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John Isaac: "Left-Shift" to Improve Complex PCB Designs
September 26, 2012 |Estimated reading time: 5 minutes
This article originally appeared in the August issue of The PCB Magazine.
As the name implies, a left-shift philosophy is one in which certain activities on the design and manufacturing timeline are shifted to the left. By considering specific issues far in advance of what is typical, re-spins of the PCB, problems that could adversely affect manufacturing, and issues that could even cause product recall are minimized, or even eliminated, before the first PCB is ever etched and drilled. That philosophy is a driving force now and into the future. Using analysis and simulation software early and throughout the design process (often referred to as virtual prototyping) can effectively result in getting a more competitive product to market faster, and reducing the development cost overall.
Figure 1.Left-shifting design and manufacturing allows the product development timeline to be compressed, hastening product introduction and saving cost.
Let’s take a look at some specific examples of how the tools today, and in the future, anticipate items once considered down the timeline.
What is Left Shift?
A left-shift philosophy may be applied to an entire product continuum, which Mentor does, and the company sees it as the future of electronic design and manufacturing. DFM is one such activity that produces higher returns when it is left-shifted so that manufacturability issues are examined during the layout, not on the first prototype board. With DFM, the manufacturing requirements (materials, tolerances, yield-enhancing practices) and component (and alternate component) sizes are all accounted for as the PCB is designed, so that the board will not have to be re-done late in the process because two components impinge each other, or traces are too close to the edge of the board, for example.
This same left-shift concept may also be applied to FPGA design, resulting in less costly and higher-quality PCBs, not to mention shortening the time to design the PCB. The traditional design process of designing the FPGA pinouts with little input from the PCB designers can result in some rather horrific-looking layouts. When the process is left-shifted, pinouts can be considered that allow easier escape and breakout for PCB routing. This often saves layers, improves the performance (shorter interconnect lengths), or even allows for a smaller form factor. Sometimes, this consideration is the difference between being able to route the PCB or not.
Another area where left-shifting produces great benefits is virtual prototyping and simulation to detect and eliminate problems with signal integrity and power integrity. If not dealt with, these issues often result in the worst problems of all: intermittent failures. These failures are difficult to troubleshoot and rectify. One of the trickiest is power problems where a PCB layout starves current or voltage to an IC to the point where it does not switch properly. And this may only happen under very specific circumstances where many ports are switching simultaneously. It is vastly better to discover that before any PCBs are made.
Advanced design systems have a seamless methodology that enables an engineer to pre-analyze (left-shift) the high-speed net classes with a set of constraints for those interconnects. During layout, the constraints are followed to meet the length (delay), parallelism (crosstalk) and tolerance (matching) specs. Once a preliminary layout is achieved, simulation software can analyze signal propagation to locate any potential for problems, allowing a re-design without ever having to spend the money for a physical prototype.
In manufacturing, certain tools can notify managers instantly when an assembly line is out of compliance, based on pre-set thresholds. No longer do manufacturers have to scrap an entire lot of product: they can discover bad component or assembly trends early – left-shifted – and stop the production line with very few scrapped products. Further, notifications can be sent, not when a part is exhausted, but at exactly the right time to allow just-in-time delivery to the machine. This saves cost by eliminating down time, and saves inventory and floor space by not storing those components near the machine.
System designers can left-shift the PCB design all the way to the IC itself. By considering the chip, the package, and the PCB all at the same time, the optimum combination of pinouts and PCB design can be realized in a shorter time, producing a higher quality design. Plus, by left-shifting the PCB design and the package design, ICs can be optimally designed into multiple products with significantly different form factors. For example, the same bare die can be used in one package for a product that requires a socket, and then designed into a package/board combination in another product that requires a small, mobile form factor. By moving the consideration of both the PCB and package to the left in the timeline, costs are saved, design time is reduced, and the resulting flexibility allows wider use of the same die, which not only leverages the hardware, but software as well.
Where is All of This Going?
Product complexity is headed in one clear direction: up and to the right! To get the design, as well as manufacturing process under control, left-shift automation must be employed to eliminate as many problems up front, where they are less costly and consume less time, rather than dealing with those issues after a design and prototype has been constructed. Employing DFM, maximizing FPGA pin-assignment flexibility, and virtual prototyping using analysis and simulation software, are ways to keep complex designs under control.
In the future, the division between design and manufacturing will be blurred and even eliminated. The most efficient way to design electronic products is not serially, not even in parallel, but rather by taking the entire process, from the spark of idea through shipping that first (and subsequent) product out the door, and left-shifting it as far forward in the process as possible by employing the power of automation.
John Isaac isdirector of Market Development, Systems Design Division of Mentor Graphicsand has worked in the EDA industry with PCB and IC technology for more than 30 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.