The FixturFab DEV260 Development Fixture is a simple, low-cost test fixture that is great for debugging or configuring PCBA’s during development. In this series, we will cover how to create a DEV260 fixture for configuring and testing a SparkFun Power Delivery Board. These same techniques can be used to create a fixture for programming, debugging, or configuring your custom PCBA during EVT and DVT.
DEV260 Fixture Series
The first part of the DEV260 Fixture Series covers designing the mechanical bed-of-nails fixture which includes selecting test probes, guide pins, pressure pins, and positioning the DUT.
To follow along with this tutorial you will need:
The SparkFun Power Delivery Board takes advantage of the power delivery standard with the use of a standalone controller from STMicroelectronics, the STUSB4500.
The STUSB4500 is a USB power delivery controller that addresses sink devices. It implements a proprietary algorithm to allow the negotiation of a power delivery contract with a source (i.e. a power delivery wall wart or power adapter) without the need for an external microcontroller. However, you will need a microcontroller to configure the board. PDO profiles are configured in an integrated non-volatile memory. The controller does all the heavy lifting of power negotiation and provides an easy method for configuration over I2C.
To configure this device using I2C, SparkFun recommends using their Qwiic system, however, this requires using a connector to configure each device. To easily configure multiple Power Delivery boards (i.e for a small production run), a simple bed of nails test fixture can be used.
To configure the board using a bed of nails fixture we’ll use a Binho Nova Host Adapter. It will need to access the following nets with Test Probes (Pogo-Pins):
If this were a small production run, we’d also want to run a series of functional tests. To allow for this, we will also add test probes for the remaining nets.
Designing the Test Fixture
To start the design process, go to fixturfab.com and click Design Now. Create a new project and enter the name, PCB thickness, and upload the Eagle .brd file. You can find the Power Delivery Board’s Eagle BRD file here. FixturFab supports Eagle’s .brd files, KiCAD’s .kicad_pcb files, Altium Test Point CSV reports, and gerbers from any other EDA software.
For this project select a DEV260 fixture. These are laser-cut for increased reliability (down to a 50 mil pitch between test points) and have a similar maximum PCBA size to our entry-level production fixture solutions.
The parsed test point information from the ECAD file will then be displayed. The PCB board width and height are automatically detected, but you should always verify that these values are correct.
Adding Test Pins
First, configure the test points for the project. While we only need the four test points that were specified above, we will include all of the detected test points in this fixture. This will allow us to wire the DEV260 fixture for other uses, such as functional testing.
For each test point, the Receptacle and Test Probe need to be configured. A receptacle is a metal tube that mounts into the probe plate of the test fixture and is wired to the Binho Nova. The test probe (pogo-pin) is spring-loaded and mounted inside of the test receptacle. We have a separate article covering this choice in detail.
These test probes contact the PCBA, also referenced as the Device Under Test or DUT, creating an interface between the board and test instrumentation. Using receptacles allows for broken test probes to be easily replaced.
For this fixture, we will use R75-1W Receptacles, and P75-LM1 Test Probes.
Adding Guide Pins
To locate the Power Delivery Board within the fixture, Spring Guide Pins will be used on the four mounting holes on the PCB as shown below.
FixturFab identified the four mounting holes, as well as the 16 “mouse bites” that are on the PCB. The mounting holes can be identified by their diameter of 3.3mm. Disable all of the mouse bites, and then select GP-2 Spring Guide Pins from the Pin Selection dropdown.
Adding Pressure Pins
Pressure pins also need to be added to the fixture. These pins are used to push on the top of the PCB to press it against the Spring Guide Pins and Test Probes within the fixture. The pressure pins that are available on FixturFab have a tip diameter of 3mm. To determine the locations of the pins, we can use the ECAD design software and place a 3mm circle in a location where it will not interfere with any other parts.
Add the coordinates for the pressure pins and select the 6x45mm Pressure Pin, 3mm Tip type.
All of the components for the fixture have now been added. Click Next to view a 3D render of the fixture and configure the location of the Device Under Test (DUT).
The location, rotation, and panel configuration of the DUT can now be configured. By default, the DUT is placed near the center of the fixture, with a panel configuration of 1 device.
The following changes can be made:
- Test point Layer
- Top/Bottom of the PCB, select which layer you like to have contacted by the test probes.
- If Top is selected, the test points are mirrored, so that the PCBA can be inserted upside down
- DUT Rotation
- Rotation of the PCB
- Add an offset to the DUT X location, this is used to center the PCBA within the fixture.
- +X moves the DUT X millimeters to the right
- -X moves the DUT X millimeters to the left
- Add an offset to the DUT Y location, this is used to center the PCBA within the fixture
- +Y moves the DUT Y millimeters towards the back of the fixture
- -Y moves the DUT Y millimeters towards the front of the fixture
- Panel Settings
- If a panel of PCBA’s will be tested at the same time within the fixture, these settings can be used to configure the panel
Add offsets so that the DUT is centered in the X-axis, and towards the front of the fixture (Y-axis).
After making all the necessary adjustments, click Generate Fixture. A summary of the fixture design will then be shown.
The fixture design can be purchased by sending an email to firstname.lastname@example.org, if you are following along with this series, send us an email and we will unlock the design files for you.
After purchasing the fixture, FixturFab will perform a design review, and then fabricate, assemble, and ship the test fixture. This entire process takes approximately 5 days.
For those with access to a laser cutter, we can supply a design file package with everything needed to fabricate your own DEV260. The next article will cover assembling a DEV260 fixture.