Incorporating Manufacturing Overhead and Production Volume Variance in Your Design Process

Key Takeaways

• The development reason production lots can vary so greatly in size.

• How to reduce manufacturing overhead by modifying inputs on production volume variance.

• General design for manufacturing (DFM) best practices to reduce cost and increase yield.

Incorporating manufacturing overhead and production volume variance is the only way to climb the ladder toward mass production.

The manufacturing of PCBs is a complex, multi-step process and the design of the board can differ greatly depending on the lot size for that production run. In general, designs are more accommodating with smaller runs and much less so with larger volumes. The reason for that comes down to per-board costs: prototypes and small batches aren’t as financially constrained as a board prepared for tens of thousands of units or more. Manufacturing overhead and production volume variance, therefore, go hand in hand as both an input to design and a targeted outcome of PCB manufacturers.

Achieving the best-built board for your design requires foresight. The more you know about how your board is fabricated and assembled, the better prepared you are to make decisions to support and facilitate the PCB manufacturing process, especially if you have an eye on the environment. These decisions, which comprise design for manufacturing (DFM), form the foundation that ensures your PCBs are built to take advantage of the equipment, processes, and capabilities of your contract manufacturer (CM). 

How Manufacturing Overhead and Production Volume Variance Interact

The manufacturing of your boards follows the same basic steps regardless of the level of production. However, the manufacturing overhead is highly dependent on the production level. Typically, there are two levels of production output:

• Low volume is universally the realm of prototyping and revisions where the design and its translation to a physical product are still being calibrated for optimal performance. Some established boards, generally non-consumer Class electronics, may remain exclusively within this realm – there’s simply no mass production demand, and this eases design and manufacturing while offering some ancillary performance benefits.

• High-volume designs must be carefully reviewed to ensure the build is optimized, even at the reasonable expense of performance. For example, eliminating even a single drilled hole for an unnecessary via reduces the tooling cost and processing time 10,000-fold or more. Component lists need to be vetted for similar reasons, and also to prepare for procurement. The board production and evaluation must be viewed as an economy of scale.

Boards undergo multiple manufacturing runs or iterations through the design ⇒ build ⇒ test cycle until the design is finalized and ready for high-volume production. At high volumes, the primary concern is the yield rate or the ratio of usable boards to boards built.

PCB manufacturing utilizes optical inspection techniques for defect analysis.

Obviously, manufacturing overhead cannot be eliminated; however, it can be managed especially during design. By following the tips below, this can be accomplished and your manufacturing process can be optimized and costs controlled.

Tips for Managing Manufacturing Overhead

Tip #1:     Acquire and institute DFM early in the design process

By applying DFM early you can afford the additional time and expenditure to bring your design to a state where your boards can be built. The fastest and most accurate way to accomplish this is by file upload, if available and your design software accommodates it.

Tip #2:     Apply DFM based upon production volume variance

DFM specifications can be relaxed or even changed to help speed up iterations during development. However, for high-volume production, your DFM should be finalized, as it should be for the overall design.

Tip #3:     Utilize circuit analysis to aid in determining DFM specifications

Having a software design program that allows you to perform signal and board analysis during design can help you make the right choices for board materials, trace routes, via types and other DFM specifications, which can reduce your manufacturing overhead.

Principles of Design for Manufacturing

For PCB design, DFM can be defined as the application of circuit board parametric specifications that enable your manufacturer to transform your design intent into an actual product that meets its mechanical and electrical objectives. The need to follow and apply good DFM guidelines and rules cannot be overstated. After all, your boards are not buildable unless and until the dimensions and specifications are within the tolerances of your CM’s equipment. DFM specifications include the following:

• Material selection: Choosing substrates and laminates with parameters that are best for the design type.

• Clearances and spacing: Ensuring the spaces between components, traces, and board edges are sufficient for fabrication and assembly processes.

• Trace dimensions: Making sure that copper weights and route widths are within CM capabilities in addition to meeting signal propagation requirements.

• Drill hole locations and via types: Utilizing the correct aspect ratios to select hole sizes and choosing locations that can be drilled based on CM equipment. 

• PCB stackup: Selecting the number of layers and layer materials based on signal needs and within CM capabilities.  

• Solder mask definitions: Ensuring there are adequate solder dams, solder mask clearances, and webs to protect the board against shorts and oxidation.

The list above is representational and does not include all design choices and specifications that can add to the fabrication and assembly of your board. For example, there are guidelines focused on assembly, known as design for assembly (DFA), which include ensuring component footprints match the bill of materials (BOM) entries and including polarity and a pin one indicator on the silkscreen. 

DFM and Manufacturing Overhead

Including good DFM as part of your design process does have costs. These costs, which can be classified as direct or indirect, are the manufacturing overhead for the design stage of development and include:

1. Time to acquire and incorporate your CM’s DFM rules and guidelines.

2. Time spent on modifications and redesign to bring your design to a manufacturable state.

3. Man-hours spent on bringing up your design to production readiness.

4. Additional SW costs, if any, to facilitate DFM implementation.

The board fabrication and PCB assembly stages of development themselves are direct costs for your development process, which are mostly determined by the level of production.

Manufacturing overhead and production volume variance are interrelated, but PCB manufacturing will typically look to emphasize one facet over the other. Whether you are designing to cost or to value, Cadence offers the right PCB Design and Analysis tools to help you manage your manufacturing overhead based on production variance. And with the advanced analog/mixed-signal simulation capabilities of PSpice, you can ensure that your DFM selections are the best for your design.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.