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Test points on a printed circuit board (PCB) are designated locations that are used for testing the functionality and diagnosing issues of the PCB during manufacturing and troubleshooting. They provide a way for test probes to make a reliable contact to the circuitry on the PCB.

Test points can come in several forms, but the most common types are:

Surface Test Points

These are typically exposed copper pads placed on the surface of the PCB. They can be probed with a spring-loaded test probe, known as a "pogo pin", in a test fixture.

Via Test Points: These are vias that have been designated as test points. Like surface test points, they can be probed with a pogo pin. Vias can be a convenient way to add a test point without using additional board space, but they may not provide as reliable a contact as a dedicated surface test point.

Edge Connectors or Gold Fingers

These are contacts placed along the edge of the PCB. They're typically used for functional testing, where the PCB is plugged into a test fixture.

Test points are typically placed at strategic locations throughout the PCB to give access to critical signals, power rails, ground, digital buses (like I2C or SPI), micro-controller pins, and other important nodes. The selection and placement of test points is an important part of the PCB design process. It needs to balance the need for test coverage with considerations like board space, cost, and potential interference with the operation of the circuit.

Once a PCB design is complete and manufactured, automated test systems (like a "Bed of Nails" tester or a Flying Probe tester) can use these test points to quickly and reliably test each board for manufacturing defects or design issues.

Bed of Nails

Bed of nails testing, also known as in-circuit testing (ICT), is a method of testing printed circuit boards (PCBs) to ensure that they were manufactured correctly.

The term "bed of nails" comes from the test fixture used, which consists of a series of spring-loaded pogo pins arranged in a pattern that matches the location of the test points on the PCB. When the PCB is pressed onto this bed of nails, each pogo pin makes contact with its corresponding test point, allowing the testing equipment to measure the electrical characteristics of the circuit.

The bed of nails tester can check for a variety of potential issues, including:

Short Circuits

The tester applies a small amount of current to each circuit and measures the voltage drop to check for short circuits.

Open Circuits

The tester checks for continuity between each pair of points that should be connected.

Component Values

The tester can measure the values of certain components like resistors and capacitors to make sure they are within their specified tolerances.

Digital Logic

For digital circuits, the tester can apply different logic levels (high or low) and check the response of the circuit to make sure it is functioning correctly.

The advantage of bed of nails testing is that it provides a quick and reliable way to test a PCB for manufacturing defects. However, creating the bed of nails fixture can be time-consuming and costly, especially for complex boards. This method is typically used for high volume production where the initial cost of the fixture can be spread over many units. It's also more effective for through-hole and larger surface mount components, as modern circuits with tiny, high-density components can be difficult to test using this method.