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What are PCB Test Points

As we all know, PCB testing and inspection will determine the quality of electronics. So, PCB Test points play a crucial role in ensuring the functionality and reliability of electronic devices during the testing and manufacturing phases. They provide access to critical points in the PCB, allowing engineers to perform various diagnostic and quality control procedures. In this blog, we will discuss PCB test points, their design considerations, and the best practices for implementing them.

What is the PCB Test Point?

A test point is defined as a terminal in a PCB that allows the user to transmit a test signal to the PCB to test its functionality or monitor parameters in the PCB to verify its integrity. Test signals are injected through the test points using probes during production to identify any possible problems. The test signal output determines whether a given signal is low or high when compared with the desired result, and if necessary, optimal changes are to be made.
 
The test points are usually placed along or beside the traces and through-hole leads. It allows the engineer to verify that each circuit part is working correctly. It uses manual probing using multimeters and oscilloscopes, automated tests like In-circuit testers, and flying probe testers. Besides, PCB tests can also be designed and analyzed using software solutions.

Importance of PCB Test Points

The purpose of setting up test points is to test whether the components on the circuit board meet the specifications and the solderability. For example, if you want to check whether there is any problem with the resistance of a circuit board, the easiest way is to take a multimeter to measure its two ends.
This involves careful inspection to find any possible problems that could compromise the operation of the final product. Test points in PCBs allow for detailed PCB testing during production. This helps to identify and address irregularities or faults. Test points also serve as access points for diagnostic tools.
This lets engineers pinpoint and address issues quickly during the debugging and troubleshooting phases. PCB testing is important in safety-critical applications like medical devices or automotive electronics. Extensive testing is necessary because a malfunctioning PCB in these applications might pose a risk to public safety. When manufacturing a PCB, automated test equipment is used to verify whether the process followed has resulted in a quality product.

Types of PCB Test Points

In a previous blog, we explored the types of PCB test methods, similarly, there are different methods of test points. The major types of PCB test points are:

Probe Test Point

A PCB test probe is a small component soldered onto the board, featuring a hook where a testing probe can attach. This setup allows devices like voltage testers to connect to the PCB. Test points are typically labeled with their corresponding net names, such as “GND” for ground or “PWR” for power. These labels are often visible on the board’s surface or silkscreen, making it easy to identify the correct points for testing.

Automated PCB Test Points

It is the second type of test point used for automated test equipment, including through-holes, via, and small metal pads to accommodate the automated test system’s probes. Automated test points can be further segmented into three types:
  • Bare Board Test
The PCB testing method is carried out on bare boards with no components. It is done to check for manufacturing defects, such as shorts and opens, in circuit boards.
 
  • Flying Probe Test
It uses moving probes to contact the rest of the points on the PCB. It is ideal for low—to medium-volume production.
 
  • In-circuit Test
It is the automated PCB testing system that checks the functionality of each PCB component. It ensures that each one is placed correctly and operates properly.

How are PCB test points Designed?

To develop the PCB test points, you need to consider the following points:
  • Identification of Test Points. It will try to determine the essential areas to test, like power and ground planes, components interfaces, and signal paths.
  • Then, position the test points in inaccessible locations for probes and automated testing equipment. Make sure no other components obstruct them.
  • Choose which test types you need. Select between surface mount, loop, pads test points, and through-hole based on the PCB design and your testing requirements.
  • Label the test points with the corresponding names, like GN for ground and VCC for power.
  • Then, you must ensure that the test points do not interface with the PCB’s electrical performance. You should also maintain signal integrity and minimize the additional inductance or capacitance introduced by the test points.
  • Design test points to withstand the mechanical stress of repeated probing, especially in automated testing environments.

Challenges in Designing PCB Test Points

Designing a PCB test point is one of the most crucial steps. The major purpose of designing the test point is to ensure functionality, potential faults, and performance. However, designing a test point on a circuit board is a challenging task for designers. Let’s check some of the major challenges while designing the PCB test points:

Space Constraints

PCBs often have limited space in compact or high-density designs. The next major challenge while designing PCB test points is further allocating the space for the test points without affecting the board’s layout and functionality.

Accessibility

The test points should be accessible for the testing equipment. In the case of densely populated boards, ensure the test points are reachable without interference from the other components.

Signal Integrity

If your circuit board has a test point, it could introduce parasitic inductance and capacitance that affect signal integrity in high-speed circuits. This can lead to issues like crosstalk and signal distortions.

Thermal Management

Test points can create a thermal issue, mainly in the case of the high-speed circuit. The placement of the test points shouldn’t interfere with the heat dissipation mechanisms. This is crucial for maintaining reliable operations.

Electrical Noise

Test points can sometimes act as antennas, picking up electrical noise. This is problematic in sensitive analog or RF circuits, where noise minimization is crucial.

Things to be considered when designing test points

There are various things we need to consider while designing PCB test points. Let’s check some of those important considerations:

Test point sizes and shape

Larger test points make it easier for the probe tips to make contact. An ideal size is 0.050 inches; however, designers often use it down to 0.035 inches. Test points are commonly designed with square pads, which can visually differentiate them, providing more surface area for the problem.
If you check out a lot of PCB CAD design software, you will see that they have this feature to convert regular holes and via into test points, adjusting the size and shapes as required.

Spacing between Test Points

Maintaining a greater distance between test points facilitates building, modifying, and maintaining test fixtures. The optimal distance is 0.100 inches from the center to the center. This allows the use of larger, more robust probes and improves the long-term reliability of the fixture. In high-density designs, the test points can be spaces as close as 0.050 inches center to center, with fixtures using smaller probes.

Spacing between test points and components

Maintaining a greater distance between test points facilitates the building, modifying, and maintaining test fixtures. When designing the test points, it is suggested that there be a clearance of 0.10 inches from the test point’s center to the component’s edge. In high-density designs, test points can be spaced as close as 0.050 inches center to center, with fixtures using smaller probes.

Spacing between Test Points and Board Edges

Test machines frequently use vacuum drawdown systems to secure the board. These systems need room around the edge to ensure a tight seal. To guarantee a good vacuum seal, test points should be placed at least 0.125 inches away from the board edge, ideally 0.100 inches.

Test Point Placement

In circuit boards, the high-density probing area could cause the board to flex and damage the solder joints. Thus, it’s recommended that the test points be distributed evenly across the board.
 
Ensure the test points are free from obstructions like mounting hardware, overhanging components, and solder masks.
Also, the high-speed design rules should be considered to prevent the test points from becoming antennas that emit interference.

Probe Point Parameters

Different from automated test points, probe points must have holes or surface mount pads large enough for the posts or loops to be soldered to them. These points should be placed in locations easily accessible to technicians.

Minimize Electrical Noise

Test points should be designed to minimize noise. It includes placement of proper grounding and avoiding placement near high-frequency components or noisy power lines.

PCB Testing at FC PCBA

PCB test points are important for verifying the functionality and quality of PCBs. Proper design and placement of test points facilitate efficient testing and ensure safety in crucial applications.

You can check FC PCBA, where we ensure high-quality PCB with comprehensive PCB testing services. Our state-of-the-art testing facilities and expert engineers are equipped to handle all the testing needed, from probe tests to automated system testing.
 
Contact us today to learn more about your PCB testing needs and see how we can help you achieve your products.

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