Test Fixture Signal Interfaces

What is a Test Fixture Signal Interface?

A bed of nails test fixture is a mechanical device used to test printed circuit boards (PCBs) or printed circuit board assemblies (PCBAs). The term "bed of nails" refers to the array of small, spring-loaded pogo pins (or "nails") that make contact with various test points on a PCB. Test fixtures are used in conjunction with Automated Test Equipment (ATE) to apply signals to the PCB and measure its response, thereby verifying the board's and its components' functionality.

The design of a test fixture is customized to match the layout of the specific PCB being tested, ensuring that all necessary test points are accessible. Pogo pins contact test points when the test fixture is closed. FixturFab uses the term ”Signal Interface” to describe how a mechanical test fixture accesses the electrical test point signals on the Device Under Test (DUT).

Pogo Pins (aka Test Probes)

Test probes (commonly known as pogo pins due to their spring-loaded mechanism) are precision components used within test fixtures to make temporary electrical connections with a PCB's test points. Each pogo pin consists of a pin housed within a barrel, with a spring applying pressure to make a reliable connection. The tip of the pin is typically pointed or rounded to ensure it makes reliable contact with the test point.

Pogo pins are favored for their durability and consistent contact pressure, making them ideal for repeated use in testing environments. They come in various sizes and tip styles to accommodate different test point sizes and densities.

Receptacles

Receptacles are components that are used to house pogo pins. They are typically fit-pressed into the test fixture’s probe plate. Receptacles allow for the easy replacement of pogo pins if they become damaged or worn out, thereby extending the life of the test fixture. They also provide a stable mounting point for the pogo pins, ensuring consistent alignment and contact with the circuit board test points.

FixturFab uses three types of receptacles.

Receptacles with Solder Cups

• Design: These receptacles have a small cup at one end designed to hold solder. This cup is where the wire is inserted and then soldered in place.
• Application: Solder cup receptacles are commonly used when a permanent, strong electrical connection is required. They are ideal for environments where the connection should not loosen over time, such as in certain types of electronic equipment or in applications where the device is subject to movement or vibration.
• Assembly: The assembly process involves inserting stripped wire ends into the solder cups and then applying solder to secure the wire in place. This process requires a soldering iron and solder, and it creates a durable bond between the wire and the receptacle.
• Advantages: Provides a robust connection that is resistant to disconnection due to pulling on the wires or vibrations.

Receptacles Designed for Wire Wrapping

• Design: These receptacles are designed with a long, rectangular post intended to have wire tightly wrapped around it. The wire is usually wrapped using a special wire wrapping tool.
• Application: Wire wrapping is a technique often used in prototyping and in systems where the ability to modify or repair connections easily is beneficial. It's suitable for creating numerous connections on a single post and is valued in applications requiring reliability and durability.
• Assembly: The assembly process involves wrapping a stripped wire around the post's rectangular section using a wire wrapping tool, which tightly coils the wire around the post to make a secure connection.
• Advantages: The connection can be modified, added to, or repaired without soldering. Wire wrapping produces reliable connections, as the tight coils around the post create a gas-tight connection resistant to oxidation and corrosion.

Wireless Receptacles

• Design: These receptacles feature a spring-loaded tip. Combined with a test probe, this is commonly called a "double-ended pogo pin" or "wireless probing." Each end of the pogo pin and receptacle combination is then spring-loaded, ensuring consistent pressure and a reliable electrical connection on both sides.
• Application: Wireless probing is useful when a temporary connection is needed between two points without soldering or permanent attachment. This design is beneficial for bridging connections between two circuit points in a flexible and reversible manner.
• Assembly: A Test Point Carrier Board (TPCB) is mounted to the bottom of the probe plate with standoffs so the spring-loaded receptacles contact test points on the TPCB. This does not require soldering or wrapping wires to each receptacle, making it a “wireless” design.
• Advantages: Without having to solder or wire individual receptacles, wireless receptacles with a TPCB drastically reduce labor and risks due to faulty solder/wire joints.

Like pogo pins, receptacles vary in length, spring pressure, and tip style to accommodate different testing requirements and to match the specific physical characteristics of the test points they are intended to contact. For more details, FixturFab wrote a dedicated article on selecting pogo pins and receptacles.

Bare Receptacles with Optional Wires and Labels

FixturFab sells test fixtures with bare receptacles and can add the labor to include wires and labels. Below are some design considerations for each option.

Bare Receptacles for Custom Integration

• Design: Bare receptacles provide the basic structure needed to accept pogo pins but without any pre-attached wiring. This design is open-ended, allowing for custom wiring solutions tailored to the specific needs of the customer's testing setup.
• Application: Ideal for customers who have the capability and prefer to build their own signal interface. This approach offers maximum flexibility, enabling the integration of the mechanical test fixture with various testing setups and equipment configurations.
• Assembly: The customer is responsible for adding wires directly to the receptacles, whether by soldering to solder cups or wrapping wires around posts designed for wire wrapping. This DIY approach allows for precise control over the wiring configuration.
• Advantages: This method is highly customizable, making it possible to create a signal interface that perfectly matches the unique requirements of the customer's testing environment and equipment.

Wiring and Labeling for Convenience

• Design: Adding wires and labels to the receptacles in a test fixture simplifies the connection process to the mechanical test fixture. Each wire is typically soldered to a receptacle and labeled according to its function or connection point.
• Application: Suitable for customers looking for a plug-and-play solution. Pre-wired and labeled receptacles save time during the setup and ensure a straightforward way to connect the test fixture to the testing apparatus.
• Assembly: This approach involves meticulously attaching wires to each receptacle and securing them in place, followed by applying labels to each wire for easy identification. The process is labor-intensive, especially for fixtures with a high quantity of test points.
• Advantages: The main benefit is the convenience and time savings for customers who need a quick and simple setup. Labels significantly reduce the risk of connection errors.

Challenges with Poor Wiring Practices

• Design Consideration: When wiring is not done carefully, it can result in a tangled mess, often referred to as a "rat's nest" of wires. This situation is not only difficult to manage but can also lead to testing errors and equipment damage.
• Application Impact: Poorly organized wiring is a common issue in bad test systems observed on manufacturing floors. It complicates troubleshooting and maintenance and can interfere with the accurate functioning of the test fixture.
• Assembly Advice: Proper planning, organization, and execution of the wiring process are essential to avoid these issues. Wire management tools and techniques, such as cable ties, wire ducts, or custom wire harnesses, can help maintain an orderly and functional setup.
• Advantages of Good Practice: A well-organized wiring scheme enhances the reliability and efficiency of the test system. It facilitates easier modifications, upgrades, and maintenance, ultimately contributing to smoother manufacturing operations and higher-quality outcomes.

Test Point Carrier Boards

While an initial investment is required to design a Test Point Carrier Board (TPCB), this approach becomes highly cost-effective for bulk production. Once the TPCB design is finalized, manufacturing additional PCBAs becomes quick and inexpensive, thanks to modern PCB fabrication and assembly services.

FixturFab recognizes the importance of making this process as efficient as possible, which is why we offer to integrate TPCBs into test fixtures at no additional labor cost, provided they adhere to our design guidelines. This policy not only encourages optimal design practices but also helps ensure that our customers can benefit from the advantages of TPCBs without incurring extra costs.

Feedthrough Plates

After deciding between bare receptacles, wires, or a TPCB, the last optional part of a signal interface is the feedthrough plate on the back of the test fixture.

Feedthrough plates accommodate a variety of feedthroughs to match the specific connectivity requirements of different testing environments. Whether it's USB for data transfer, ethernet for network connections, HDMI for audio/video output, or any other necessary interface, the feedthrough plate can be customized to include just the right collection of ports.

FixturFab provides a selection of standard feedthrough plates that cater to common testing requirements. However, we understand that one size does not fit all when it comes to testing setups. We offer the option to design a custom feedthrough plate for those with unique or specialized connectivity needs.

Conclusion

Designing a signal interface involves decisions between pogo pins, receptacles, wires, a TPCB, and the feedthrough plate. Each selection comes with its own set of advantages, price, and lead time.

During FixturFab’s Request For Quote process, we’ll help select the most effective option to access the circuit board’s test point signals so you can quickly begin testing with a bed of nails test fixture.