The concept of design for manufacturability (DFM) has been the subject of significant thought and investigation resulting in a number of published references and case studies touting its success in a variety of manufacturing environments. DFM is an integrated component of the design process that bridges the gap between design and manufacturing considerations ultimately affecting the cost, performance, and producability of a product. DFM demands a commitment to establish a model that systematically addresses manufacturing, assembly, testing, and performance concerns concurrent with the development of the design (Figure 2-1).
A company must be organized to support the DFM discipline. Best manufacturing practices suggest a team approach that includes customers and suppliers. Design projects must be organized to address producability. This can be accomplished by forming multi-disciplinary, integrated teams that facilitate communication by including as much knowledge from key stakeholders as possible. Seven to ten member teams can effectively examine and disseminate the relevant information and provide problem solving resources while still allowing for unplanned changes. It is crucial to identify and manage risk. A risk is the potential inability of achieving product goals, and is quantified by the probability and consequences of a failure. DFM moves some risk to the initial phases of the design cycle where it is less likely to have a negative impact.
The impact of design decisions vary depending on the methods and materials in use at each process step. At the EMPF, the engineers specialize in electronics manufacturing and assembly, particularly at the packaging level. In the design and production of electronics, producability is best addressed before the project builds inertia. At its heart, DFM has the following guiding rules:
- Estimate the cost of manufacturing to identify cost reduction areas.
- Reduce the number and cost of components.
- Reduce the number of assembly operations by reducing the number of parts.
- Make the assembly operations easy to perform.
- Reduce the cost of supporting production.
- Consider the impact of DFM decisions.
- Employ Design Guidelines.
- Establish rules based on expert knowledge and lessons learned.
- Base guidelines on specific capabilities including equipment and human resources.
- Employ benchmarking by comparing best practices of others in related fields.
Properly implemented, the DFM will reduce design time, reduce the cost of manufacturing, reduce the cost of quality, improve performance, and reduce the cost of service and maintenance of the end product. Some key DFM considerations in the electronics assembly design process follow.
Assembly Process Overview
DFM rules should be established for each process step. As a part of the design sequence, the design review should ensure that every manufacturing process step is known. The most rigorous implementations would include validation of manufacturing process steps as the design is being developed.
Electronic assemblies can be designed with a variety of components from different suppliers. Some components have leads that pass through holes in circuit boards. Some are attached by means of surface mount leads, while others are leadless. There are also components with hidden terminations that must be secured to assemblies with special adhesives. Some components are compatible with standard epoxy circuit cards, while some are better suited for exotic materials where high temperature or high frequency performance demands preclude the use of standard circuit substrate materials
.Consumer demands for increasing performance in smaller systems have driven most designers to place components on both sides of circuit cards. Designers must consider the consequences of component exposure to multiple thermal excursions.
Designs that include components with leads that pass through holes drilled in the circuit card should be evaluated carefully. The following is a list of considerations one should keep in mind.
- Service environment
- Reliability requirements
- Whether or not the holes are plated, as part of the board manufacturing process
- Designability of the land patterns and through holes to allow pin in paste soldering
- Ability of the components to withstand reflow soldering process temperatures
- The availability of automated insertion equipment
- Limitations imposed such as lead to hole ratios, proximityof adjacent components, or height of adjacent components
Remember to consider any surface mount components on the side of the board adjacent to through hole components that will be cut and clinched by automated equipment.
Surface Mount Clearances
Surface mount components and associated land patterns must be considered to ensure optimum manufacturing performance. If you fail to design with component clearances in mind, you may prevent the use of automated equipment that has clearance limitations. You may also have to remove parts that were not directly affected by the anomaly. Re-work should be considered as well, because BGA re-work equipment may have some limitations, and placing large components in proximity to BGAs increases re-work costs dramatically. It is possible to design a board that cannot be re-worked at a reasonable cost.
PWB and Panelization Strategies
The size and shape of PWBs are important aspects to consider when designing. IPC-2221 provides some guidance to alleviate uncertainty. As a rule, designers should calculate the usable surface area of the circuit card, then calculate the space required by components in the design. Next, the ratio of area required for components to usable surface area should be determined. Very complex designs may populate over 75% of the usable surface area. Therefore, complex, multi-layer boards may be required. Reducing the ratio can result in less expensive, and more reliable designs.
Component Selection and Packaging
Components should be considered based on manufacturing process requirements. Two considerations are whether or not there is a cleaning process, and the solvency of the components. The component must be able to withstand temperature excursions. It is interesting to note that reflow temperature excursions are relatively brief when compared to the dwell times in a water based, batch cleaner.
Components that are unusually sensitive to static discharges and mechanical shock and are prone to absorb moisture require a "select on test" value or adjustments. They should receive special scrutiny before they are included in a final design.
Component designers are providing the industry with a variety of new package designs. Since there are many considerations to examine, new packages that have been released to the industry are not necessarily appropriate for use in any application, on any substrate, or in all service environments.
Some future DFM considerations in electronics manufacturing include 1) the impact of lead-free soldering process temperatures, 2) smaller component packages, 3) directly attached semi-conductors, 4) chip scale packages, 5) high density printed circuit board technology, and 6) embedded components. The DFM process will ensure that designers pay close attention to the requirements of component and board suppliers, their own manufacturing processes and capabilities, and the interface requirements of the end user.
The EMPF Learning Center offers a two-day comprehensive course on DFM principles. For more information please contact the EMLC registrar by e-mail at email@example.com or by telephone at 610.362.1320.
1) Product Design and Development; Karl T. Ulrich, Steven D. Eppinger, McGraw Hill, 1995
2) Product Design for Manufacture and Assembly Second Edition Revised and Expanded; Geoffrey Boothroyd, Peter Dewhurst, Winston Knight, Marcel Dekker, 2002