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Design for Manufacture and Assembly of PCBs and General Rules it Conforms to

Up to 80% of a final product's cost is determined by how it is designed (with the rest typically being owed to overhead and capital costs). Naturally, it follows that reducing a product's cost at design time is of critical importance to produce a successful and cost-competitive final product. Design for manufacture and assembly is a formal approach to examine a product's components and assembly cost and aims at cost reduction before real production starts. This article will begin with a general discussion of design for manufacture and design for assembly concepts and then this discussion will be carried on in detail in subsequent entries, which will discuss specifics with regards to PCB design for manufacture and design for assembly. Finally, a final entry will conclude the series with a discussion of the most commonly-seen PCB design issues.

Before continual depiction, it is necessary to discuss how the term "design for manufacture" is used when speaking in more general terms and when discussing PCB production more specifically. Design for manufacture and design for assembly can refer, in a general sense, to the simplification and optimization of a prototype or conceptual design in preparation for its manufacture. When those terms are used to discuss PCBs, they often mean a more direct examination of potential manufacturing issues. The first entry in this series will use the former definition as we discuss the concepts in a broad sense and the second and third will use the latter definition as we shift our focus to PCB manufacture and assembly.

Overview of Design for Manufacture and Assembly

Generally speaking, the goal of discussing design for manufacture and assembly is to determine how to design a product that can be manufactured and assembled in the most cost-effective way. Design for manufacture (DFM) is concerned with reducing the overall production cost and, more obviously, design for assembly (DFA) is concerned with the reduction of material inputs, capital overhead costs and reduction of labor. Both focus on the application of standards to reduce production costs and both also seek to shorten the product development cycle time. The combination of the two methodologies is also commonly referred to as design for manufacture and assembly (DFMA). The later section will discuss both types of analysis in combination, as they are so closely related and both terms are often used interchangeably.

General Rules PCB Design for Manufacture and Assembly Conforms to

DFMA analysis starts after a conceptual design has first been created. A conceptual design may involve the creation of a prototype or the development of a new version of a product. After a conceptual design has been created, this design's bill of materials (BOM) can be examined by way of a DFMA analysis. The rules DFMA sticks to are depicted as the follows:

Minimize the number of parts in a design

Reducing the number of components in a PCB design is a straight-forward goal with obvious benefits. It will reduce that design's cost and the complexity of assembly, though not as apparent, it is of great benefit. For example, when pick and place machines are used to populate PCB assemblies, they are limited to the number of components they can support in a single pass. Being mindful of the number of components the pick and place machine make use of in assembling circuit board can lead to non-obvious cost reductions. If for instance, a design requires a resistor of 20K and resistors of 10K have already been used in the design, it may actually be cheaper to use two resistors of 10K in series when that can reduce the number of times pick and place machine runs. Along the same lines, looking for standard integrated circuits that can consolidate a portion of your design into a single IC can speed up assembly time and shift portions of testing requirements onto the IC manufacturer. As such, being mindful of PCB component count and type is probably the most important step to reduce overall PCB production cost. In a word, if a part is not required for the final design, eliminating it will lower BOM cost, reduce purchasing cost, processing time, testing time and assembly labor input.

Develop a Modular Design

Consider breaking apart PCB designs into functional blocks if you can use those blocks across a number of different products. Increasing the quantity of a particular module that is ordered from a manufacturer can greatly reduce that module's per-unit cost. Also of note, using modules can reduce the cost and complexity of testing a completed assembly by simplifying the test process. Smaller systems are inherently easier to test and repair than larger ones. Obviously the cost benefit that you can obtain from a modular design application must be weighed against the increased interconnection costs associated with using several modules. Other benefits modular design features include ease of design updating, standardization of subsystems across multiple products and simpler trouble shooting of product subsystem design failures.

Strive to Use Standard Components

Using standard components is drastically capable of reducing design development time and cost. It goes without saying that specifying a complex custom solution will greatly increase the upfront cost of any product and may make a design infeasible. Using more common components can also simplify a product's supply chain and alleviate component supply concerns. Another benefit to prefer standard components lies in the fact their foot prints are more easily verified before being used in a PCB design.

Depend More on Multifunctional Components

Whenever an electrical component can serve multiple purposes in a design, it behooves the PCB designer to take advantage of. For example, using an enclosure that can also serve as a heat sink in a design can offer significant savings to a design's cost. Another example of a dual-use device is using a standoff as a connection to earth ground from the PCB to the PCB's enclosure through a connected mounting hole on the PCB.

Design Modules for Use in Multiple Products

Using standard parts across a range of products can reduce handling costs and allow for high volume purchasing costing. This concept can also be extended to product modules. If a module can be used across a number of products, higher production volume can reduce said module's cost and ultimately lead to lower finished product cost.

Design for Ease of Fabrication

Selecting PCB materials that require less processing during fabrication can greatly streamline product manufacture. Avoiding operations such as having to paint an enclosure by using an appropriate enclosure material can eliminate entire manufacturing steps and lower product cost. Also, making sure that design components are not produced with overly large tolerances can eliminate time consuming and costly part rework during assembly.

Reduce and Avoid, if possible, Using Fasteners

When a PCB is to be assembled, as with all products, it costs more to use fasteners to mount components than to use press fit type mounting technique. To take advantage of this, try to reduce the use of fasteners in your assembly. One way to do this is to use surface mount versions of power ICs and integrate heat sinking into the design of your board. For example, switching from a TO-220 version of an IC that uses an external heat sink to a D2PAK version using the PCB as an integrated heat sink, can save a substantial amount in your final design.

Minimize Assembly Directions

If possible, all parts should be installed along one axis starting from the same side of an assembly. This is often referred to as a “Top Down” assembly, where all components are installed from the top, down into the final assembly. Using this sort of single-sided assembly process saves the time associated with turning and rotating a product during assembly. Thus, as with all design decisions, PCB design engineers will have to weigh whether it is better to produce a smaller PCB with components placed of both sides of the board versus designing a larger PCB with components placed on only one side of the board (PCBCart has the capabilities to handle both single-sided PCB Assembly and double-sided assembly).

Maximize Component Placement Acceptance

Engineers should design PCBs in such a way that component mounting errors can be reduced. This can be achieved by using components that have higher dimensional tolerances (higher pin spacing) or avoiding issues such as tomb-stoning. Using parts that are designed with high levels of placement tolerance can greatly reduce the failure rate of an assembly. Additionally, using base structures that are rigid and predicable in dimension can also improve the rate of correctly placing a component. Furthermore, machine vision type feedback systems and other forms of feedback enable placement automation processes that can greatly improve production yields.

Minimize Repositioning and Handling during PCB Assembly

Any time that a PCB be repositioned during assembly process will increase the amount of time required to assemble components on that PCB. It is easy to understand that repositioning is incurred whenever a PCB features two sides and components are installed on the front and back face of the PCB. When possible, use all surface mount components on a single side of a board. Using only surface mount devices will limit soldering portion of assembly process to a single reflow step, while the inclusion of through-hole components may require an additional wave soldering step or manual soldering.

Advantages of PCB Design for Manufacture and Assembly

  • • Fewer parts need handling and documenting.
  • • Bill of materials cost can be reduced.
  • • Handling cost can be to some extent cut down.
  • • Labor and energy input can be decreased.
  • • Overall manufacturing time can be shortened so that manufacturing efficiency can be greatly improved.
  • • Lower complexity leads to higher reliability.
  • • Products can be more competitive.
  • • Higher margins will be obtained.

DFMA is a clear path to reducing the cost of your next design. The benefits of reducing the number of in a design are apparent. Products will be more viable if they are lower cost and less prone to failures but, by reducing the amount of materials that go into making products handling cost are also reduced, documentation requirements are minimized, and the required assembly labor is lowered. All of these factors lead to lower production cost and allow for either higher margins for products or price products at more competitive price point. Furthermore, production time is reduced, allowing for delivery of product to customer to be reached within less time. DFMA formalizes the implementation of these goals.

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