When designing a printed circuit board, it’s natural to focus on functionality, layout, and component placement. Test points might seem minor, as they are small pads that provide little direct benefit to the overall operation of the product beyond manufacturing and testing.

However, thoughtfully placed test points can contribute significantly to the overall quality and reliability of a product. They make it easier to verify that each board has been assembled correctly, isolate and diagnose faults, and provide critical access throughout the life of the product.

Here’s how test points contribute to the value of a product and how to design them effectively.

What Are Test Points?

Test points are exposed conductive pads or features on a PCB that provide access to specific electrical nodes within the circuit. During PCB manufacturing, test points are often accessed by test probes to measure voltage, signal integrity, resistance, and continuity.

A test point can be a discrete feature added specifically for testing or may use an existing package lead, board via, or other conductive element. Test probes access these points to verify that the PCB electronics function as designed.

How Are Test Points Utilized?

Test points on PCBs enable a few different types of testing across the lifecycle of a product:

• In-circuit testing (ICT) is a common form of testing where a custom fixture, typically with spring loaded probes, contacts test points on the PCB. These probes can test hundreds of points at one time, making this a fast and high-yield test for boards that require high-reliability.
• Flying probe testing is similar to ICT, but uses probes that move around the board, as opposed to a custom fixture. Because of this, flying probe testing tends to be less expensive than ICT, but offers lower yield and is better suited for prototypes or low-volume production boards.
• Functional testing verifies that the board performs as intended under real operating conditions. Test points allow technicians to monitor power rails, signals, and outputs during these tests.

Test points can also help during prototype bring-up, debugging, firmware programming, and field service or repair.

Why Are Test Points Important?

Better Defect Detection 

Manufacturing defects, such as open connections, shorted connections, or components placed incorrectly or with incorrect values, can be difficult to see on a populated board. By utilizing test points on a PCB, test equipment can more quickly detect these issues before a board moves onto the next stage of manufacturing or gets shipped to a customer.

Faster Troubleshooting

After a board fails testing, a technician often needs to identify the cause of the failure, and isolate the faulty module or component.

Test points provide convenient access to the signals and power nets of a PCB, making it easy to identify suspected faulty components or connections and speed up the debugging process. Without test points, debugging can become extremely tedious and time-consuming, sometimes requiring technicians to probe surface-mount component leads or use potentially destructive debugging methods.

Smoother Prototype Development

As a designer or engineer works through the bring-up of a prototype board, being able to access signals and power nets can accelerate debugging and help determine what, if any, design changes are required.

Lower Manufacturing Costs

Test points aren’t necessarily a value-add in every design, as they can take up valuable board space. However, faster testing can help accelerate the manufacturing process, keep production moving, and reduce labor costs when a board fails testing and requires additional debugging.

By working with manufacturers to make assemblies more testable, engineers and designers can help manufacturers develop better fixtures and procedures that support a more efficient manufacturing process.

Support Throughout the Product Lifestyle 

Test points can also support product maintenance throughout its lifetime. They can help isolate faults on a PCB, whether on the manufacturing line or in the field, and can also facilitate firmware updates and other maintenance or support tasks.

Best Practices for Designing Test Points

Test points on a PCB can help add a level of confidence that each assembled board is working correctly, as well as provide a means for rapid diagnostic testing and manufacturing support. There are a few best practices when designing test points.

• Plan Early. Test points should be considered throughout the design process, not at the last minute. Discuss test strategy with your manufacturer early so the board supports the testing methods that will be used.
• Prioritize Critical Nets. Wherever possible, test points should be available on critical nets, such as power rails, ground, clocks, reset lines, programming interfaces, and other critical signals. For in-circuit testing, a higher percentage of nets can provide better fault detection coverage.

• Keep Test Points on One Side. Whenever possible, test points should be made available on only one side of a PCB, typically at the bottom. This allows for reduced fixture cost, as double-sided test fixtures are more complex and expensive. 
• Provide Adequate Size and Spacing. Test pads on a PCB should be sized and spaced to allow reliable contact with test probes. Spacing between test points is just as important as size because adequate mechanical clearance is needed between probes. 
• Avoid Obstructions. Keep test points away from tall components, connectors, board edges, and other features that could block probe access. Test pads should also be free of solder mask and silkscreen.
• Label Test Points Clearly. Clear silkscreen labels make it easier for technicians to identify test points during debugging and service.
• Consider Signal Integrity. On high-speed signals, test points and the traces leading to them can affect signal quality. In these cases, engineers may use carefully placed inline pads or alternative testing methods, such as boundary scan, to minimize impact.

Balancing Test Access and Board Space

As components become denser and PCBs shrink in size, opportunities for test point access also decrease. Working closely with your manufacturer to identify the most important test access points and other design considerations can help support an effective testing strategy as a PCB moves from design to manufacturing.

The best PCB designs consider test points in relation to the product life cycle and work with the manufacturer to balance design and testing requirements. This process will vary by product and should be developed around the specific needs of each design.

Frequently Asked Questions About Test Points

Are test points necessary on every board?

Test points aren’t necessarily mandatory for every board, but should be considered for power and critical signals. An effective test strategy can vary per product.

What are the differences between ICT and flying probe testing?

ICT generally uses a custom fixture with spring loaded probes to test a PCB. Flying probe uses multiple, individually controlled test probes. ICT allows for high-yield, high-speed testing, but is expensive. Flying probe allows for lower cost testing with lower yields, and is best suited for low-volume or prototype boards.

Can test points affect signal integrity?

On high speed signals, test points can degrade signal integrity. This can be mitigated with proper test access methodologies.

Build Better Boards with Sonic Manufacturing 

Sonic Manufacturing works with engineers to design and manufacture reliable printed circuit board assemblies. Our team can work with you to determine the best test strategies for your PCB, as well as manufacture quality PCBs from prototype to production. Contact Sonic Manufacturing today to learn how we can help you build great boards.