This webinar explores the design and importance of Printed Circuit Board Assembly (PCBA) functional test fixtures. It highlights that functional tests, performed after assembly and in-circuit testing, are crucial for identifying board issues early in the manufacturing process, saving significant time and cost. The discussion covers:
The typical PCBA manufacturing process, including fabrication, assembly, inspection (AOI, X-ray), and electrical testing (in-circuit, functional).
The traditional approach to functional test design, which often involves manual wiring and expensive equipment, versus FixturFab's modern, automated approach using Python and custom test point carrier boards.
Key components of a bed-of-nails fixture, such as pogo pins, probe plates, and pressure plates, along with design guidelines for test points (spacing, location, size).
A demonstration of FixturFab's automated design service, which allows users to upload PCB design files to generate 3D renders, Bills of Material (BOMs), and quotes for custom test fixtures.
An overview of creating a PCBA functional test system, including developing test specifications, identifying necessary test points, and designing the mechanical and electrical interfaces.
FixturFab Turnkey Test Systems overview
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Introduction to PCBA Functional Test Fixtures and FixturFab [00:00]
Welcome and Introductions [00:05]: Joe Selvik and Duncan Lowder of FixturFab introduce themselves and the webinar's topic: designing custom PCB test fixtures.
FixturFab's goal is to modernize functional test fixture design through a software platform.
The platform automates the design of bed-of-nails fixtures by processing circuit board design files (e.g., Altium, KiCad) to identify test points and guide pin locations, generating mechanical files.
This automation aims to simplify the mechanical design portion and assist in developing test cases, such as checking for short circuits on power rails.
In-Circuit Testing (ICT) [05:18]: Performed with the board unpowered, checking resistance and capacitance between test points. Can use flying probes for low runs or bed-of-nails fixtures for mass manufacturing.
Functional Testing [05:58]: Occurs when the board is powered up, checking voltage rails, programming processors, and testing functional blocks (sensors, motor drivers, etc.). This step is performed after ICT to prevent damage from power-up failures.
Understanding and Justifying Functional Tests [06:55]
The primary goal is to verify all functionality of the assembled PCB.
Example: For an Arduino clone, tests include verifying voltage rails (e.g., 5V, 3.3V), processor programming and execution, and USB-to-serial communication.
Example of a functional test for an Arduino clone
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Failure Debugging [21:01]: Test specifications include test cases that correspond to specific circuit blocks (e.g., a 5V regulator test). If a test fails, a debug guide directs the operator to the relevant component for rework.
Controlling Net Signals [22:55]: For injecting or intercepting signals, collaboration between electrical and firmware engineers is crucial.
Methods include holding processors in reset for high-impedance states or using test-specific firmware for direct microcontroller control.
Boundary scan technology can also be used to check net connections between ICs.
Power Control [28:56]: Essential to prevent damage during probing.
Use programmable power supplies controlled by software.
Implement physical sensors (inductive, microswitches) to detect when the fixture is closed and enable power.
Probe Plate: Made of FR-4, holds receptacles into which pogo pins are inserted.
Test Point Carrier Board PCB [25:38]: A custom circuit board that solders to the receptacles and routes signals to the instrumentation.
Guide Pins [25:59]: Dalton pins or spear-tip spring-loaded pins are used to accurately locate the DUT on the cartridge. Spear-tip pins are preferred for varying hole diameters.
Test Point PCB Functionality [30:20]: Can implement digital I/O checking using multiplexers or measure resistance between test points with a DMM or voltage divider setup.
An assembled interface cartridge showing the probe plate, moving plate, pogo pins, and test point PCB
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A test point carrier PCB with various components
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Pressure Plate [31:40]: Contains pressure pins that ensure even pressure distribution across the PCB to maintain contact with test probes and prevent board bending or component dislodgement, especially with high cumulative forces from many pogo pins.
Demo of FixturFab Automated Design Service [34:00]
Overview [34:00]: The automated design service on FixturFab.com allows users to upload test point lists and Gerbers to design bed-of-nails fixtures.
Steps in the Demo:
Select a Test Fixture [34:50]: Choose from various types like 3D-printed development fixtures (for first revisions, quick turnaround) or Ingun production fixtures (for mass manufacturing).
Choose a Receptacle Type [35:50]: Select how probes connect (solder cup, pre-attached wire, wire-wrap) and tip styles (spheres, stars, concave).
Configure the Device Under Test (DUT) [37:10]: Upload test point files and Gerbers for 2D and 3D rendering.
Test Point Configuration [37:50]: Review and adjust test point, pressure pin, and guide pin (Dalton) locations on a 2D render of the PCB.
3D Test Fixture Render [40:10]: Generate a 3D model of the fixture with the DUT, allowing for visual verification and adjustment of the DUT's position within the fixture.
2D render of a PCB with identified test points and pressure pins
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3D render of a test fixture with the device under test
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Create BOM and Quote [43:00]: Generate a Bill of Materials and an estimated cost for building and assembling the fixture. Development fixtures can be shipped in 3-5 business days, while production fixtures take 4-8 weeks.
How to Create a PCBA Functional Test System [46:00]
6-Step Process for Functional Test System Creation [46:00]:
Create Test Specification [49:00]: Develop a detailed document outlining all necessary tests, descriptions, and expected outputs, typically from the PCB's schematic or product requirements.
Identify All Nets That Need Test Points [52:10]: Due to increasing board density, it's no longer feasible to probe every net. Prioritize critical nets such as voltage rails and programming pins.
Create a Block Diagram and Identify Instrumentation [53:00]: Map out the test system components (e.g., fixture base, test point carrier board, power supplies, digital I/O, multimeters) and their interconnections.
Create Mechanical and Electrical Design [48:10]: Design the physical fixture and the electrical interface (wiring diagrams or custom carrier boards).
Fabricate and Assemble the System [48:29]: Build the physical test fixture.
Develop the Automated Test Software [48:33]: Write the test scripts, ensuring hardware abstraction to allow for software development before physical hardware is available.
Sample test specification in a spreadsheet format
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Block diagram of a functional test system for a macro pad
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Test Point Spacing [53:16]: Place test points at least 1.9mm (75 mil) apart. While smaller spacings are possible, they lead to more delicate and breakage-prone probes.
Single-Sided Probing [53:43]: Place all test points on a single side of the PCB to reduce complexity and risk in fixture design.
Clearance from Components [54:05]: Ensure test points are at least 1mm away from any other component to prevent probe deflection and false failures.
Test Point Pad Diameter [54:31]: Test point pads should be greater than 0.8mm in diameter for reliable contact.
These are guidelines, not absolute minimums, intended to facilitate easier and cheaper test fixture creation.
Ingun fixture bases can integrate automated opening mechanisms, and robotic arms can be added for closing, loading, and unloading.
Fixtures can be integrated inline with the PCBA assembly line, where conveyors load boards into automated functional test systems.
Double-Stage Probing [57:47]: A technique used to integrate in-circuit and functional testing in one fixture. It involves two different device heights and probes of different lengths, allowing only longer probes to contact in one stage (functional test) and all probes to contact in another (in-circuit test).
FixturFab can handle high voltage applications (e.g., high-pot testing) by integrating safety features like interlocks that prevent the fixture from being opened when high voltages are present.
For line voltages, interlocks ensure power is cut if the fixture base is opened.
