which allows the designer to view the Gerber data.

Finally, Altium recently bought Morfik Technology a provider of cloud-based software applications. Altium expects that cloud technology will pervade future electronics and embedded systems. Even though the technology has a lot of potential, it is yet to be seen how it will assist the board designer.

Like other tools, Altium Designer also has its limitations. First and foremost are its limirws capabilities for handling high-speed designs. The simulation tools that come with the package are not powerful enough to handle speeds above 2GHz. So designs that incorporate speeds in excess of 5GHz must be simulated in other tools like HyperLynx or AnSoft.

A second drawback is its overly comprehensive query system, which is based on the query language. Even though it is a powerful system, the designer will have to write the queries to do simple tasks much of which is automatic in PADs.
Finally, some tools that are incorporated into Altium Designer have been acquired from existing software, e.g. CAMCASTIC. The drawback of this approach is that its GUI is rather different with the layout tool, which can sometimes be frustrating to the designer. A better approach would have been to redesign the CAM tool such that the GUI and hotkeys were identical to the layout tool.

All type of boards can be designed by any of the layout tools. It is a question of preference, cost and the limitations of each tool. However, for every board, I am sure there will be a tool that best suits its complexity and requirements.

References:

1 – David Lieby. “The constraint Manager from a User Perspective.” Web. 22 Oct. 2011
2 - Wikipedia contributors. "Altium Designer." Wikipedia, the Free Encyclopedia. 14 Oct. 2011. Web. 28 Oct. 2011.
3 – Gabe Moretti. “Altium releases 3D PCB visualization”. EE Times. 26 Nov. 2011. Web. 25 Oct. 2011

 

About the author:

Syed Wasif Ali is an advanced certified designer (CID+) and a layout engineer at NexLogic Technologies, Inc., San Jose, CA. He received his BSEE from N.E.D. University of Engineering and Technology in Karachi Pakistan.

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