Today’s BOM is a roadmap of the engineer’s design intent up and down the manufacturing supply chain.

The role of the bill of materials (BOM) as an engineering tool is changing. Not in the sense that the BOM is transforming, but that the application and audience have evolved, as changes to the design cycle and manufacturing supply chain have occurred.

When electronics companies both designed and manufactured their finished goods in-house, a BOM was an important internal process document. Armed with sales forecasts and a BOM that defined the components needed to build the product, the purchasing department could schedule the procurement of parts, while manufacturing supervisors could plan staffing levels ahead of time. In fact, this sort of planning was the entire purpose for MRP/ERP systems during this period. But that was then, and this is now.

With the current trends in electronics, design has increasingly become separated from manufacturing. Companies have redistributed functions, preserving critical expertise as in-house departments and moving non-critical roles – like manufacturing – to an outside vendor. The BOM still defines the components needed to make a product, but now it also communicates critical build information throughout the manufacturing chain, this includes contracting companies who may not have a connection back to the original designers or to each other. The BOM serves as a two-way communication tool, telling manufacturing what the designer intended. It provides a way to communicate deviations from the designer’s intent. More than ever before, a BOM baseline is critical to these processes.

The importance of this two-way communication can be seen in the interchange between engineering and the purchasing department. Prior to placing an order for subcomponents, purchasing will usually send a request to engineering for any changes to the BOM that should be accommodated in the upcoming order. If the BOM has been changed, but those changes are not communicated in a timely manner, purchasing may order the parts kits as if there are no changes. The result will be a subcomponents order containing unusable/obsolete parts, kits with errors and a significant amount of re-ordering and rework to rectify the mistake.

From the designer’s point of view, the key to effective configuration control within the overall design flow is the presence of a complete design baseline. Without this document, the subsequent changes in the BOM will be much more difficult to reconcile.

From the manufacturer’s point of view, the BOM is the primary source of information for the work to be performed. When coupled with the coordinate data needed to program the assembly pick-and-place machines (commonly called the centroid data), and provided that the BOM is current, it provides nearly everything needed to program machinery and to assemble the boards.

In the current design/manufacture work flow, with an increasing number of outside vendors delivering to the manufacturer even as early as prototype, the BOM is much more than an internal document. Increasingly, it is at the center of process validation and product lifecycle management. As the industry begins to recognize this change, innovative solutions are coming to the aid of design teams. To help make changes in the BOM’s role clearer, let’s start with some background in PCB design and its relevance to the bill of materials.

Innovation/Prototyping. This is engineering’s most creative phase of the design, often heavy on research and innovation. Activities that distract from the vision of the circuit will make it more difficult to innovate. It is during these initial stages that innovation is facilitated by technology such as SPICE simulation to emulate circuit behavior. With simulation-critical components, the BOM can be verified during the schematic capture stages, rather than having to wait until the physical-prototyping stages. Later, the simulation data will help drive the identification of tight-tolerance components. At this early stage, prototype functionality (and to a lesser degree, component specifications) often takes precedence over individual component costs. The BOM is essentially a running notepad for the designer’s vision.

Optimization. This is where designers refine the prototype for use as a manufacturable product. Design changes might include redesigning the board to optimize real estate, densities and manufacturing process parameters. It’s during optimization that costs first face deep scrutiny. Engineers who follow Demming’s theories on quality, for example, might begin leveraging simulation results to specify critical-tolerance components at this point. The BOM becomes the mechanism for analysis, decision-making and product budget planning. With critical-tolerance components identified, purchasing knows which devices can enjoy a wider tolerance and, perhaps, a lower cost. As a design approaches manufacture, the BOM will split into two variants: the engineering BOM and the manufacturing BOM. While the engineering BOM begins the configuration management process, the manufacturing BOM reflects the needs of manufacturing. As such, it may contain additional pieces (a consumable stencil, for example) needed for manufacture but not an actual component on the board. It is critical that both BOMs always reconcile to each other.

Design Maintenance. Electronic devices, especially commodity electronics, continue downward price tendencies until they near the end of their useful lifetimes. So, common engineering wisdom says to expect at least 10 revisions to any single part in a system throughout the system’s product lifetime; therefore, version-by-version changes must be tracked on the BOM. Without an effective method for capturing the engineering BOM for comparison against the corresponding manufacturing BOM, the maintenance function can quickly become overwhelming and unworkable. This configuration tracking becomes a primary task for the maintenance of the design configuration. If design maintenance is less onerous, then the device maker can continue to lessen the raw materials cost during the product’s lifetime, adding to the overall profit margin.

Design Phase-Out. This is the end of the product’s lifecycle. Whether because of declining customer demand, obsolescence, unavailable key components or redesign, production of the board is scheduled to come to an end. Some electronics companies switch over to stockpiling spares and even reworking failed boards with replacement components to keep customer equipment running. In these circumstances, the BOM is once again crucial information for the repair and maintenance engineers.

As we move through these four main phases, the importance of the BOM increases, and later in product lifecycle, it is even more important for the engineer to have access to both the as-designed engineering BOM and the as-built manufacturing BOM.

Simulation in the Design Flow

It is not new ground when speaking about the need for faster, more efficient and more productive
design. Anything that can be done to improve design performance helps facilitate that needed increase in efficiency and productivity. If designers can make BOM component choices earlier, they can spend a greater amount of time (and effort) on optimizing the design, rather than iterating on critical components. Design decision and analysis can be pulled into the design flow earlier by using SPICE (for analog parts) and IBIS (for digital parts) simulations.

Device manufacturers now create SPICE/IBIS models for many of their components, allowing engineers to proactively emulate and evaluate components in a virtual, simulated environment. Thanks to SPICE/IBIS modeling work, engineers have a detailed and advanced introduction to information that typically has been either buried in datasheets or has required physical device interrogation.

In order to facilitate this kind of improved design and proactive BOM generation, National Instruments has taken a major role in making SPICE simulation technology accessible to engineers. Through the NI Multisim schematic capture and simulation environment, engineers can access thousands of SPICE models that then be evaluated in various design topologies. Multisim has been developed in order to meet the needs of prototyping, and engineers are empowered to use that innovation/prototyping stage to research and to experiment. The environment has been designed for easy use, allowing the proactive creation of a BOM that will quickly and effectively transfer to latter stages of the design flow.

Bhavesh Mistry, marketing manager at NI, notes, “Using simulation models at these early conceptual stages provides immediate feedback on the performance of components in a proposed design topology. The various analyses and graphical visualization of a circuit simulation permits insight into performance that may not be possible on a physical prototype. Powered by computers, simulation provides flexible and in-depth analysis capabilities, more than is afforded by just an oscilloscope and arbitrary waveform generator on a physical prototype.”

The Weak State-of-the-Art for Design Tool BOMs

The BOM output that comes from most CAD tools really is insufficient and missing many of the key pieces of information required by manufacturing says Duane Benson, marketing manager at Screaming Circuits, a subsidiary of MEC in Milwaukee. The CAD tool BOM is only as useful as the information put into it. Either the design team or purchasing department will have to convert this parts list into a complete BOM manually. There's a lot of room for improvement in this process.
The more power and capability to fully define the components in the BOM itself, the more control designers will have over the profitability of their designs. Keith Ackermann, lead engineer with Sunstone Circuits’ PCB123 design team, agrees, “The old flow of ‘Now that I’ve simulated successfully, what’ll it cost to build?’ is reversing itself. Engineers would like to include cost decisions into their parts selection from the very beginning.” Of course, translating the schematic into a physical layout has its own implications on the BOM management process.

Ackermann explains that once a component is identified, designers have to record the part number in some way. After, they have to examine the mechanical drawing for the part and attempt to locate the proper PCB footprint in the system library, or possibly make a new footprint for the part. If the designer is not intimately familiar with the part library structure, this can be a frustrating process. So many parts look the same or use inconsistent naming conventions. While this sparse amount of data may work for the as-designed engineering BOM, it increases the work involved in rectifying the as-built manufacturing BOM to the engineering BOM.

The impact of resorting to this lowest-common-denominator scenario is that the component specification process, from layout designer to the entire manufacturing chain, will be working from ambiguous or even erroneous information. This lack of clarity will make it much harder to rectify engineering and manufacturing BOMs.

Creating a better BOM involves the entire supply chain. Digi-Key Corp. sees the gap in the transition from design to manufacturing as a potential showstopper in today’s short time-to-market environment. Given that one of the key actionable items performed with a BOM is the ordering of parts to support the kitting and manufacture of the design, the company recognized the functional need to streamline the process of researching, specifying and ordering components.

By making a web query service available to CAD tool developers industry wide, its opened the door for developers to integrate Digi-Key data directly into the design environment. Companies have integrated the Digi-Key query into design tools. EMA Design Automation added the Digi-Key query to its Component Information Portal tool designed for the OrCAD tool suite that specifically targets the schematic side of the design process, enhancing parts parameter data with availability information from Digi-Key. In this way, specifying engineers can be alerted to any potential deviations that would be caused by a specific part, even as early as the conceptual design phase.

Sometimes, a designer starts already knowing some of, if not all, the parts to be used in the design. In a design environment where a parts library contains a large number of components specified down to the manufacturer’s or distributor’s part number, users could start their design process by shopping the library for parts then dropping them into the BOM view of the design.

“You really have to feel the pain an engineer goes through repeatedly on every design to understand the value BOM-based design can bring. A seemingly simple task, such as adding a resistor to the design, can be a nerve-fraying activity,” says Ackermann.

NXP Semiconductor recognized the parts selection issue as a key driver for BOM-related maintenance issues downstream and has been developing a complete library of NXP parts. These parts consist of: known-accurate schematic symbols, defined with properties; certified-correct, layout-side footprints; attached property data; current pricing; and availability for every NXP component.

By providing the design team with better control of the BOM, the configuration management process improves, and this brings us back to the opening question. Why would attention to the BOM in the PCB design flow really matter? Other than the fundamental changes to communication between design and manufacturing, how could incremental optimization of the BOM possibly have a significant impact on a designer’s work? A leading CAD/CAM software company found that out of each 10 designs completed, eight were redesigns. Only two could be classified as design creation. The implication is that most of a designer’s work is a modification of something pre-existing. In the future, if a design can be edited by just changing the part entry on the BOM, it will improve design ease and speed time to market.

Conclusion

Today, the bill of materials is much more than an internal document. It drives communication both up and down the manufacturing chain, starting as early as initial prototype and proceeding to the very end of the product’s life cycle. Where design tools have optimized design/simulation, physical layout and design verification, the time has come for similar attention to be paid to the BOM’s management function. Visionary features and components are stitching together the key benefits from each company into a collaborative whole, allowing the engineer’s design intent to flow more freely to and through each specialist in the manufacturing supply chain. As the industry turns its attention, finally, to rethinking the BOM so that it can provide a much-needed roadmap for the supply chain, designers will undoubtedly cheer the transformation. PCD&F

Nolan Johnson is CAD/EDA marketing manager for Sunstone Circuits; This email address is being protected from spambots. You need JavaScript enabled to view it..