The design phase is the most important part within a new product development cycle. Investing enough time on it, is fundamental for good quality products. Nevertheless spending enough time in testing a new system is also as much important as the design phase.
Only by testing a real system is possible to properly validate how the integration of the sub-modules are affecting each other. Last but not least, during the test phase it is possible to check how the system usability is. In this article it will be presented a 3D printed fixture to test new assembled PCB that come back on our desk. The fixture has been designed to be flexible enough and be used in various scenarios enabling a quick PCB Assembly (PCBA) test and replacement.
In Figure 1, is possible to see a typical ϯxture setup. The picture directly highlights that the test setup is made of a main plate to which multiple parts can be added as building blocks.
Figure 1: Simple fixture use case
The main plate offers flexible attachment points to extend the setup with columns and
arms, adding the extra ϲexibility to fit multiple PCB sizes and shapes. This also allows to either test the PCBA horizontally or vertically.
The table can be printed with 3mm thickness or 5mm, the one shown is made of 3mm
thickness, offering a good trade-off between printing time and rigidity.
The main plate is attached to the lab table via the feet that can be screwed on the side. The feet holder can be simply attached to the lab table via a mounting tape, which is sticky on both sides. All the join points are made with custom knobs so that the fixture setup can be done without the screwdrivers.
It all starts with power and signal lines, indeed every system requires power and may have signal lines or digital input/output going in and out. The fixture offer the option to screw on each side of the plate, a connector panel. This allows an easy connection with the lab power supply or connect and disconnect the signals. The project has today two simple connector panels. The first model is without holes, so it can be used to get a custom model without making a new design. The second connector panel has six banana female holes, to host enough power lines and signals. The assembled connector panel can be connected to the PCBA under test with the required connectors/adapters. The connector panel approach allows an easy PCBA plugin, without the need of making each time a new cable setup.
Testing a system, does not means only keeping it fixed while it is powered. Testing it
means having the ability to also take proper measures while the system is operating the functions for which it has been designed for. The connector panel helps here further, since it allows an easy connection for the multi-meter and power supply. Measuring the current consumption is typically one of the ϯrst test you run on a system, by limiting the maximum current of the power supply to a known current the system should not exceed. Often in complex systems, before even powering it, you can run some tests by measuring potential shorts or known impedance between known critical points. Having the PCB firmly anchored on the table is a neat help that fixture offers.
Once the test moves forward the number of measures can get more complex, so
holding multi-meter probes on known test points is useful to monitor voltages or currents on speciϯc points. The simple C arm holder, can easily work for multi-meter probes as well for scope probes. Figure 2 shows a more complex setup where the probes are used to check multiple signals on the PCBA.
Figure 2: Probe mounting example
Different building blocks have been used to make more complex arms. You can easily add also springs on the arm side to have more holding force pushing on the test point.
The way the blocks are attached, really depend on the use case. Additional ϲexibility can be added by printing custom parts that fit special needs.
Below are listed all the available building blocks that can be printed out using the .stl files.Other building blocks can be easily designed using a CAD such as FreeCAD. The files can be downloaded for free and can be freely shared. The parts can be used for private or commercial applications, maintaining the original Author credits under the terms of CC (Creative Commons license CC BY).
Main plate
File Name: Main Plate – 3mm
Printing Count: 1
Description:
This is the main plate that has holes and rails to comfortably hold the PCB and the 1xture parts. It can be printed either in 3mm or 5mm thickness (25% 1lled). 3mm has been tested with success, achieving a good compromise between rigidity and printing time.
Foot
File Name: Foot – A
Printing Count: 5
Description:
The foot is placed on all 4 corners of the main plate and in the middle of it. It has a 6mm diameter hole on top that allow M4 brass knurled threads to be pressed in. On the side there is a second hole for the same purpose, which allows to 1x the foot to the foot holder. On the bottom side there is a third hole where the M4 threads can be directly made with the apposite tool. This would allow using M4 plastic screws by EMC test (described below), avoiding any metal.
Foot holder
File Name: Foot holder – A
Printing Count: 4
Description:
The foot holder allows to screw the feet on the table. While the plate and feet can be moved, the foot holder is designed to be either screwed on the lab table or sticked to it via a mounting tape.
Connector Panel
File Name: Connector Panel – B
Printing Count: 1-2
Description:
The Connector Panel allows interfacing the signals and power from the
bench instruments to the PCB under tests. The type A is a blank plane
that can be drilled, while type B has 6 holes for standard banana
connectors.
Knobs
File Name: Knob – A and B
Printing Count: 20 Type A and 25 Type B
Description:
There are two types of knobs that must be properly assembled before using it. Type A has the 6mm diameter hole for the M4 brass knurled threads. Type B has a through hole with a 4M nut holder. In this knob you should 1rst 1x the nut and then screw the 4M screw from the other side. You should assemble diIerent type of screw length to accommodate diIerent building scenarios (18-25mm length).
PCB holder
File Name: PCB holder
Printing Count: 4 for each type
Description:
The PCB holders allow to 1x the PCB to the diIerent arms. You can make a setup for horizontal or vertical mounting. The type A, allows to keep the PCB at 5cm from the ground plane, in case EMC conducted tests are run accordingly to the CISPR 25 standard.
Probe arm
File Name: Probe Arm – base
Printing Count: 4
Description:
The vertical arm base allows holding the probes but also the PCB. Under the base there is a 6mm diameter hole for the M4 brass knurled threads.
Probe arm front
File Name: Probe Arm – front – A
Printing Count: 4
Description:
The arm extender, allows to add a free join to the arm. It can be also used to hold the PCB horizontally or vertically.
Probe arm
File Name: Probe Arm – L
Printing Count: 4
Description:
To make more complex structures or simply get in the areas where the other blocks may have challenges, changing direction with an L is as easy as fundamental.
Probe holder
File Name: Probe holder – A
Printing Count: 2
Description:
To hold the scope probes or simply the multi-meter probes in position, the C arm makes the job. This is used as last block on the arm.
The probe holder could be screwed on the main plate directly, also to properly route the cables on it. Under the base there is a 6mm diameter hole for the M4 brass knurled threads.
The PLA 1xture can be handy in case EMC (ElectroMagnetic Compatibility) tests must be done on the PCB. In particular by using the PCB holder - horizontal - A, the PCB height is compatible with the requirements of CISPR 25 that asks for 5 cm distance between the subsystem or PCBA and the ground layer. The dielectric constant, for the PCB holding material, required for CISPR 25 conducted test, is Ԑr<1.3. While PLA typically have Ԑr in the range of 2-3, since the PLA table thickens is either 3-5mm, and underneath there is only air, the averaged Ԑr, depending on the PLA that is used, could be less than what required by CISPR 25. While it is critical to respect the standard, to have a comparison with the limits shown in the standard itself, having higher dielectric constant it means that the capacitive coupling that causes the common mode conducted noise to pass from the PCBA to the ground layer, is higher. Thus, if the test setup has higher dielectric constant material, you would measure higher noise than it would be measured with the required Ԑr<1.3, which is still acceptable for a pre-compliance test, since it would be a worst case scenario and not a better one.
If CISPR 25 tests must be performed either for pre-compliance tests or simply to check the conducted noise to properly dimension the input EMI filter, it is important that all the screws and threads are made of non conductive material.
Another interesting EMC scenario, it is using the fixture to hold electric and magnetic probes. Also in this case, it is important to use plastic screws to limit influencing electric and magnetic fields. In case the magnetic field is the most relevant one, the brass threads are non a big problem as the ferroelectric screws would be. If the electric field is relevant, any metal should be avoided. Typically high current systems with high current rate variations are more a source of magnetic field. High voltage rate variations are causing a dominant electric field. Nevertheless, the Maxwell’s equations link the electric and magnetic fields, anytime there is a change over time of any of the two. So, be careful if you decide to ignore one component of the electromagnetic field.
Below I’m listing some other considerations that could be useful to properly use the PCB fixture.
• Standard PLA material is not ESD approved material, thus for sensitive PCBA it is recommended to either paint the material with ESD spray coating or use ESD PLA variants.
• Metal plates are not ESD approved material neither, since the discharge of accumulated charges is too fast, due to the low impedance path that is offered by a metal surface. Thus metal coating should not be used to transform standard PLA to ESD material.
• The PLA, the same way it can be melt for being printed, can melt in case you have short circuits or the system can dissipate enough energy to rise locally the temperature to the melting point. To avoid fire hazard:
You should never leave unattended a powered PCB under test.
The power supply should be set to the minimum power required by the system.
Protecting the system with an extra fuse would help in case of short.
Local national regulations and lab rules must apply in any circumstance. UL regulation requires for instance “as a must” a fuse protection in case the power goes beyond a certain limit and ϲammable material are used. To limit this problem you can also consider making the fixture using printing ϯlament that are UL 94 approved.
Easy things, such as a 3D printed test fixture, can help out. Building blocks can make the PCB test fixture quite handy. The project presented, is open and can be used for commercial and not commercial applications. It can be further extended to speciϯc needs that go beyond the features that have been described.
Bibliography
[1] www.LaurTec.it: official site where you can download the “EMC Testing” series.
[2] www.PCBWay.com: Service provider for PCB manufacturing, Assembly and 3D printing.
[3] https://www.laurtec.it/tutorial-54/emc-testing/400-3d-printed-pcb-assembly-test-fixture
Written by Mauro Laurenti
Copyright © 2021 Mauro Laurenti
https://www.laurtec.it/tutorial-54/emc-testing/400-3d-printed-pcb-assembly-test-fixture
