Virtual Versus Physical Prototyping — Get it Right FasterIn the Aberdeen Group survey report1, “Why PCB Design Matters to the Executive”, best-in-class electronics companies pointed at virtual prototyping as one of the best strategies to improve their PCB design best practices and enable them to meet their aggressive business goals of getting more competitive products to market faster and at lower cost. These companies understand that simulating and analyzing the products in software during the design process has significant time-to-market and quality advantages over older practices of producing multiple physical prototypes and testing them in the lab.
The Advantage of Virtual Prototyping
Let’s be up front and acknowledge that virtual prototyping will usually lengthen the time it takes to get that first piece of hardware. It requires that the design team perform multiple tests in software prior to building the first hardware, and on the surface may seem like slower progress on the delivery schedule. It is human nature for designers to want to get that physical PCB in their hands so they can see the fruits of their design efforts. But bottom line: there are at least three advantages to virtual versus physical prototyping.
The first advantage is time to market. If a design team is relying on physical prototypes, they typically will manufacture a board and take it into the lab for testing. This testing will usually highlight several design errors that require a re-spin through the design process. This build–test–re-design process can take several weeks per iteration and if done entirely with physical boards, adds months to a development schedule.
The second advantage is design quality. Building and testing a prototype can miss some corner-case errors, errors not in the test portfolio, or long term reliability problems that may not materialize in test chambers or in the test board, as manufactured. The most expensive fix to make to a product is one that is discovered in the field and requires extensive warranty costs or recall.
The third advantage is a more competitive product. If a design team can quickly create a sandbox design and test it virtually, they have the opportunity to experiment with many “what-If” scenarios and develop a product that is functionally and performance rich. Limiting themselves to physical experimentation with ideas is time consuming and probably will reduce the number of passes they take. Another advantage to performing virtual prototyping is the ability to design just to spec and not take an overly conservative approach. Conservative design can add PCB layers, passive components, heat escape paths, etc. that all add to the cost of the product.
Figure 1 – Virtual versus physical prototyping can help companies meet their competitive business goals.Broad Application, Limited Resources
The first application that most people think of for virtual prototyping is signal integrity. Circuit simulators have existed since the 60’s (then analog) and have become a necessity as the frequencies and number of high speed nets on a PCB have increased. Today, many of the boards designed have 40-90% high speed interconnects. But virtual prototyping analysis examines many parameters. Among the analyses that can be executed are: power integrity, thermal management, vibration and shock, electromagnetic interference, and manufacturing for yields and reliability, to name a few. So during the design process, the pressure is on the design team to perform many types of analysis yet still meet their schedule.
Figure 2 – Virtual prototyping should be used throughout the design process to reduce cycle time and cost, plus produce a reliable product.
In the past, simulation and analysis was the job of a few highly skilled specialists. The designer would make some progress on the design and then pass it to the specialists who would perform their highly complex analysis and suggest some design changes. Two problems existed with this methodology. The first is the limited resources of these experts. The analysis tools they used were highly technical and required very detailed knowledge to run so only they could do an adequate job of virtual prototyping. The design might have to wait in a queue waiting for some specialist’s cycles. Meanwhile the designer is limited on the progress they can make on design changes.
The second issue is the volume of design elements that needed to be analyzed. In the past, analyzing a few key high-speed nets, for example, was something a specialist could do quickly. Now, using the same example of high-speed nets, performing analysis on thousands of interconnects for signal integrity can take extensive time even for a specialist.
Fortunately, EDA and MCAD suppliers like Mentor Graphics have responded to this challenge. They have focused not only on increasing the accuracy and speed of the analysis tools but also on usability. Many of the analysis tools are intuitive to use and well integrated/embedded with the design systems. Now the majority of the virtual prototyping functions can be performed by the designers themselves. This has not completely eliminated the need for the specialists but has significantly reduced the need for the “over the wall and wait” methodology. As well, it affords the designers the opportunity to experiment with ”what-ifs” and produce a more competitive design.
Let’s look at two examples of virtual prototyping that are used in today’s design of electronic products.
As ICs get faster and denser they dissipate more heat. This is compounded by increasing densities of those ICs on the PCB and in the system. Having ICs exceed a critical temperature has two affects. First, it reduces performance. The ICs literally slow down and the performance of the system suffers. It may also result in timing errors that result in functionality failures. The second effect is reliability. Above a critical temperature the reliability of the IC decreases exponentially and may result in a long term failure and warranty costs to the product. This second affect may not be discovered through physical prototyping as the failure may take months or years to materialize and building/testing a physical prototype may not run long enough to cause the failure.
So analyzing the product for proper thermal management is absolutely necessary. But virtual prototyping during the design process is a multi-tiered process requiring the collaboration of both electrical and mechanical designers, as viewed in Figure 3.
Figure 3 – Thermal management of an electronic product requires virtual prototyping at all levels of the product and participation by both electrical (ECAD) and mechanical designers (MCAD).First, the IC supplier usually analyzes the component package and delivers a model of the thermal characteristics. Next we want to analyze the standalone PCB as the design is being performed. Typically a PCB designer wants an analysis of their active component placements to determine if they are creating a section of the board that will be difficult to cool. But this requires more than a rough estimate of the board with the component dissipations and locations. Since the heat dissipation paths are many (heat sinks, copper in the inner layers of the board, convection, conduction and radiation), the data passed from the PCB design system to thermal analysis must be complete. The setup and execution of the analysis software must also be rather intuitive since you want this to be performed by the PCB designer who is not necessarily a thermal expert, without stalling the design process.
But the ultimate virtual prototyping must be performed with the PCB or multiple PCBs in the final product enclosure under the conditions that can be expected in the end-user’s environment. This type of analysis is typically done in the mechanical design domain where the MCAD system has the complete physical definition of the product: enclosure, mounting methods, heat sinks and rails, the PCBs, etc. The PCB designer must transfer the design data for the PCB(s) to the mechanical designer where they can be inserted into the enclosure. The MCAD system needs to have a full 3D physical definition and thermal properties of the components, their leads, etc. and all elements of the complete product. The mechanical designer then uses software employing computational fluid dynamics to perform a combination of convection, radiation, and conduction analysis to determine if the ICs exceed critical temperatures and may cause a reliability or performance problem.
Prototyping for Performance
As the performance of ICs continues to increase, so do the downstream effects of interconnects on the PCB. But getting the interconnects correct between the components is only part of the challenge for high performance systems. Challenges include: signal integrity, timing, crosstalk, power distribution network (PDN) design and integrity, mixed signal analysis, RF circuitry design and analysis, and electro-magnetic interference. And overcoming all of these challenges requires close collaboration between the electrical engineer and layout designer as well as the use of powerful, accurate, and fast virtual prototyping software.
Looking closer at just the signal interconnect integrity challenge we see the world changing rapidly. In the past, interconnect buses were multiple nets running in parallel at multiple hundreds of megahertz frequencies and what might be considered fast edge rates. Today’s high-speed serial busses, such as PCI-Express, DDR2/3, and Serial ATA, running at frequencies from several hundred megahertz to beyond a gigahertz, make for very tight timing margins. The fine-geometry silicon required for these speeds makes for extremely fast edge rates (rise and fall times of 100s of picoseconds). And not only do we have a delay challenge (get the interconnects short so the signals get there fast) but also a tolerance challenge where the interconnects in a bus structure must get there at the same time (extremely tight tolerances of 10 picoseconds translating to tenths of mils in interconnect length). This is all compounded by the fact that many high performance systems have up to 90% of their interconnects that must adhere to these specs.
In this case we not only need efficient and easy-to-use virtual prototyping capabilities but also communication and collaboration throughout the entire, front to back, design process (Figure 4). It starts with engineers defining the constraints for all of the high-speed nets and supported by analysis software that virtually simulates the characteristics of the signals and the PCB board physical stack-up. These constraints are entered into a management system that will feed into the rest of the design process, controlling the layout designer’s routing, and finally in the post layout verification of the design’s most critical nets.
Figure 4 – Virtual prototyping of the PCB for high performance design integrity occurs throughout the complete design process from definition through layout to verification.Deploying a New Methodology
The extensive use of virtual versus physical prototyping is a change from the norm in many companies. Software is available that makes virtual prototyping (VP) possible but this is not enough. The company has to commit to this new methodology:
- Understand that the initial design process will take longer than it used to before you have a piece of hardware in your hands. Realize that your goal is to cut the TOTAL design process time and virtual prototyping can make that possible. Management and the design team members have to be sold and commit to the change.
- Define the design process with virtual prototyping as an integral component with functionality from start to finish of development. Making VP continuous from design conception right through to manufacturing delivery and not an afterthought will enable you to truly shorten your design cycle times and get better product to market faster.
- Invest in the education of the design team in the use of the VP software. Engineers and designers will be stepping out of their normal job responsibilities and need proper training in methodology and specific tool use.
- Get the right tools in place. Understand that these tools cannot just be useable by the few company specialists but also by the designers themselves. The software has to be easy to learn and use, seamlessly integrated with the product design functions, fast and accurate.
1 A copy of the Aberdeen report and a company assessment are available here: http://www.mentor.com/products/pcb-system-design/promotions/aberdeen
John Isaac has worked in the Electronic Design Automation (EDA) industry with PCB and IC technology for over forty years. His career started with IBM where he managed the development of EDA systems for IBM's internal design of their high-end ICs and PCBs. He then joined Mentor Graphics where he has held marketing positions in both PCB and IC product areas. He is currently responsible for worldwide market development for the Systems Design Division.
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