Design for Manufacture (DFM)
The principles of design for manufacture and design for electronic assembly have been widely been used in industry through design guide- lines and DFM systems for effectively measuring the efficiency of designs for manufacture and cost. The most important guidelines for
DFM design for parts are:
1. Use minimum parts types
2. Use standard components
3. Use parts that fit or snap together with no fasteners
4. Tools are not required for product assembly
DFM analysis results in reduced production time and need for operator skills. The DFM design guidelines，such as the ones mentioned above, are based on common lessons learned while developing electronic products. Prior to formal DFM systems, checklists used by major electronic companies as a repository for the wisdom of their successful design engineers.
DFM design guidelines emphasize the design of electronic products using self-locating and self-aligning parts, built on a suitable part. The number of parts should be minimized by using standard parts and integrating functionality and utility. Several cost saving techniques should be used, such as standard and automatic labeling self-diagnosis capability at the lowest level, and using symmetrical and tangle-free part designs.
In the formal methodology of DFM, a scoring system is used measure the design efficiency, based on the performance object' and the manufacturing capability. Several alternate designs can be created using the principles of DFM, and the best design can then be chosen based on the scoring system. A conceptual view of a DFM scoring system is shown in Figure 1.5. A typical output of well-designed DFM products is shown in Table 1.2, which compares the design of a new product to older non-DFM designs. Such a product is the Hewlett Packard (now Agilent) 34401A Multi-meter. This case study was authored by Robert Williams and published in a book edited by the author (Shina, 1994). The product was designed using six sigma and QFD. It can be seen that the number of parts and assemblies have been reduced significantly over previous generations of multi-meters through the application of DFM as well as QFD principles during the design stage. In addition, the new product was introduced to manufacturing without any engineering change orders (ECOs) in the first year of production. A typical successful new development project for a new product using DFM could include the following activities:
• Score product and part designs in breadboard or early prototype stage, prior to initiating CAD drawings. This is important, since once the drawings are completed, it is difficult for design engineers who invested valuable time in the current drawings to redraw them based on DFM evaluations.
• Identify difficult assembly steps and determine if part design changes can make them easier to assemble.
• Test for redundant parts and review the use of nonstandard parts.
• Based on the DFM review, simplify and redesign the parts or final product, using competitive benchmarks, especially if the competition is successfully applying DFM. This design review may include changing process plans or assumptions. Generate a new design that is more efficient by eliminating redundant parts, making parts symmetrical and minimizing assembly motions.
• Rescore the new design and weigh benefits of redesign versus cost and quality adverse consequences, if any. Consider the impact on schedule, tooling, production, and part cost.
• Pursue chosen design approach.
Figure 1.5 Use of a DFM scoring system.
The objectives of DFM are more focused on design for low cost. This is accomplished through fewer parts, parts that are standardized, or parts that are easier for operators or production machines to assemble, hence requiring lower operator skills. The result of DFM analyst could be very beneficial toward achieving the goal of six sigma. A well-designed DFM part or assembly can have a much wider tolerance, or it can be easier to manufacture, resulting in reduced assembly defects. In addition, the design team can focus better on a smaller number of parts.
An interesting consequence of applying DFM to new designs, which will be discussed in the next chapter, results from the reduction in the number of parts. Each additional part carries with it a potential for more defects. A smaller number of parts reduces the opportunities to generate defects, hence making the part design more robust and closer to the six sigma goal.