The physical design of a PCB has the largest role to play in determining how economical the functionality test of a completed unit will ultimately be. Design for manufacturing practices incorporate the strategies DFT (Design for Testability) into a PCB design from it’s inception to make the test process as straightforward as possible.

DFT guides the PCB layout and design processes so that detecting electronics manufacturing defects is as easy and cost-effective as possible down the line. Conventional PCB assembly manufacturing testing procedures can make up a significant percentage of overall cost for a project; therefore DFT encourages PCB designers to build-in profitability from the start.

There exist three overarching stages for the implementation of DFT during a PCB design and layout process:

  • Establish what connections on the circuit board will be necessary for an in-circuit/MDA test system from an electrical standpoint.
  • Determine the mechanical requirements of board alignment, and how to provide areas for test probes to make contact with the PCB.
  • Generate the appropriate data for fixture fabricators and test programmers (in an efficient format) to equip testing machinery.

DFT affects how components are selected, arranged, and integrates circuit-level testability for a PCB. Its goal is to provide electronics manufacturers with a reliable, easy means by which to functionally test the workings of a fully assembled board – without requiring excessive amounts of time, cost, or labor.

For full coverage of all components and connections, a test system needs electrical access to each node (or ‘net’) on the circuit board. A node is one electrical connection on a PCB, it consists of either a pin or a lead – and is connected, (via conductors) to at least two discrete components on the board.

Conventional in-circuit testing equipment uses a series of carefully placed spring probes sitting inside a ‘bed of nails’ style test fixture which is uniquely paired to a single corresponding PCB unit, or a multiple-unit panel.

A well-developed level of communication between product development and test engineering teams remains an essential aspect for the effectiveness of a DFT project strategy. In regards to the specific guidelines of DFT, fundamental operations used by hardware engineers during the PCB design and layout process include:

  • Appropriate placement, spacing, and sequencing of probe targets which provide a reliable electrical connection to each board ‘network’.
  • Detailed examination of each component body size, spacing, height, and verification of visual/mechanical accessibility to all components.
  • Integration of tooling holes, also known as ‘alignment holes’ – which mate with test fixture guide pins, for accurate positioning within the system.
  • Adopting as compact a board size as is possible; fewer test points result in less costly testing equipment and reduction of large-board related problems (such as board-flex during vacuum fixturing).
  • Optimizing the shape of the PCB; board outline shapes that are rectangular in nature are more economical to fixture.
  • Restricting the number of necessary routed openings on the PCB – so minimal gasketing is required.
  • Elimination of open holes (such as unmasked or unsoldered vias).

DFT methodologies function as a comprehensive set of techniques to design a PCB layout for the reliable, economical, and rapid testing of finished circuit boards. It provides an important way for electronics manufacturers like Milwaukee Electronics to rely on consistent results from the various stages of PCB design, electronic assembly, and functional test operations which are relied upon to deliver quality product to every customer.

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