Reaping the Benefits of Design for Assembly

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Reaping the Benefits of Design for Assembly

Posted by: Mike Desrochers
Category: Checklists, Guides, Technical Posts

Creating a new product to market can be an exhilarating experience. With the significance and reliance on new technology we are seeing unprecedented growth for new products. For many start-ups and entrepreneurs the culmination of taking a concept to reality presents unique learned experiences. This is also why achieving a successful launch is the most rewarding. With so many hurdles and considerations during the business planning process it’s easy to overlook critical elements that could have streamlined the process as it relates to manufacturing. The ability to retrospectively analyze the design process post build is important but clearly more beneficial if you captured them ahead of time.

As we indicated in an earlier blog post, Design for Manufacturing: Contemplate, Anticipate and Optimize your PCB, design for manufacturability (DFM) is the scientific practice of designing products, so that they are easy to manufacture in medium to high-volume runs, as well as in low volumes where non-recurring engineering (NRE) can be reduced.

Design for Assembly (DFA) is a slightly different concept. It is the designing of components to optimize proper assembly and correct functioning, to reduce wastage of parts, and allow a higher degree of reliability and operational efficiency. DFA extends across the full product lifecycle, from the conceptualization stage through production for both PCBs and box builds/electro-mechanical assemblies. In the excitement of getting your product to market this can be an oversight at times.

Benefits to OEMs

Design for Manufacture and Assembly (DFMA) is a blend of the DFM and DFA approaches, which facilitates a product design to be manufactured and assembled in the most efficient manner in terms of cost, labour, materials and other resources. DFMA allows a company to identify, quantify, eliminate (or at least mitigate) wastage and inefficiencies within a design, by pooling the abilities and perspectives of both design and manufacturing engineers in developing the design, manufacturing and assembly methods concurrently.

By eliminating various revisions or changes in engineering, it minimizes wastage in assembly time and processes, making it more cost-efficient, and thereby offering better product reliability.

Aesthetics & Assembly Design Elements

Creative Thinking

For many OEMs, aesthetics of the final product can be a significant consideration. This necessitates creative thinking to offer visual appeal while ensuring correct functionality. E.g. marking the correct positions of assembly on mating parts lessens the probability of incorrect assembly and misalignment. Using orientation features in injection-molded components can reduce glitches in assembly. Chamfers may be used to enhance visual appearance. The design must not have hollows or overhangs that are impossible to machine.

Environmental Factors

The design must also consider the environment in which the manufactured product will be used. Environmental conditions such as temperature, moisture, vibration, and other extraneous factors will have an impact on the correct functioning and reliability of the product. Good design must account and build for these parameters.

Component Materials

Often materials with different coefficients of thermal expansion are to be joined. When we consider that these materials can range from metals, plastic and glass to wood and fabric, the expansion issue becomes much more significant. When exposed to the same heat level and temperature rise, different materials will exhibit different expansion behaviour. Therefore, the design must allow for the potential of uneven and unequal expansion.

Component Quantities

When the product design involves fewer parts and components, the cost of assembly and the risk of incorrect assembly are both lowered. So good design must optimize the number of parts used. The cost savings from such an optimization process will far exceed the extra costs accrued by any adjustments being made to moulds and any additional materials. Combining functions is one way to achieve component quantity optimization while still meeting all product requirements.

Disassembly Considerations

Often, disassembly may be required for a variety of reasons – for repair or to recycle at the end of the product life. There is a labour cost involved in disassembly. DFA also plans ahead for these disassembly considerations, in addition to assembly considerations. Some examples would be replacing mechanical fasteners with living hinges or snap-fit joints; opting for screws with suitable shaft-to-head diameter ratios and head styles designed for automated feed, or screws with washers under the head to preempt loose washers, or self-tapping screws to avoid a tapping stage.

Fastening Methods

The choice of fastener and corresponding assembly method can affect labour time and costs. The skilled designer would explore alternative ways of fastening. E.g. replacing threaded bolts, washers and nuts with weld nuts or nuts that can be captured in part. Other tactics could include minimizing the array of hardware, using snap-fit or tab-and-slot, and testing bonding with adhesives.

Enclosures

The materials commonly used in enclosure production are:

  • Thermoplastics (injection-molded polycarbonate and/or ABS)
  • Aluminum (extruded, sheet metal or die cast)
  • Steel (sheet metal)
  • Composites (fiberglass)

They can be either standard (purchased off the shelf), or custom designed.

Standard enclosures have a lower engineering cost because they are already designed. But they have a higher unit cost because of the need for holes to machine/thread, painting, graphics, assembly, etc.

Custom enclosures, on the other hand, offer better product branding/differentiation, lower unit cost (in volume), and overall better design optimization.

Rapid prototyping or 3D printing technologies allow manufacturers to quickly and cost-effectively test a design before committing to expensive, time-intensive machining of molds.

Circumventing Tight Tolerances

Tighter tolerances typically results in higher costs, therefore, they must be avoided where possible. In the design stage, it is important to have process controls in place to monitor Key Control Characteristics (KCCs) or Key Product Characteristics (KPCs). Determine if process capabilities need to be enhanced early on in the design stage in order to build in the required time.

While designs of components and assemblies must account for tolerances on all dimensions, only a fraction of tolerances will impact the functioning of the product. So tight tolerances for non-critical dimensions are unwarranted; additionally, they represent an unnecessary cost. Another issue to avoid is tolerance “stack-up” which results from interactions between component parts. A solution is to dimension parts in the center of the tolerance range to allow for the greatest variance while still serving as a conforming functional part of the assembly.

Cable Assembly & Routing

Cable harnesses are used when two or more components have to be electrically connected. A typical cable harness consists of bundles, ports, and transitions. A good DFA process can review and analyze such a harness assembly.

Cable assembly and routing are critical for optimal signal routing in various applications used in products. Products that contain electrical wiring will most likely involve wires, cables and connectors installed on a harness assembly.

In both the assembly stage and maintenance or repair stage, managing, routing and connecting bundles of different wires can become very complex. Simplifying the internal wiring design will make for a more elegant and efficient solution. For example, connecting wires must be kept as short and straight as possible; but they must allow enough play for debugging and maintenance. Another example is replacing discrete wire with ribbon cables, which would allow conductors to be placed side by side in their respective insulations for the entire length of the cable.

Replacing twisted pairs with ribbon cables can yield significant cost savings. Ribbon cables can also reduce crosstalk, and increase signal density. A manufacturer may also consider flat cables instead of wires. When they are feasible, as they tend to be more reliable and more cost-effective than even ribbon cables.

In instances where the cable cannot be changed, the connectors could be changed to modular versions.

Handling Requirements & Part Orientation

The DFA process must decide and specify how parts will be oriented and handled during manufacturing and assembly, because it can cause many problems such as non-value-added motion, part movement, and safety issues. Certain guiding principles include:

  • Establishing symmetry of parts along both axes can make manufacturing and assembly easier.
  • Designing parts for grasp and orientation in an assembly.
  • Avoiding the use of parts that have a risk of entanglement, are sharp or pointed, and are difficult to handle, can cause more wastage and slow down production.
  • Avoiding heavy parts that may necessitate lifting devices and thereby end up increasing worker fatigue and the risk of injury.

Testing & Programming

DFA must also plan for testing the assembly and parts for quality and correct functionality. Designing the product so that the PCB assembly can be tested using a bed of nails test fixture can reduce the test time and improve the test coverage, while using Automated Test Equipment (ATE) to capture and document test results effectively.

The bed of nails test can also be used to program integrated circuits. When mixing low-power, fine pitch, high-density components with higher-power, lower-density components, it can be beneficial to split the design into multiple PCBs. This makes for easier testing and a lower overall defect rate.

Modular Product Design

An OEM gains many benefits when adopting modular design in the DFA process, including:

  • Lower costs
  • Ease of installing products quickly which in turn reduces labor and time inputs
  • Shorter learning curves for new workers on the assembly
  • The flexibility to deliver and install customized combinations of modules
  • The possibility of improving individual modular assemblies with minimal effect on the rest of the product
  • Greater outsourcing of parts and assembly modules increases on-time product delivery
  • Better maintenance of products and the reduction of the number of service parts that a dealer needs to stock

The August Partnership

Nearly 75% of the product cost is determined in the engineering phase. DFA allows the identification of hidden areas of wastage and inefficiencies to be eliminated. It is a tool used by many companies such as Texas Instruments, Ford, GM, Chrysler, and Lockheed Martin.

At August, we understand that as an OEM, you need to be creative and technically sound simultaneously when designing your products’ manufacturing and assembly. We can help you navigate the pros and cons of box builds/electro-mechanical assembly and testing across fasteners, enclosures, tolerances, cable assemblies, cable routing, and environmental hardening. Please contact us today.

Interested in DFM for PCBs? Read our related blog post Design for Manufacturing: Contemplate, Anticipate and Optimize your PCB.