I am a bit of a movie buff, but it is rare that a movie both captivates and inspires me. However, Limitless which chronicles the exploits of a man who has been able to unlock the full potential of his brain surely did! Not only does it star one of the most up and coming actors in Bradley Cooper, but Robert DeNiro, one of my favorite actors of all time, also appears in it. Although the premise of this film may seem a bit far-fetched, it accurately reinforces the essential elements of achieving the best future results. Namely, basing today’s activities on a solid foundation that makes good use of foresight.
Achieving the best-built board for your design also 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, form the foundation that ensures your PCBs are built to take advantage of the equipment, processes, and capabilities of your contract manufacturer (CM). Let’s review the key principles of DFM with an eye toward optimizing the manufacturing overhead of PCB development based on production volume variance.
Principles of Design for Manufacturing (DFM)
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 that distances between components, traces, and board edge 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 upon signal needs and within CM capabilities.
- Solder mask definitions: Ensuring there are adequate solder dams, solder mask clearances, and webs, if necessary to protect board against shorts and oxidation.
The list above is representational and does not include all design choices and specifications that can add 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 pin one indicators 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:
Manufacturing overhead of design:
- Time to acquire and incorporate your CM’s DFM rules and guidelines.
- Time spent on modifications and redesign to bring your design to a manufacturable state.
- Man-hours spent on bringing up your design to production readiness.
- 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.
Managing Manufacturing Overhead Based on Production Volume Variance
The manufacturing of your boards follows the same basic steps regardless of the level of production. However, the manufacturing overhead is highly dependent upon the production level. Typically, there are two levels of production: low-volume and high-volume. Low-volume production ranges from a few boards to hundreds.
At this level, proof of concept and prototyping are done, which may require multiple manufacturing runs or iterations through the design ⇒ build ⇒ test cycle until the design is finalized and ready for high-volume production. At high-volume, the primary concern is the yield rate or the ratio of usable boards to boards actually built. Production numbers here may reach the tens or hundreds of thousand or more.
An example of what PCB manufacturing looks like
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.
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.