Happy’s Essential Skills: Design for Manufacturing and Assembly, Part 1
Advances in interconnection technologies have occurred in response to the evolution of component packages, electronic technology and increasing complex functions. Therefore, it comes as no surprise that various forms of printed wiring remain the most popular and cost-effective method of interconnections.
Manufacturing, assembly and test technologies have responded by improvements in their technologies. These increased capabilities have made selection of technologies, design rules and features so complex that a new function has developed to allow for the prediction and selection of design parameters and performance versus manufacturing costs. This is the planning for design, fabrication and assembly. This activity has also been called design for manufacturing and assembly (DFM/A) or sometimes predictive engineering. It is essentially the selection of design features and options that promote cost-competitive manufacturing, assembly, and test practices. Later in this column, I will offer a process to define producibility unique to each design or manufacturing process.
The purpose of this column is to provide information, concepts, and processes that lead to a thoughtfully and competitively designed printed circuit, ensuring that all pertinent design and layout variables have been considered.
Originators: Dewhurst & Boothroyd
Modern DFMA stems from the ideas of university professors Goefrey Dewhurst at University of Massachusetts -Amherst and Jeffery Boothroyd at University of Rhode Island . These Manufacturing Engineering professors came up with the concept that you could predict the assembly time, dificulty and cost by cateloging and summing all the kinematic actions it takes during assembly. This concept became so accurate that it was used to predict assembly while a product was still in the design phase. Thus as a predictive metric, it became Design for Manufacturing—DfM. Today this technology is taught in universities and used by most large OEMs around the world. The Dewhurst & Boothroyd software (Table 1 and Figures 1a & b) consists of:
The D&B techniques consist of analyzing an assembly for these features:
- The need for this part
- How many fasteners are required
- The number of different fastener types
- The number of difficult-to-assemble parts or subassemblies
- The number of motions and twist/turns involved in each assembly step
- The role of tooling and fixturing
The resulting analysis shows a:
- Total part count
- Theoretical minimum of parts or preassembled item (Pmin)
- Assembly efficiency (ease of assembly—AE)
- Assembly time
The analysis uses software to measure:
- Prototype evaluations based on either actual or 3D models using Boothroyd and Dewhurst method.
- Pmin is a measure of the complexity of the product. In general, the more functionality there is in a product, the higher the Pmin value.
- Assembly Efficiency (AE) is a ratio of the theoretical minimum number of parts (Pmin) to the estimated assembly time. An approximation is used to compute AE (2.933 seconds per part), so AE values are used for relative comparisons only.
Table 1: The Dewhurst & Boothroyd DFMA software.
Figure 1a: The Dewhurst & Boothroyd DFMA software.
Figure 1b: The Dewhurst & Boothroyd DFMA software.
In general, these are some of the guiding DFM/A principles:
- Know Your History—Learn from the past: Returns, corrective action processes, recalls, etc. Know and understand problems and issues with current and past products.
- Standardize Design Methods & Tools: Standardize design, procurement, processes, assembly and equipment. Don’t redesign the wheel—use existing parts and assemblies and limit exotic or unique components.
- Simplify the Design—Methods for Part Reduction: Parts reduction is one of the best ways to reduce the cost of fabrication, assembling a product overhead and increase quality and reliability.
- Simplify the Design—Parts Commonality via Multi-use/Multi-functional Parts: Develop an approved or preferred parts lists or a standardized BOM. Use one-piece structures from molding, extrusions, castings and powder metals. Use multi-functional parts that perform more than one function.
- Design for Total Quality Management—Fundamental Principle of Lean: Lean supply, fabrication and assembly processes are essential design considerations. Develop and use standard guidelines appropriate for the process being performed. Know and apply lean principles to design manual operations for the capabilities of the operator. Practice ergonomics to maximize productivity and reduce operator fatigue and discomfort.
- Eliminate Waste: Overproduction, delays—waiting, transporting/moving, process inefficiencies, queues-inventories, unnecessary motions and defective products.
- Design for Parts Handling: Minimize handling to correctly position, orient and place parts to avoid multiple or complex assembly orientations.
- Design for Efficient Joining and Fastening: Avoid threaded fasteners when possible, consider alternatives; if used, minimize variety. Screws, bolts, nuts and washers are time-consuming to assemble and difficult to automate.
- Use Error-Proofing Techniques: Mistakes will happen. What can go wrong will go wrong. Use error-proofing techniques in product design and assembly.
- Design for Process Capabilities: Make unnecessary the tight tolerances and tolerances that are beyond the inherent capability of the manufacturing processes or operators in a continuous production situation.
- Design for Test, Repair & Serviceability: Defects will occur. Designing for ease of test and repair will make these processes more efficient, cost effective, and reliable. Failed products are often returned to the manufacturer for service and failure analysis. Where possible, use the production test equipment/setup for return analysis.
The foundation of a robust DfM system is a set of design guidelines and tasks to help the product team improve manufacturability, increase quality, reduce life cycle cost and enhance long term reliability. These principles need to be customized to your company's culture, products, and technologies, and based on a solid understanding of the intended production system—whether internal or external. The basis for these principles is “Measures of Performance & Metrics” where design choices have a score that can be shared by all.
Develop and use standard guidelines appropriate for the process being performed. Know and apply lean principles to design manual operations for the capabilities of the operator. Practice ergonomics to maximize productivity and reduce operator fatigue and discomfort.
Design Planning and Predicting Cost
The need for cost reduction in order to remain competitive is a principle responsibility of product planning. On the average, 75% of the recurring manufacturing costs are determined by the design drawing and specifications . This was one of the conclusions found by an extensive study General Electric conducted on how competitive products were developed. Manufacturing typically determines production set-up, material management and process management costs (Figure 2), which are a minor part of the overall product cost.
Time-To-Market along with competitive prices can determine a product's ultimate success. The first of a new electronic product in the market has the advantages. By planning the PWB layout and taking into consideration aspects and costs of PWB fabrication and assembly, the entire process of design and prototyping can be done with minimum redesign (or respins).
Figure 2: Design determines the majority of the cost of a product.
Design Planning and Manufacturing Planning
Electronics is one of the biggest enterprises there is globally. It is common for design to be done in one hemisphere and manufacturing in another. It is also common for manufacturing to be done in a number of different places simultaneously. An integrated approach must be adopted when the intention is to rationalize fabrication and assembly as part of the entire production system and not as individual entities as shown in Figure 3. This dispersed manufacturing must be taken into consideration during the design planning and layout process. No finished product is ever better than the original design or the materials it is made from.
Figure 3: Fabrication and assembly rationalized by planning and design.
The planning process central focus will be the tradeoffs between the loss and gain in performance for layout, fabrication, assembly and test versus the costs in these domains. Therefore, some major considerations will be the following sections:
- New product design process
- The role of metrics
- Layout tradeoff planning
Planning for design, fabrication and assembly (PDFA) is a methodology that addresses all those factors that can impact production and customer satisfaction. Early in the design process, the central idea of PDFA is to make design decisions to optimize particular domains, such as “producibility,” “assemble-ability,” “testability,” as well as fit to a product families, etc. in manufacturing. Planning takes place continuously in the electronic design environment (Figure 4). The data and specifications flow in one direction, from product concept to manufacturing. During the design process, 60%of the manufacturing costs are determined in the first stages of design when only 35% of the design engineering costs have been expended. The typical response is shown in Figure 5.
Figure 4: The electronics design environment.
Producibility is now regarded as an intrinsic characteristic of a modern design. Like the concept of quality in manufacturing, it must be built in, not inspected in. Producibility must be designed in; it cannot be a "checkpoint" in the design process or inspected in by tooling.
NEW PRODUCT DESIGN
The keys to superior producibility in new product design can be found in the expanded design process. One of those key items is the role of metrics or data-based analysis of planning tradeoffs.
Figure 5: Design costs accumulate early in the Life Cycle compared to intrinsic manufacturing costs.
Expanded Design Process
The new expanded design process that incorporates planning, tradeoffs and manufacturing audits is shown in Figure 6 and Table 2. The process is made up of 12 separate functions that incorporate the planning and tradeoffs sections in this book.
Figure 6: Specifications determine product partitioning and producibility.
Table 2: The expanded electronics design process and its stages.
This differs from the more conventional design process (Figure 3) by the inclusion of four important functions:
- The formal Technology Tradeoff Analysis during Specifications
- Detailed Tradeoff selection of features for layout, fabrication and assembly
- Design Advice during component placement and routing
- Manufacturing Audits to review the finished layout for producibility, time-to-market, and competitiveness
The initial new product design stage is specification and product definition. This key step takes ideas, user requirements, opportunities and technologies and formulates the executable specifications of a new product. During this operation, the ability to predict what will happen in manufacturing by technologists that may not have any manufacturing experience can affect both time-to-market and ultimate product costs. Figure 6 shows the technology tradeoff analysis that requires the balance of loss and gain in various domains performance versus costs. Size and partitioning for IC and ASIC must be balanced with overall packaging costs and the resultant electrical performance. All of these factors affect the manufacturing and product cost.
Another definition of this process could be called a "verified design". A verified design is defined as one that was predicted from models or measures that have been correlated to past designs.
This is compared to the traditional approach which is a "non-verified design" or “trial-and-error.” This is diagrammed in Figure 7. The advantage of the "verified design" can be significant reduction in redesigns in order to achieve the original product objectives.
Figure 7: Design incorporating tradeoffs versus traditional design.
Editor’s Note: Part 2 of this article will be published tomorrow, June 30, 2016.
Update: Click here for the part 2 of the article.