design_for_manufacture_and_assembly

2.1Design for Manufacture and AssemblyDesign for Manufacture andAssemblyA set of guidelines developed to ensurethat a product is designed so that it can beeasily and efficiently manufactured andassembled with a minimum of effort,time, and ts designed using DFMA principles should have higher quality and reliability than thosedeveloped using traditional design methods. DFMA also ensures that the transition from thedesign phase to the production phase is as smooth and rapid as Use DFMA?••••Lower Assembly CostShorter Assembly TimeIncreased ReliabilityShorter Total Time-To-Market

Lower Assembly CostDFMA lowers assembly costs by using fewer parts, eliminating unique parts wherever possible,and decreasing the amount of labor required for assembly.

Shorter Assembly TimeDFMA shortens assembly time by utilizing standard assembly practices such as vertical assemblyand self-aligning sed ReliabilityDFMA increases reliability by lowering the number of parts, thus decreasing the chance of r Total Time-To-MarketSince products developed using DFMA make the quickest and smoothest transition into theproduction phase, the time for a product to go from conception to the consumer (total time-tomarket) is reduced. The result is a more complete and workable design the first for Manufacturability ConsiderationsMaterials1. Is material available in standard stock configurations (e.g., bar stock, sheet, standardextrusion)?2. Is material compatible with the most desirable manufacturing process (e.g., ease of forming,casting, machining)?3. Is the material available from reliable sources?4. Do material prices fluctuate widely over time?5. Are special alloys and exotic materials used only for environmental or functional demands?Fabricated parts1. Are specified tolerances reasonable for functional requirements?2. Are tolerances attainable within normal capability of the manufacturing process to be used?3. Are data points, surfaces, and tooling points clear and accessible?4. Does parts configuration minimize the need for special processes and special tooling?Product Assembly1. Are tolerance dimensions realistic?2. Is marking and stenciling defined and visible?3. Are assembly notes complete and definitive?4. Is internal wiring layout critical? If so, is the location and routing specified?5. Is harness development required? If so, can the harness be fabricated outside the unit andinstalled as a subassembly?6. Does the design lend itself to automated assembly?7. Are component parts accessible for assembly?8. Can testing be performed without disassembling the unit?9. Are standard connectors and assembly hardware used?10. Are circuit cards, if used, designed to plug in?

11. Has the assembly been analyzed to meet electrical, thermal, vibration, and shockspecifications?12. Can printed circuit flex cable or molded ribbon be used in place of hard wiring?13. Can plastic tie-wraps be used in place of lacing or spot ties?[Source: Practical Engineering Guides for Managing Risk: Design for Reduce Technical Risk,AT&T, McGraw-Hill, Inc., New York, New York pp. 503-528]Principles of DFMA forMechanical Design•••••••Minimize Part CountMake Parts Multi-FunctionalReduce the Number of Screws and Screw TypesFacilitate Parts HandlingUse Standard Parts and HardwareEncourage Modular AssemblyUse Stack Assemblies/Don’t Fight GravityPrinciples of DFMA forMechanical Design••••Design Parts with Self-Locating FeaturesMinimize Number of SurfacesAssemble in the OpenSimplify and Optimize the ManufacturingProcess•Eliminate Interfaces•Design for Part Interchangeability•Design Tolerances to Meet Process Capability

Minimize Part CountThe final cost of a product is directly proportional to the number of parts. As the number of partsis reduced, product quality and reliability typically increase. This comes about because fewer partsare present to break or work loose and reduces the chances of misalignment. The following threequestions can be used to determine the necessity of a particular part.A Simple Test to Determine if a PartCan Be Eliminated•Must the part move relative to other partsin performing its function?•Must the part be made of differentmaterial?•Must the part be a separate component?If the answer to these three questions is "NO", then the possibility of combining the part withother parts should be considered.

Make Parts Multi-FunctionalMulti-functional parts combine several functions into one part and reduce The Number of Screws and Screw TypesDecreasing the number of fasteners reduces assembly weight, cost, and complexity. Thealternative to fasteners is to incorporate self-aligning features into parts design. If fasteners arenecessary then use a common size and/or self-tapping tate Parts HandlingParts should be designed with handling in mind. They should be designed to minimize thepotential for becoming tangled or stuck together. The goal is quick and easy parts handling thatdoes not require special tooling or fixturing for assembly. Making parts easily identifiable alsospeeds handling. Where possible we would like to make parts symmetric so that orientation is notan issue. However, if a specific orientation is necessary, the use of asymmetric features will forcecorrect alignment, eliminating the potential of incorrect assembly.

Use Standard Parts and HardwareThe use of standard components in a design reduces the number of tools required for assemblyand lowers assembly cost. Unfortunately, standardization requires increased communicationbetween the multiplicity of design teams working on complex products and systems. Often thebest way to handle this is to create a list of standard parts for each project. Generally, fastenersand hardware can quickly get out of hand. A good rule of thumb is to never design a part thatyou can get out of a catalog. In order to facilitate the use of standard parts, the design teamshould utilize various resources such as the preferred parts lists, standard parts manuals, vendorcatalogs, trade magazines, age Modular AssemblyModular designs simplify assembly operations and make problem identification easier by reducingthe number of parts. Modular assembly also simplifies inventory and improves maintenance andserviceability. By designing parts as separate, self-contained modules, disassembly time isreduced, fewer tools are required, and overall repair time is reduced.

Use Stack AssembliesAssembly operations should use, not fight, gravity. One way to do this is to design parts for stackassembly (i.e., assembly components from the bottom up). This makes automatic assemblypossible. Stack assemblies require less reorientation of the components which speeds theassembly process. Not using gravity generally requires the use of additional tooling and fixturing.

Design Parts With Self-Locating FeaturesSelf aligning parts can be placed into an exact location with no adjustment required. This makesassembly easier and faster for the assembly workers. Examples of self-locating features includeprojections, indentations, chamfers, molded keyways, ze Number of SurfacesMinimizing the number of surfaces can reduce the manufacturing and assembly requirements of asystem or subsystem. This is realized by minimizing the number of times that an item must bepicked up and le in the OpenAssemblies should be designed so that assembly operations are accessible and in the open. Blindassembly can increase assembly time, cost, complexity and can contribute to quality fy and Optimize the Manufacturing ProcessSimplification and optimization of the manufacturing process reduces recurring direct andoverhead costs. In designing the production process the focus should be on: smoothing materialflow; minimizing the number of material moves; using processes that are easily controllable;avoiding, where possible, processes that are difficult to control (i.e., welding, brazing, etc.);performing like operations simultaneously; and separating manual and mechanized ate InterfacesInterfaces increase the cost of an assembly. Each interface doubles the amount of informationrequired and increases overall assembly time (i.e., “2 sets of dimensions, 2 sets of tolerances, 2sets of interface features, assembly labor, assembly materials, etc.”).

Design for Part InterchangeabilityUsing interchangeable parts is a simple way to reduce part numbers, simplify the assemblyoperation (i.e., part orientation is not an issue) and ultimately minimize assembly time and cost. Inparticular, left-handed and right-handed parts should be avoided; they often, unnecessarily,increase the potential for confusion and complexity of Tolerances to Meet Process CapabilityManufacturability is enhanced when part designs take into consideration process ication of unnecessarily tight tolerances is a significant waste of time and money. Overlytight tolerances may require additional fixturing, processes, and/or inspections. Therefore,designers must consider manufacturing and supplier capability when specifying tolerances toensure that tight tolerances are truly necessary.

Principles of DFMA forElectronics Assembly•••••••••Use Standard Parts and Material Whenever PossibleDetermine Capabilities of Each Process StepDesign for Ease of Part Orientation for AssemblyEliminate Multiple Solder and Cleaning StepsEliminate AdjustmentsMinimize the Number of PartsPublish and Maintain Up-To-Date Design GuidelinesDesign for Component VariabilityIdentify Defect Rates by CauseUse Standard Parts and Material whenever PossibleUse of standard parts and material reduces the cost, delay, and risk of processing problems. Theprimary advantage of standard parts and materials is that these are known quantities. As suchthey should minimize the potential for ine Capabilities of each Process StepProcess capability defines the ability of the process to produce within acceptable specificationlimits. By documenting process capabilities we can provide the design team with usefulinformation for choosing processing technologies and the appropriate processing for Ease of Part Orientation for AssemblyUsing uniform part orientations eases inspection and assembly tasks. Maintaining the correctorientation is important because of potential impact on quality, particularly the quality of solderjoints. Orientation can be facilitated by using notched packages, chamfered packages or specialmarkings to ensure correct ate Multiple Solder and Cleaning StepsWe would like to minimize the processing technologies utilized. The greater the number oftechnologies used, the greater the chance for errors and reduction in process yield. Likewise,each additional processing step increases cost, risk, and potential for ate AdjustmentsAdjustments are costly and complicate processing. Process controls should be in place tominimize misalignments. Note, however, that not all misalignments must be repaired. Inparticular, minor misalignments may only affect the look of the product and not its functionality.

Elimination of adjustments could also include replacement of potentiometers and manual switcheswith phase lock loops, feedback loops, voltage regulators, ze the Number of PartsMinimizing the number and sizes of parts makes assembly (particularly automated assembly)easier. A potential strategy is to consider combining part functions. Reductions in part counts canalso have a positive impact on system reliability. In addition, it can reduce machine , placement machines have a finite number of feeders).Publish and Maintain Up-To-Date Design for Manufacturing GuidelinesAn organization utilizing IPD should establish Design for Manufacturing (DFM) guidelines. Theseguidelines should address reliability, quality, and manufacturing requirements of the should also include: board parameters (i.e., coating, plating, materials, etc.), critical processsteps, process capabilities, and important machine parameters that impact component for Component VariabilityWe cannot address every aspect of variability but the designer should realize and make allowancesfor uncontrollable variations (i.e., what Deming would call common cause variation) that areinherent to any system. For example, the lead-width of fine pitch packages is controlled but stillvaries from item to item. Knowing this we should design pads that are large enough to reliablyhold the leads regardless of ts of DFMA••••••••Reduced part number and part countsReduced assembly operationReduced product lead-timeReduced packaging costsIncreased productivity and efficiencyReduced material costReduction in overall system/product costImproved product quality and reliability

Many companies have used DFMA with great success. One example is Digital EquipmentCorporation. Digital used DFMA principles when it decided to re-design its mouse. Some of theimprovements realized because of DFMA were:• The number of parts used was reduced by 50%.• The number of assembly steps was reduced by 33%.• The amount of time required for assembly was reduced by 53%.• The volume of material required was reduced by 47%.• The cost of packaging was reduced by 41%.In addition, the project was completed in half the average time normally for Manufacture/Assembly SoftwareBoothroyd Dewhurst, Inc. has developed a software package that allows engineers to implementDFMA techniques within a user friendly, graphical environment. The software can quicklycalculate the costs involved for different materials and manufacturing processes as well as identifyareas where the number of parts can be software allows engineers who are not familiar with the DFMA process to reap the benefitsof implementing DFMA. Engineers can change the materials or processes used and quicklydetermine if the costs are reduced or not. The system incorporates databases of parts andsubassemblies with Design for Assembly (DFA) information associated with the manufacturingsite. Subassemblies are graphically represented along with information on unnecessary parts,estimated assembly times, and approximate cost data. The system also provides an accurate costestimate for a variety of materials and associated manufacturing processes including: injectionmolding, machining, sheet metal work, die casting, and powder metal ial users of the Boothroyd Dewhurst software should note that they must have a detailedknowledge of their manufacturing process plan before they begin using the software. Thesoftware is not knowledge-based, and does not aid the user in developing a process plan. Thisdrawback can be frustrating to new users of the tool; however, it does not diminish theimportance of the software as it relates to DFMA. The primary benefit of the system is that itallows design engineers to quickly and easily consider the impact of using alternative materialsand manufacturing processes. Boothroyd and Dewhurst, Inc. have documented significantreductions in parts count and cost (51% and 37%), time to market (50% faster), assembly time(62%), and manufacturing cycle time (57%) as well as improved quality and reliability (68%) bysystem users.(Illustrations in Section 2.1 were adapted from Machine Design Magazine)