Happy’s Essential Skills: Design for Manufacturing and Assembly, Part 1


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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 [1]. 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 ManufacturingDfM. 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:

  1. Prototype evaluations based on either actual or 3D models using Boothroyd and Dewhurst method.
  2. 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.
  3. 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.

AE (2).jpg

2_Table1.jpg

Table 1: The Dewhurst & Boothroyd DFMA software.

3_Fig1a.jpg 

Figure 1a: The Dewhurst & Boothroyd DFMA software.

4_Fig1b.jpg 

Figure 1b: The Dewhurst & Boothroyd DFMA software.  

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