Chapter 1: Increase your knowledge in In-Circuit Testing (ICT)

In-circuit testing (ICT) is widely used as a hardware verification method for populated Printed Circuit Boards (PCBs) before the final manufacturing stage. More exactly an ICT machine is capable of measuring component properties (voltage, current, resistance, capacitance, frequency) and checking for various problems found on industrial PCBs, such as open and short circuits. Although ICT is considered an example of white-box testing, it is not able to verify the software functionality of PCBs. Instead, ICT makes use of a fixture (which holds the PCB in place), and a batch of spring-loaded pins (Bed of Nails) for checking the correct placement of components (e.g., capacitors), measuring component values (e.g., determining wrong resistance values) and verifying solder joint quality.

On the other hand, ICT is less effective for multilayer PCBs with dense component packing where very small passive components are difficult to verify due to the physical size of the test probes. While ICT can assess the functionality of Integrated Circuits (ICs) by checking the logic states of the IC pins, it might not be recommended for certain types of PCBs where test access (physical connection) is limited due to high component density. Consequently, ICT is unable to perform functional testing of components under all operational conditions. Despite its limitations, ICT can significantly increase production throughput by early detection of faults.

In conclusion, ICT focuses on evaluating electrical values to identify open and short circuits effectively. It utilizes a ‘bed of nails’ tester, spring-loaded pins, and custom-designed fixtures to assess the integrity of PCBs. Despite its limitations with multilayer PCBs and small components, ICT plays a vital role in the manufacturing industry by significantly increasing production throughput and ensuring overall quality. Complementary testing techniques must be employed to perform functional testing under all operational conditions.

Chapter 2: Increase your knowledge in Flying Probe Testers (FPT)

Flying Probe Testing (FPT) is an advanced ICT method that is commonly used in PCB manufacturing to test electrical connectivity and components. Its main advantage over the Bed of Nails technique is the fact that it requires PCB design files for the test program setup, thus eliminating the need for a test fixture. FPT machines can reach different test points on a PCB by programming specific test paths for each design and then moving the probes via servo-electric motors.

Besides test ‘access’, another important term in FPT is ‘agility’ and refers to the speed of the flying probes. The test speed affects the throughput and is generally slower than Bed of Nails testing. A key factor in reducing test time with an FPT machine is increasing the number of probes used concurrently. Moreover, an advanced feature that could assist FPTs in handling complex PCBs is using multiple probes operating simultaneously on both sides of the PCB. Another way to increase the efficiency of an FPT machine for a specific PCB is to optimize the test sequence and path for the specific PCB layout. Besides component verification, FPTs are capable of testing the quality of solder joints by using optical inspection strategies. Ultimately, FPT is preferred over Bed of Nails testing due to its ability to calibrate probe pressure and use soft-touch techniques to avoid damage to delicate components during the testing of PCBs.

In conclusion, FPTs are valuable tools in PCB manufacturing, allowing for efficient and flexible testing of electrical connectivity and components. Through their ability to adapt to varying PCB designs and eliminate the need for physical fixtures, flying probe testers offer significant advantages over traditional bed-of-nail testers. While they may have slower test times, flying probe testers maintain accuracy and can perform limited functional tests. Their agility and ability to handle complex PCBs contribute to their appeal, particularly in low-volume or prototype production.

A summarized view of Chapters 1 and 2 can be seen in Figure 1 which can be used as a learning diagram to understand the basic notions of ICT and FPT, their advantages and disadvantages on both sides.

Figure 1. In-Circuit Testing Methods used in PCB Manufacturing

Chapter 3:

Increase your knowledge about our FPICT #1 (initial FPICT research paper here)

The Flying Probe-Inspired In-Circuit Tester (FPICT) presented in the paper serves multiple purposes, with its main objective being to offer an affordable and user-friendly educational tool for PCB testing. The FPICT is not designed to replace traditional computer-based testing, but rather to complement it with a more interactive and practical learning experience. It provides a solution for students and educators who require a hands-on approach to learning PCB testing, without the high costs associated with industrial solutions.

The FPICT is based on the Arduino MEGA2560 microcontroller, making it a versatile and widely used platform. This choice of microcontroller ensures compatibility and ease of use for users familiar with the Arduino ecosystem. One of the primary educational benefits of the FPICT is its ability to aid in interactively learning PCB testing. It provides students with a practical platform to apply theoretical knowledge and gain hands-on experience. This type of interactive learning is crucial in modern education, promoting a deeper understanding of concepts.

The FPICT utilizes ULN2003A motor drivers, which contribute to its precise and controlled movements. These motor drivers help facilitate the movement of various components, such as the testing probes, which are essential in the testing process. Stepper motors in the FPICT drive components like the testing probes. Having stepper motors allows for high-precision positioning, which is crucial for accurate and reliable testing. This advantage ensures that the FPICT delivers precise results, increasing its value as an educational tool. With the rise of hands-on and practical learning, the FPICT supports the educational trend of providing students with real PCB parameter values. This trend encourages students to move away from textbook-only education and engage with real-world scenarios, giving them a more comprehensive understanding of PCB testing.

In conclusion, the FPICT presented in the paper serves as an affordable and user-friendly educational tool for PCB testing. It offers precise measurement testing, low power consumption, accessibility in terms of cost, efficient testing speed, Arduino-based versatility, interactivity for learning PCB testing, and high-precision positioning with stepper motors. These features make the FPICT an ideal choice for educational institutions and individuals looking to enhance their understanding of PCB testing practically and engagingly.

At this link https://www.youtube.com/watch?v=50I1wAC3zyY, you can see a demo video of the initial proposed FPICT.

Chapter 4: Increase your knowledge about our FPICT #2 (improved FPICT research paper)

The primary purpose of the improved FPICT prototype is to verify the interconnects of PCBs. Unlike traditional manual testing methods, the FPICT offers a more efficient and reliable solution. It replaces the need for laborious and time-consuming manual testing, ensuring accurate and precise results.

The FPICT test platform studies were primarily aimed at high school students. Introducing this advanced testing platform to students, provides them with an exceptional opportunity to gain hands-on experience in PCB testing. This not only enhances their technical knowledge but also sparks their interest in the field of Test Engineering Education (TEE). The microcontroller used in the FPICT prototype is the Arduino MEGA2560. This microcontroller provides the necessary computational power and control capabilities to effectively operate the testing platform.

One of the reported educational benefits of the FPICT prototype is its ability to increase learning rates in Test Engineering Education (TEE). By engaging students with a practical testing platform, the FPICT enhances their understanding of key concepts and fosters skill development. The FPICT is primarily utilized in technological high schools, where it serves as an invaluable tool for hands-on learning in the field of PCB testing. Its usage in educational environments contributes to the overall improvement of TEE. One of the features of the FPICT prototype that is emphasized as beneficial for educational use is its portability. This feature allows for ease of transport and setup, enabling students to use the testing platform in various locations, both in classrooms and laboratories.

Chapter 5: Increase your knowledge about Test Point Measurements

First, we will be looking at our six Devices Under Test (DUTs) and will establish the order in which we will realize the Test Point Assignment. It is worth mentioning that for our practical part, we will only need the actual DUTs and a digital caliper, as seen in Figure 2. Since we have a total of six PBCs, it would require constructing the same amount of Bed of Nails fixtures to verify all test points simultaneously. From the cost point of view, we need to consider a large sum to cover the expenses for the fixture materials and pogo pins. Additionally, we need to pay attention to the die size of the devices located on the Arduino UNO board. Proper placement of the pogo pins is hardly achievable being very close to each other. Another reason is the presence of Surface Mounted Devices (SMDs) on the board, which would involve assigning telescopic pins on both sides of the circuit (an aspect not found in practice). Following we will explain what we are required to do to test these PCB devices. For our demonstration objectives, we consulted the schematics of all DUTs and then pinpointed all test points necessary for the FPICT software code. In this tutorial, it is out of scope to show all of the pinpointed schematics since the user will only see how the flying probe will make contact with the physical hardware platform via the webcam feed.

Figure 2. IoT Hardware Platforms and Digital Caliper Overview.

1. ESP 32 Microcontroller Unit

ESP32 is the smallest of the selected MCU batch with a length of 48.20 mm and a width of 25.40 mm (see Figure 3), but far superior to Arduino UNO or its predecessor, ESP8266, since it incorporates Wi-Fi capabilities, Bluetooth 4.2, and Bluetooth low energy, leading to an energy-efficient IoT hardware device.

To verify its functionality, we selected three critical test points for this MCU, namely the main 5V power supply and a pair of 3.3V voltage supplies. First, we choose a reference point which could be any edge of the MCU board. It is, however, recommended to select the reference point as close as possible to the test points to reduce the overall test time of the FPICT.

Figure 3 – Measuring the physical distance from the reference point to the 5V pin (left side), 3.3V pin same grid line (middle), and 3.3V pin opposite grid line (right).

The above measurements establish the two cartesian coordinates (x, y) as follows: TP1(0, 5.31), TP2(0, 15.74) and TP3(5.31, 22.40). The Z-axis coordinate was omitted because it varies depending on the board’s height, and can be later measured when the device is fixed on the hardwood board. In the software program seen in Fig. 4, the three pairs of coordinates will be stored as follows:

Figure 4 – Configuration Structure for the ESP32 Test Points

2. L298N Motor Driver

The L298N Dual H-Bridge is among the most popular motor drivers for controlling peripheral engines used in a plethora of projects. More specifically, the L29N IC is capable of driving two DC motors or one Stepper motor with a maximum current draw of 3A. Since the voltage and current values can vary depending on the motor’s power requirements an additional heatsink is installed on its surface to properly dissipate heat. The L298N board (see Fig. 5) has a square shape measuring 43 mm in length. To verify its functionality only one test point was considered sufficient, namely the common point of the Dual H-Bridge, which was measured according to the method described above: TP4(0, 16.37).

Figure 5 – L298N Motor Driver Test Point Localization using Digital Caliper

3. STM32 Development Board

The STM32 MCU is a great alternative to Arduino UNO due to its more compact electronic packaging and the capability of being programmed from the same Arduino software interface. Additionally, thanks to its energy-efficient design it consumes less power than other MCUs in its category, making it the ideal choice for any home or school project. Similar to the L298N, the STM32 (seen in Fig. 6) presents itself in a square-like shape with a parameter length of 50 mm. Test point localization is much easier since, compared to other development boards, the STM32 has only a 3.3V power supply scattered around a few pins on the edge of the device. By using the digital caliper, we could determine the two test points with the following coordinates: TP5(0, 18.89) and TP6(0, 24.16).

Figure 6 – STM32 Development Board Test Point Localization

4. Arduino UNO R3

The Arduino UNO R3 seen in Fig. 7, is the most iconic development board due to its wide use in a variety of projects, ranging from simple sensor readings to solar tracking devices and other automated systems. It offers a total of 6 analog GPIO pins (A0-A5) and 14 digital GPIO pins (D0-D13). Additionally, some Arduino UNO releases have a different pin configuration on the backside allowing for a better evaluation concerning the Atmega328 MCU. In this example, I will show you how to use a simple but efficient trick to measure all test points by using the digital caliper only twice.

Figure 7 – Arduino UNO Test Point Localization

Our strategy, as illustrated in Figure 7, is to measure only the distance from the reference to the two rows of contact pins (which denote the Atmega328P). By determining the minimal distance between two neighbor pins (in our case 2.19 mm) it is extremely easy to find the coordinates of any test pin from the two grid lines. The selected test points are: TP7(16.33, 0) – RESET 5V, TP8(16.33, 2.19) – RXD 5V, TP9(16.33, 13.4) - ANALOG POWER 5V, TP10(9.04 ,15.33) – GND 0V, TP11(9.04, 17.52) – AVCC 5V. All 11 test points that were chosen for this tutorial were organized in the following table.

While FPICT is a potent tool for testing PCBs, it has these limitations:

• Electrolytic components can be tested for polarity only on specific configurations (e.g. if not parallel connected to power rails) or with specific sensor
• The quality of electrical contacts cannot be tested
• It is only as good as the design of the PCB. If no test access has been provided by the PCB designer, then some tests will not be possible