All Systems Go! Supply Chain Woes—Which Comes First, the Design or the BOM?

Niten_Bhagwath_250.jpgIn an ideal world, when developing a PCB for an electronic product, decisions made during the design process should drive the bill of materials (BOM). We may think of this as an example of “the dog wagging the tail.” In the real world, however, there has always been some small amount of the BOM driving the design, which we may think of as “the tail wagging the dog.” A classic example of this is when an engineer’s calculations indicate the need for a resistor of 123 kΩ—a 40-cent part—while a 120 kΩ resistor—available for only 4 cents—will provide an almost identical response.

Current realities have made such BOM-driving-the-design decisions more inescapable to ensure product manufacturability. Everyone around the globe—from small companies to mega-enterprises with trillion-dollar valuations, all the way to the U.S. government—is currently facing unprecedented supply chain challenges. Supply chain optimization has long been under pressure, involving as it does a sophisticated balance of low cost, future availability, and product needs. While it’s true that difficulty obtaining all the components that are perfect for the task at hand is not totally unprecedented, alternative parts used to be plentiful. This meant that, even if you couldn’t get exactly what you wanted, you could get something similar. But with supply chain disruptions around the globe, even next-best parts are hard to find, making it critical to ensure component availability for the design.

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Figure 1: A design with a rigid BOM requirement may result in an incomplete product.

One part of the solution is for everyone in the organization to have visibility with respect to stock levels. It’s important for this visibility to commence at the earliest stages of the design while the engineers are working on the initial high-level block diagram, thereby allowing for the design architecture to accommodate parts that can actually be sourced. You shouldn’t be relying on a spreadsheet that resides on the engineer’s desktop and is only updated and distributed by email “every now and then.” Stale information is no good to anyone. It shouldn’t be necessary for anyone to need to dig around to uncover the data they need. However, if given this information at the appropriate time, design solutions and workarounds are possible.

For example, you may be considering a certain component, only to discover this component is not currently available. There may be alternatives that are equivalent, or not as good, or better (some may say “overkill”). Take DDR4 memory, for example. It might be that you really require only an 8Gb DRAM for this version of the device. However, early in the design, you discover that it has a 30-week lead time, but 32Gb DRAMs are available in quantity. You could decide to add a few extra control signals to your design to enable this part, thereby allowing you to manufacture your product within an acceptable timeframe.

A similar approach may be employed with connectors. There may be a specific connector you wish to use that’s in short supply. You think you’re going to be all right... you’re sure you’re going to be all right… you know you’re going to be all right… but, there’s always the off-chance things will go pear-shaped. If you have the board real estate available, it may be a good idea to include footprints for both your preferred connector and an alternative that’s more easily sourced. Using this approach, in a worst-case scenario, you can ship the board populated with your backup connector and a conversion cable. Even if you never end up using it in production, you can at least bring the board up using the backup connector for the purposes of test and verification.

Adding no-load parts as a backup can be used in other situations too. Consider a function like an on-board power supply. Your options are to construct this function out of discrete parts or to purchase a pre-built module. If you have sufficient board real estate, you may decide to design both options in, thereby allowing you to change tack in the future if needed.

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Figure 2: Allowing the BOM to drive the design helps mitigate supply chain woes.

Shortages and supply chain issues are not confined to components—they are also prevalent with respect to the materials used to fabricate the boards themselves. You may be okay with standard FR-4, but what is the situation with respect to something like FR408HR or even more specialized substrates? To determine the current state of play, it’s important to engage with your contract manufacturer (CM) or circuit board fab as early as possible in the design process so that they can guide you in your material selections. The board’s interfaces to the outside world are “set in stone,” but it may be possible to modify the internal implementation of the board. If the material you want won’t be available for 16 months, you might have to shorten the trace lengths to support an alternate dielectric, which may have higher dielectric losses at the rates you need to run at.

And, of course, it’s not just the board substrate itself. Other factors that could drive design changes may also need to be considered, such as the availability of the desired solder paste and finishes like immersion silver (IAg), immersion tin (ISn), electroless nickel immersion (ENIG), and organic solderability preservative (OSP).

Many of these changes may end up with a small incremental cost for the board, but it’s important to contrast this against the potential opportunity cost of millions of dollars in lost sales caused by the fact that you don’t have all the parts you need to complete the assembly.

The bottom line is that your design methodology should include a powerful and highly integrated product lifecycle management (PLM) capability. What is required is a tool that makes fresh data immediately available to everyone in the organization, so whenever buyers, engineers, and designers make changes, the others get to see those changes in real time, thereby allowing them to make decisions accordingly.

When it comes to allowing the BOM to drive your own designs, have you used any of the approaches presented here? More importantly, do you have any additional tips and techniques that you would care to share with the rest of us?

Nitin Bhagwath is director of product management, PCB front end at Cadence Design Systems.

Download The System Designer’s Guide to… System Analysis by Brad Griffin along with its companion book The Cadence System Design Solutions Guide. You can also view other titles in our full I-007e Book library here

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2022

All Systems Go! Supply Chain Woes—Which Comes First, the Design or the BOM?

04-21-2022

In an ideal world, when developing a printed circuit board (PCB) for an electronic product, decisions made during the design process should drive the bill of materials (BOM). We may think of this as an example of “the dog wagging the tail.” In the real world, however, there has always been some small amount of the BOM driving the design, which we may think of as “the tail wagging the dog.” A classic example of this is when an engineer’s calculations indicate the need for a resistor of 123 kΩ—a 40-cent part—while a 120 kΩ resistor—available for only 4 cents—will provide an almost identical response.

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All Systems Go! Find and Fix Thermal PCB Problems Sooner Than Later

03-17-2022

In an earlier column titled "Bridging the Gap Between Design and Analysis with In-Design Analysis," Brad Griffin discussed how the “shift left” that’s happening with electronic design means it is no longer sufficient for signal integrity (SI) and power integrity (PI) analysis to be performed in isolation. Designing, analyzing and verifying the design in its entirety is key. Another facet of this shift left is the need to address thermal integrity (TI) sooner rather than later. In other words, finding and fixing thermal PCB design issues early in the design process is necessary to save costs, reduce design spins, and maintain your own sanity.

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All Systems Go! Ensuring Power Integrity—Explore, Design, and Verify

02-17-2022

When designing an electronic system, ensuring power integrity (PI) is all about making sure that the power you are putting into the system via the voltage regulator module (VRM) reaches the downstream components in an efficient, sufficient and stable manner. In the not-so-distant past, ensuring the PI of an electronic system was a relatively simple and pain-free task. Many products involved a single PCB populated by readily available off-the-shelf ICs, such as the classic 7400-series devices from Texas Instruments. For the purposes of PI, these ICs, which were presented in low pin count, coarse pin pitch packages could be treated as closed boxes represented by simple power models.

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All Systems Go! Bridging the Gap Between Design and Analysis

01-20-2022

Electronic designs are increasing in capacity, complexity, and performance. This is coupled with increasing pressure to get new products to market as quickly as possible while, at the same time, ensuring that these products are robust and will not fail in the field. The only practical way to address all these diverse requirements is to make design and verification tools and methodologies more powerful, intuitive, and easier to use. In-design analysis provides a way forward.

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All Systems Go! Meet Power Delivery Requirements Upfront with Power-First PCB Implementation

01-06-2022

The drive for faster throughput, increased mobility, and maximum efficiency in modern electronic devices has made power delivery a critical piece of design success. However, meeting the power needs of modern designs is anything but simple. To achieve a robust design, each supply must be capable of delivering sufficient current to every dependent device. In addition, those supplies must be both stable (able to maintain narrow voltage tolerances) and responsive (capable of adapting to transient current demands). Identifying and resolving power delivery problems late in the design process is incredibly difficult. If design power requirements aren’t considered upfront, it can lead to schedule delays and a significant amount of debugging time in the lab. Implementing a power-driven, PCB layout methodology ensures the design process addresses critical power and signal integrity (SI) issues collectively at a time they can be easily solved.

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2021

All Systems Go! Simulating Wirebonded CoB on Rigid-Flex

11-18-2021

There are many good reasons to use a chip on board (CoB) implementation. When this is combined with wirebonding and the use of rigid-flex PCB, challenges mount. An application that demands all three—CoB, wirebonding, and rigid-flex PCB—is a camera module that goes into a mobile application, the sample design used to illustrate the design and analysis challenges in this article. If you are not aware of and prepared for the potential pitfalls, it is highly likely that your project could fall short or even fail.

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All Systems Go! Signal Integrity Signoff of 3D-IC Systems

10-14-2021

3D-ICs meet the demand for integration of disaggregated system-on-chip (SoC) architecture built from multiple chiplets and heterogeneous architectures such as analog, digital, optoelectronics, and non-volatile memory.

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All Systems Go! Comprehensive Thermal Analysis of a System Design

09-23-2021

In recent years, driven by the demand for smarter electronics, device designers have witnessed enormous scaling of large and hyperscale integrated circuits (ICs) and embraced development directions toward high density and reliability. These devices have increasingly higher thermal performance requirements—both transient and steady-state—and meeting them is becoming increasingly complex and time consuming.

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All Systems Go! Challenges in Analyzing Today’s Hyperconnected Systems

07-26-2021

Today’s data-thirsty world is looking forward to the next-generation communication systems beyond 5G, the promise of massive connectivity to the internet with extreme capacity, coverage, reliability, and ultra-low latency, enabling a wide range of new services made possible through innovative and resilient technologies. The exponential growth in data speed and networking has introduced numerous design and analysis challenges across a system design. Design teams are challenged to deliver new, differentiated products faster and more efficiently, despite the ever-growing complexity of silicon, package, board, and software for many complex applications in the hyperscale computing, automotive, mobile, aerospace, and defense markets.

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All Systems Go!: Thermal Compliance of 3D-IC

06-24-2021

In the packaging world, we have been designing heterogeneously integrated multi-chip products for decades. As we know, smaller process nodes enable higher frequencies and save on die area. However, for minimizing the system size, we need to use advanced packaging technologies.

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All Systems Go! Ensuring Signal Integrity of DDR5 Interface

05-25-2021

The double data rate synchronous dynamic random-access memory (DDR SDRAM) has evolved from a data rate of 0.4 Gbps to the next generation, DDR5, scaling to 6.4 Gbps. With DDR5, we can achieve higher bandwidth using less power per bit transferred, enabling us to do more computing on larger data sets.

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All Systems Go! EM Analysis for Today’s System-Level Designs

04-30-2021

There are two main reasons to do EM analysis: to see if the signals in the design will meet your performance specifications, and to see whether the design has unintended EM interactions in the circuit or system. Since domain-level requirements vary, not all EM solvers are the same.

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