Design for Testing (DFT) Guidelines for PCB Manufacturing

Testing and examining a PCB after manufacturing is a pivotal factor in procuring a flawless design. Design for testing (DFT) evaluates the board’s accuracy based on functionality and manufacturability.

DFT is similar to reviewing your answer copy in an examination just before the final submission. It does require some extra effort and time but eventually, DFT helps in creating an error-free board.

What is design for testing?

DFT is a method of operational and functional testing of a board and layout optimization. This methodology identifies any short, open circuit, wrong placement of the components, or faulty components. This testing method is performed to validate three major questions:

1. Is the board designed precisely?
2. Is the board fabricated flawlessly?
3. Do all the components, ICs, and connections operate perfectly?

The other major issues considered are:

• Components should be spaced properly to mitigate the risk of defective testing.
• If the solder masks between pads are not provided correctly, the electrical connections can degrade.
• Optimization of drill bit size.
• Improper surface-mount pad size.
• Detection of acid trap.

Why DFT is required?

In the earlier days, the number of components was hardly around 100 to 200. Hence, the board had enough room to place test points. Now, the entire fabrication technology has gone through a revolutionary change. Some PCB designs include thousands of components and solder connections, especially in HDI boards. The space constraint becomes a concerning issue in deciding proper test positions on a dense board. If any component or connector hampers the design, it would be a nightmare for fabricators and designers.

DFT engineers and product developers establish a set of testing methods to find out any inaccuracies and produce a high-quality, operational circuit board.

Parameters to include in DFT

Here are some of the parameters that you can include in testing to ensure efficiency and precision.

• Test points: Test point insertion is a requisite technique in DFT to increase test efficiency. The position of a test point is decided based on how many components it can cover. The signal integrity issue can be mitigated by arranging accurate power and ground test points.
• Test traces: You can position test points on the traces that imitate the sensitive traces. These test points can be connected to oscilloscopes, TDRs, or signal generators to know the behavior of the signals. A test point can be placed in the auxiliary clock output for trigger or synchronization during testing.

• LED: You can incorporate LEDs in testing methods to determine whether the power is switched on or off.  Debug LEDs are a suitable choice for FPGA or microcontrollers since they require debugging errors in the code.
• Selection of the test method: Choose the right method as per your design. The flying probe test is preferable for small production because of its simple setup and slow testing speed. The in-circuit test (ICT) involves fixed programming and is suitable for large production.
• Headers: These are a kind of test point connected to vias to measure the voltages across it.
• Additional circuit features: If the board has enough room, some circuitries are introduced to check the voltage and current of the components. These are non-essential yet helpful in validating the components’ rating.

What are the design for testing techniques?

Here are the two important testing techniques:

Bare board testing

Bare board testing is performed to check the PCB’s connectivity before assembling the components. Following are the two ways to perform this kind of testing:

• The isolation test verifies the resistance between two electrical connections.
• The continuity test checks if there is any presence of an open circuit within the board.

Assembled board testing

Assembled board testing is executed after assembling the components. This process ensures the circuit board’s integrity and the correct functionality of the components.

7 major testing methods in PCB

Flying-probe test

The bare board and assembled board both can incorporate the flying probe test in passive and active mode respectively. The probes comprise needles for checking. The test points can include passive components like resistors, capacitors, inductors, untented vias, or the terminating end of the components.

It can detect the value of non-powered elements, and open or short circuits, measure voltages, and check the placement of diodes, and transistors. Flying probe test is the most preferred choice over ICT in recent years.

At Sierra Circuits, we use Seica Spa Pilot V8 for flying probe testing. This latest machine provides maximum performance, increased test speed, and low to medium volume run. Simultaneous vertical probing on both sides of the circuit board quickens the debugging process and enhances the flexibility for prototyping.

Benefits

• Less expensive.
• Greater test coverage.
• Do not require a fixture.
• Quick implementation.

Drawbacks

• Time-consuming since probes move between measurement points.
• Tough to set up if the board does not include any test point, test via, or masked via.
• The lump-sum capacitance can only be tested for the capacitors in parallel.

Design for Testing Handbook

7 Chapters - 28 Pages - 45 Minute Read

What's Inside:

• PCB testing strategies
• Guidelines to design and place a test point for FPT
• Directives to make your board ICT compatible
• Benefits and drawbacks of various testing methods
• Defects that you can identify through board testing

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In-circuit testing

The in-circuit test is also known as the bed of nails testing method. This process incorporates some pre-mounted, electrical probes aligned under the board through the preset access points. The accurate, and stable electrical connection between the probes and the PCB can be established in this way. The testing probes can cause to flow current on the pre-determines design test points.

ICT can check for shorts or open circuits, solder mask shortcomings, misplacement or absence of components, etc. This method comprises testing fixtures to hold the board with the probes correctly, and test jigs to check multiple components on a board simultaneously. This test method saves time.

Benefits

• Used for large production volume.
• Provides coverage up to 90%.
• Accurate. Free from human error.

Drawbacks

• Not suitable for small volume production.
• Voids or inadequate solder masks can not be detected.
• Expensive. Technologies like test jig add to more costs.

Functional testing

It is implemented for quality control and ensures the intended operation of a device. The test parameters are provided by the customers/designers depending on the design. The technique often incorporates simple switch on/switch off tests, and sometimes it requires complex software and precise protocols. Functional testing directly checks the board’s function in real environmental conditions.

Benefits

• Low cost.
• Versatile and can be customized as per the design.
• Doesn’t impact the lifespan of the board, unlike other tests that exert excessive stress on it.

Drawbacks

• Requires experienced technicians.

Automated optical inspection

AOI incorporates 2D or 3D cameras that click high-resolution images and verify the schematics. It also compares with the perfect and imperfect designs available in the database. This method can find out all the visible errors with great accuracy. AOI is used with another type of testing method to ensure the correct results, for instance, AOI with the flying probe, and AOI with the in-circuit test. It can be included directly on the production line to prevent any premature board failure.

Benefits

• Fatal defects can be detected with accuracy.
• Consistent approach.

Drawbacks

• Only surface defects can be detected.
• Time-consuming process.
• Setups are subjected to change based on design.
• Detection based on the database is not always 100% accurate.

Burn-in testing

The burn-in testing is an early check-up of the board to prevent dangerous failures after fabrication is done. This method involves exceeding the specified operating limits to trigger the failures. This is an efficient way to detect the maximum operational rating of the board.

The various operating conditions involve voltage, current, temperature, operating frequency, power, and the other factors pertinent to the design.

Benefits

• Increases product reliability.
• Verifies the functionality of the board concerning the ambient conditions.

Drawbacks

• The exerted stress beyond the rating can reduce the board’s lifespan.
• The process involves more time as well as more effort.

X-ray inspection

X-ray inspection detects errors in the hidden components, solder connections, BGA packages, internal traces, and barrel with the help of X-ray imaging.

Benefits

• It is not required to check each layer of the PCB. An X-ray machine can easily check the internal layers from the top of the circuit.

Drawbacks

• This requires experienced and skilled technicians.
• The process demands more labor and cost.

Visual inspection

Here a technician inspects with bare eyes or by using a magnifier. This method can determine the unveiled components’ alignment, absence of components, and other defects.

Benefits

• Easy and basic method.

Drawbacks

• Subjected to human errors.
• Minor and invisible defects can not be detected.

PCB testing strategy

Width of border edge

This is essential to keep sufficient, unoccupied space along the opposite edges of the board. These clear edges are helpful to hold the testing machine perfectly. Generally, the standard width of the unavailable edges is maintained at 3 mm. Often, panel wastes are also incorporated for the same purpose.

Fiducials

The machine needs some reference points to know the exact position of the probes. The reference points, known as fiducials, are located on the panel waste, and on the PCB itself, if the waste has been removed. The most suitable and recommended fiducials are top-left and bottom right of the board.

Vias

The vias incorporated in the board design need to be non-masked to position a probe on the edge of them.

Component legs

Often, it is better to place the test probe near the component legs to achieve a good solder joint, though the component legs need not be tested. This technique connects any potential open circuit at the point of the test by pushing the component leg onto the pad.

Size

Place the test access points as close together as possible if you design a large board.

Cleaning

Cleaning the assembly is crucial to ensure the removal of unwanted flux. It is because sometimes the tester has to move the probe position to get better contact, undesired flux can cause failure and increase the test time.

Probe points

You can introduce accessible probe points in the ground and power rails on the bottom, perhaps on the unprobed side of the PCB. This allows fixed probes to act as temporary fixtures that speed up shorts testing and can reduce the overall test time and cost.

Test access

It would be great if you can maximize test access on one side of the assembly if possible, at least have one probable point for each network. If you are using a double-sided machine, the cost increases to turn the board over to test both sides.

Component height

There is a maximum component height allowed for both the probed and the unprobed region of the PCB, typically around 40 mm and 90 mm, respectively. The tall components are placed on the bottom side if it is unprobed or fitted after the test. They can create a no-fly zone on the assembly and also hinder test access.

Design for testing (DFT) is fundamentally a reviewing process to find and fix any manufacturing error, check the customer-provided specifications, and minimize excess process. Without this periodic testing, PCB manufacturers can face serious production errors. Assured DFT/DFA/DFM checks are one of the many benefits of choosing turnkey PCB manufacturing and assembly from a single fab shop.
DFT ensures fabricating a board with a high yield with the minimum cost. Please do comment below if you have any questions regarding this. Check out our design guide to understand how design for manufacturability works.

Design for Manufacturing Handbook

10 Chapters - 40 Pages - 45 Minute Read

What's Inside:

• Annular rings: avoid drill breakouts
• Vias: optimize your design
• Trace width and space: follow the best practices
• Solder mask and silkscreen: get the must-knows

Download Now