Achieving an optimally designed board also requires foresight. The more you know about circuit board manufacturing and assembly, the better prepared you can be to make decisions to support and facilitate the PCB manufacturing process, especially if you are concerned about the environment. These decisions, including design for manufacturing, form the basis for ensuring that your PCB will take advantage of the equipment, process and capabilities of your contract manufacturer (CM). Let's review the main principles of DFM in order to optimize the manufacturing overhead of PCB development based on yield variance.
Design for Manufacturing Principles (DFM)
For PCB design, DFM can be defined as the application of board parameter specifications, enabling your manufacturer to translate your design intent into an actual product that meets its mechanical and electrical goals. The need to follow and apply good DFM guidelines and rules cannot be overstated. After all, your board is not buildable unless the dimensions and specifications are within the tolerances of the CM equipment. The DFM specification includes the following:
Material Selection: Select substrates and laminates with parameters best suited to the type of design.
Clearance and Spacing: Ensuring that the distance between components, traces and board edges is adequate for the manufacturing and assembly process.
Trace size: In addition to meeting signal propagation requirements, ensure that copper weight and trace width are within CM capabilities.
Drill location and via type: Choose the size of the hole with the correct aspect ratio and select the location where the hole can be drilled based on the CM equipment.
PCB Stackup: Select the number of layers and layer materials based on signal requirements and within CM capabilities.
Solder Mask Definition: If necessary, ensure that there is sufficient solder mask, solder mask clearance, and connection pads to prevent board shorts and oxidation.
The above list is representative and does not include all design choices and specifications that may increase board fabrication and assembly. For example, there are guidelines for assembly, so-called “design for assembly” (DFA), which include ensuring that the package of the component matches the bill of materials (BOM) entry, and includes polarity and a first indicator on the silkscreen.
DFM and Manufacturing Overhead
Including a good DFM in the design process does come at a cost. These costs can be classified as direct costs or indirect costs, which are manufacturing expenses in the development and design phase, including:
Design and manufacturing costs:
1. Time to acquire and incorporate DFM rules and guidelines for CM.
2. Take the time to revise and redesign to bring your design to a manufacturable state.
3. The man-hours required to bring your design up to production-ready.
4. Additional software costs (if any) to facilitate DFM implementation.
The board fabrication and PCB assembly phases of development are themselves a direct cost of the development process, which largely depends on production levels.
Manage Manufacturing Overhead Based on Yield Variance
Regardless of production level, the fabrication of circuit boards follows the same basic steps. However, manufacturing costs are largely dependent on production levels. Typically, there are two levels of production: small batches and large batches. Low-volume production can range from a few boards to hundreds of boards.
At this level, proofs of concept and prototypes are done, which may require multiple manufacturing runs or iterations of the entire design⇒build⇒test cycle until the design is finalized and ready for high-volume production. In high-volume production, the main concern is yield, or the ratio of usable boards to actually manufactured boards. The production numbers here can be tens of thousands or hundreds of thousands or even more.
Obviously, manufacturing overhead cannot be eliminated; however, it can be managed during the design process. By following the tips below, this can be achieved and the manufacturing process can be optimized and costs controlled.
Tips for Managing Manufacturing Overhead
1. Obtain and establish DFM early in the design process
By applying DFM early on, you can afford the extra time and expense to get your design to a state where you can build a board. The fastest and most accurate way to achieve this is via file upload (if available), and the design software can accommodate it.
2. Apply DFM according to yield variance
DFM specifications can be relaxed or even changed to help speed up iterations in the development process. However, for high volume production, your DFM should be complete, as is the entire design.
3. Utilize circuit analysis to help determine DFM specifications
Having a software design program that allows you to perform signal and board analysis during the design process can help you properly select board materials, trace routing, via types, and other DFM specifications, thereby reducing manufacturing overhead.
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