Noah Sinclair
The bed of nails test fixture is a commonly used testing fixture for in-circuit testing of PCBs (Printed Circuit Boards). It is a crucial tool that offers unique testing and design features, making it ideal for processes requiring high accuracy and reliability. This technique is also cost-effective and helps ensure reliable products. The fixture gets its name from its structure, which resembles a bed of nails with small, spring-loaded pogo pins. The design of the nails can be customized according to the board's requirements, and it simplifies the inspection process by eliminating the need for additional wiring. This DIY project can be achieved by following specific steps, such as using a stable base material, defining test points, drilling holes, wiring, and creating custom software.
| Characteristics | Values |
|---|---|
| Purpose | To test components online to find manufacturing faults of the board and faulty components on boards through the use of electrical features of fixtures. |
| Other names | ICT test fixture |
| Basic purpose | To find open circuits, short circuits or inspect the welding on the board |
| Testing points | Approx 50 |
| Board size | 100mm x 40mm |
| Base material | Stable material such as acrylic or epoxy-phenolic glass cloth laminated sheets |
| Nails | Pogo pins, conical at the tips and longer than the holes in the board |
| Alignment | Alignment holes for larger pins in the bed of nails that will mate with mounting holes on the PCB |
| Test sequence | Automated tests are started once the electrical probes of the hood make a connection with the PCB board |
| Test data | Recorded using high-speed data acquisition |
| Test report | Data for pass or failure of the test |
| Design | Can be customised according to board design requirements, e.g. low or high pins |
| Testing points | Easily accessible, eliminating the need for additional wiring |
| Nail design | Easily customisable according to testing points, ensuring optimal connections to the board |
| Spring pin array | Checks the current flow in the circuit and finds any short or open components on board, as well as solder bridges |
| PCB placement | Placed in the testing fixture, and a button is pressed to lower the mechanical hood onto the fixture using a pneumatic slide |
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What You'll Learn
Use a 3D printer to create the tester
Using a 3D printer to create a bed of nails test fixture is a cost-effective way to ensure quality control when manufacturing PCBs (Printed Circuit Boards).
Firstly, you will need to generate a 3D model of your PCB. This can be done using OpenSCAD, which allows you to export a 2-dimensional image of the board and then "extrude" it to the desired height. Other PCB design software may have similar capabilities, or you may be able to export your files into a compatible format.
Once you have your 3D model, you can use a 3D printer to create the test jig's 3D-printed parts. This includes the top board, middle board, and any necessary mounts. It is important to prepare these parts for use with the test jig, such as by installing heat-set inserts or drilling holes for components.
The next step is to choose the right type of pogo pins and their receptacles. Different types of pins, such as flat head or pointy tips, are used for making contact with soldered or unsoldered pins on the PCB. You will also need to ensure that the holes for the pins are the correct size to hold them securely during soldering.
Finally, assemble all the components together to create your custom bed of nails test fixture. This will allow you to quickly and efficiently test your PCBs for defects, ensuring that your manufacturing process is cost-effective and reliable.
By following these steps and adapting them to your specific requirements, you can utilize the power of 3D printing to create a customized and functional bed of nails test fixture.
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Design and software for testing boards
Designing and building a bed of nails test fixture for automated testing of circuit boards requires careful planning and execution. Here are some key considerations for the design and software aspects of creating a reliable and effective testing solution for printed circuit boards (PCBs):
Design Considerations:
- Board Requirements: The design of the nails and the fixture should be customised according to the unique requirements of the PCB. This includes considerations such as the number of pins, their arrangement, and the need for testing points. The nails should be designed to connect directly with the testing points on the board, ensuring optimal connections.
- Materials: Select appropriate materials for the base and structure of the fixture. For example, acrylic glass or epoxy-phenolic glass cloth laminated sheets can provide stability and durability.
- Alignment: Ensure precise alignment between the PCB and the test fixture. Drill alignment holes for larger pins in the bed of nails that will mate with the mounting holes on the PCB. These guide pins should be conical at the tips and longer than the pogo pins to ensure proper alignment before the board makes contact with the test pins.
- Stiffness and Stability: The base of the fixture should be strong, thick, and stiff enough to support the pins and the PCB. Consider using materials that provide mechanical stability, such as linen-impregnated phenolic material.
- Connections: Use spring-loaded pogo pins for making connections with the testing pins on the board. These pins can be soldered directly to a PCB or connected using point-to-point wiring. Ensure that the connections are secure and stable.
- Nest Design: The design of the nest where the circuit board rests should be customised according to the specific circuit board being tested. This includes considerations for the size, shape, and location of the probes in relation to the circuit board components.
Software Considerations:
- Programming: Bed of nails fixtures require programming to sequence and perform a large number of circuit board tests in a short amount of time. This involves defining the test sequence, parameters, and measurements to be taken during the tests.
- Data Acquisition: Utilise software platforms like LabVIEW to streamline data acquisition during circuit board testing. This software can also accelerate the programming of test sequencing and provide automated data analysis.
- Test Automation: Consider using automated test equipment and software to reduce testing time and cost. This includes integrating hardware and software solutions, such as NI CompactRIO hardware and LabVIEW software, to enable automated testing and data acquisition.
- Test Development: Develop reliable automation software for hardware testing using frameworks like Python's pytest. This allows for the transformation of manufacturing data into actionable insights and enables the automation of test analytics.
- DFT Analysis: Tools like FixturFab Studio offer digital concierge services for custom test solutions, providing guidance and expertise in DFT analysis, remote monitoring, and custom test equipment development.
Overall, the design and software considerations for creating a bed of nails test fixture involve customising the fixture to the specific PCB requirements, ensuring proper alignment and connections, and utilising appropriate software tools for programming, data acquisition, and test automation to streamline the testing process and improve accuracy and efficiency.
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Use a stable base material, like acrylic glass
When making a bed of nails test fixture, it is important to choose a stable base material. Acrylic glass, also known as plexiglass or Perspex, is a great option for several reasons.
Firstly, it is a strong and durable material. Acrylic glass has excellent impact strength and is up to 20 times more impact-resistant than conventional plate glass. This makes it a safe option, as it will not shatter into sharp pieces if it breaks. Acrylic glass also has greater tensile strength than glass, meaning it can withstand more stress before breaking.
Secondly, it is a lightweight material. Acrylic glass is about half the weight of glass, which makes it easier to work with and also reduces shipping costs.
Thirdly, acrylic glass has excellent optical clarity, allowing 92% of white light to pass through. This makes it ideal for applications where transparency is important, such as windows or eyeglasses.
Additionally, acrylic glass is resistant to damage from temperature changes. It will not warp or break due to fluctuations in temperature, making it suitable for applications where temperature changes are common, such as medical devices or food and beverage testing equipment.
Acrylic glass also offers chemical resistance, especially when treated with a scratch-resistant coating. This makes it a good option for applications that require exposure to chemicals, such as medical manifolds or microfluidics.
When choosing acrylic glass for your bed of nails test fixture, consider the thickness required for your specific application. Acrylic glass may require a greater thickness than glass to withstand the same wind loads or support the same weight.
Overall, acrylic glass is a stable and versatile material that is well-suited for use as a base in a bed of nails test fixture.
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Use spring-loaded pogo pins for nails
The "bed of nails" test fixture is a standard testing process that is used to test components online to find manufacturing faults on the board and faulty components through the use of electrical features of fixtures. This process is also known as the ICT (In-Circuit Testing) test fixture.
The nails used in this fixture are usually spring-loaded pogo pins. The pins are designed to be easily customised according to the testing points, ensuring optimal connections to the board. Pogo pins are easily recoverable and reusable. The pins are connected to the testing points on the board, eliminating the need for additional wiring and simplifying the inspection process.
When creating a DIY bed of nails testing fixture, it is important to define the test points on the layout and get a 1:1 printout. The printout should then be stuck onto a plate of stable material, such as acrylic glass, and the holes should be drilled accordingly. The base should be strong, thick, and stiff enough for the pins being used.
The nails used for the bed of nails should be conical at the tips and longer than the pogo pins so that the board lines up before making contact with the test pins. The taller guide pins are crucial for alignment. The seal between the top and bottom plates (with spring return) can be created using duct tape around the outside edges.
Overall, the use of spring-loaded pogo pins for the "bed of nails" test fixture allows for customisation, optimal connections, and ease of recovery and reuse.
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Test jig wiring and placement
When creating a test jig for a bed of nails fixture, there are several important wiring and placement considerations to keep in mind. Here are some detailed instructions and guidelines for the wiring and placement of your test jig:
Firstly, it is crucial to select the appropriate materials and prepare your setup. Acquire a stable base material, such as acrylic glass or epoxy-phenolic glass cloth laminated sheets, to serve as the foundation for your test jig. This base should be thick, stiff, and strong enough to support the pins and components. Define the test points on your layout and create a 1:1 printout. Secure this printout to your base material and accurately drill the holes for the test pins. Ensure that the holes are aligned correctly with the test points on your PCB (Printed Circuit Board).
For the wiring process, you can opt for point-to-point wiring or use a PCB to connect the pins. If you choose point-to-point wiring, carefully route the wires to a central location, ensuring they are secure and don't interfere with other components. If using a PCB, you can solder the pins directly to the PCB, creating a neat and organised setup. Make sure the PCB is securely attached to your base.
When placing the pins, it is essential to use the correct types of pins for your specific application. The most commonly used pins are pogo pins, which are spring-loaded and provide good electrical contact. Drill alignment holes for larger pins that will mate with the mounting holes on your PCB. These alignment pins should be conical at the tips and longer than the pogo pins to ensure proper alignment before the PCB makes contact with the test pins. You can also utilise HEX spacers to connect different levels of PCBs, providing a stable and secure setup.
Additionally, consider the placement of other components and connectors. The motors and endstop wiring can be positioned below the plate for a cleaner look and easier shipping of the test jig. The proto board, which contains extra components and the voltage divider, should be placed strategically, ensuring easy access and functionality. For ATX-style power supplies, connect the PS-ON wire (green wire) to the PowerPin on the controller, allowing for power management during testing.
Lastly, pay attention to the placement of test pads and tooling pins. Test pad diameters should typically be between 0.015" to 0.040", with a nominal value of 0.035". Stagger the test pads on fine-pitch devices to allow for greater accessibility and a 50-mil spacing. Place tooling holes with a tolerance of ±0.002" between holes to ensure accurate fixture placement. Minimise tooling pin flex by using holes with a diameter of at least 0.125". By following these wiring and placement guidelines, you can effectively create a functional and organised test jig for your bed of nails fixture.
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Frequently asked questions
A bed of nails test fixture is a commonly used testing fixture that performs in-circuit testing. It is called so because it resembles a bed of nails with small, spring-loaded pogo pins.
The bed of nails test fixture offers unique testing and design features that help make reliable and high-quality boards. It is used for processes where high accuracy and reliability are required. This technique also provides low-cost testing features and reliable products.
The materials needed include a stable base plate, incircuit test pins, a printout of the testpoints on your layout, a PCB board, wires, and software to drive the test.
