DFT, DFM, DFA Design Considerations in PCB Design

Introduction

The consumer electronics sector largely affected by the reliability factor of the devices. Simulation and experimentation are the two tools that allow a designer to ensure the operational quality of a product. A good design has to anticipate the requirements of a complete PCB and provide necessary provisions to carry out those functions. Good DFT DFM DFA PCB Design techniques as always being practised at TronicsZone are important for a robust PCB that can be taken into manufacturing.

For example, a designer must anticipate the functionality test points and incorporate them in a PCB design to facilitate the experimentation during testing stage. Similarly, a good design must satisfy all the parameters to ensure ease of PCB manufacturing and assembly. Thus, putting in the effort during the schematic and simulation stage helps the designer to reduce the product development time, and also increase the product reliability.

Design for Test (DFT)

Testing and Experimentation are indispensable steps in the product cycle of a PCB. DFT is the process of supplementing the PCB’s operational design with elements such as test points to facilitate the functional testing of the board. This added test points help the designer to check for functionality tests after the physical manufacturing process is over. This is to check and validate that the product hardware is devoid of any manufacturing defects that degrade the product’s correct functioning.

Design for Manufacturing (DFM)

Component availability and fabrication technology will vary with different companies and countries. Also, Electromagnetic compliance is an important criteria that a device has to satisfy to be launched in the market. Hence, the designer has to make sure that the design is compatible to the available fabrication method, the available components perform the device functions, the final design is in the required size, and so on. Thus DFM is the engineering practice of designing products in a planned manner so that they are easy to manufacture. Good DFM practices facilitate the manufacturing process and reduce the overall manufacturing costs and time.

Design for Assembly (DFA)

The electronics industry relies on the ease of assembling components. Devices are built by sourcing indigenous and global parts and assembling them as per our requirements. If the device has fewer parts it will take less time to assemble, and if the modules within the device are designed with DFA implemented then the assembling becomes a lot easier. Thus DFA is the process of designing a device or a PCB with considering ease of assembly as one of the key criteria. The cost-benefit is DFA is high.

DFT Provisions

The key parameters to be considered during DFT are Controllability and Observability. Controllability is the ability to pre-set/reset some circuit nodes/Inputs to a certain states or logic values. Observability is the ability to observe the state or logic values of internal nodes or outputs. This allows the designer to put the design into a known initial state and then control and observe the internal signal values. This helps in deciding on the functional reliability of the device. Testing can be done to verify functional defects or manufacturing defects. The important functional and manufacturing defects that occur in PCB design are given below.

Functional Faults

Functional defects are Error and Failure. Error is a wrong or undesired output that is produced by the defective performance of the system. Failure is a combination or errors repeated defect in the functionality of the device. Error can be rectified, but failure is a serious threat and it signifies the need for improvement in the design. The procedure to diagnose failure is termed as failure mode analysis (FMA).     Trace currents, Pin voltages, Power supply levels, Switching and timing signals, and board temperatures are also verified during functional testing.

Manufacturing Defects

Manufacturing defects are due to extra-metal deposition, improper doping, contamination in soldering, and dielectric problems.

There are two techniques to implement DFT. They are Ad-Hoc Technique and Structural Technique

Ad-Hoc Technique

As DFT implies, specific design planning has to be performed before it can be applied during the manufacturing stage of product development. Ad-hoc testing is a temporary technique. In this strategy, the design testability is enhanced without making many changes in the original design. That is, instead of embedding dedicated test points, temporary test points shall be used for functionally testing the device

Structural Technique

This is a permanent solution for testing. Dedicated test points are provided in the device. Structural technique is costlier than ad-hoc but the debugging, in case of any defects it is easier debug by following this technique. Manufacturing defects are targeted using structural technique.

ICT Testing

ICT is the abbreviation for In-Circuit testing. Commonly bed of nails test setup is used for ICT testing. Using this method the measurement of parameters such and resistance, capacitance and many more shall be done and the functional verification of   analogue components such as amplifiers and oscillators can be performed.  Typical problems such as short circuits, open circuits or wrong components can be captured in ICT test. A typical ICT kit has an in circuit tester, a fixture, and a measuring software.

Fly-probe testing is a type of ICT test and is simple and efficient. The probes can move around the board and make contact as required on any specific part of the PCB. Fly-probe does not require a fixture hence it is cost efficient. Another advantage of this method is that engineering changes to the PCB may not require any changes in test points, rather it requires software programming changes only.

DFM Provisions

DFM ensures faster manufacturing of PCB devices. DFM reduces product cycle time and production cost. The important checks to be followed for DFM good practices are given below.

Board Component Selection

Board Size, shape and component placement plays an important role during manufacturing. Standard components are more reliable and less expensive than custom-made components. They add to the overall value of the product. The use of standard components also helps in logistics as in case of an error or defect replacing the standard ones is lot more easily than searching for a custom made component. Further, they have significant tolerance and solderability features also.

Board layout, shape, and size are also important while designing. Specific shape and size requirements given by the customer should be taken into account. Specific location of the connectors, grouping circuits together based on power, frequency and routing requirements are considered during DFM.

Fabrication

The reduction of the number of parts in a PCB is the best practice for reducing manufacturing costs and ease out the fabrication process. Less number of components implies less layers. Based on the requirement criteria such as total board area, power routing, signal integrity, isolation requirements, and the number of high speed signals the number of layers required for the PCB shall be decided.

Designing the components for multi-use minimizes cost. For example, a well-designed ground plane can act as a structural component, an EMI blocker and a signal integrity device. Such design can reduce the time and money during fabrication. A designer should also check for minimal trace width, trace-to-trace distance, and proper via-hole clearance and so on for DFM.

Compliance design

Electromagnetic compliance (EMC) and power usage compliance are two important factors that decide the reliability of a product. Planning for these compliance parameters during the initial stages of design improves the quality of the manufactured device. Further, after manufacturing is over, during assembly errors can happen due to variation in PCB dimension or on the accuracy of the component positioning. Size and shape compliance should be considered.

Handling

Handling refers to manual or mechanical process of positioning, orienting, and fixing a part or a component. Asymmetrical design can cause damage to the PCB. This can lead to failures. Flexible parts can be avoided for better performance. Safe and compact packing is recommended for lasting robustness of a PCB device.

Conclusion

In the production cycle of a PCB, 70% of the manufacturing costs of a product is determined at the initial stages that is the design stage of the cycle. Hence practicing DFM ensures quicker time to market at lower cost. DFT ensures the functional reliability of the product. Thus, designing a PCB to be reliable and at the same cost-efficient can be achieved by following DFT and DFM good practices.