Test PCB: Everything You Need to Know

Introduction

Testing is a crucial part of the PCB manufacturing process. It is ideally conducted throughout the production cycle, to prevent errors in the final product. Some PCB design analysis techniques can be applied early on to help minimize major issues during the manufacturing process, but numerous PCB testing methods can be used on physical boards. These tests, performed on prototypes or small-scale assemblies, focus on potential shorts, solder joint issues, and functionality, ensuring that each tested PCB functions properly.

PCB standards are built on pre-established design guidelines that direct manufacturers to create boards with the required capabilities and functions. To achieve this, the circuit boards are tested and examined using electric board testers and other circuit board testing techniques. Testing PCBs ensures the production of qualitative boards, invariably saving cost by increasing efficiency.

How Do You Test a PCB Board? 

A PCB is made up of different elements, each of which affects the overall performance of the electronic circuit. Tests are carried out to check and optimize the status of these elements, using computerized machine software, simple electronic testing equipment and tools, and even basic human inspection. These checks include but are not limited to the following:

• Electrical tests, which also include measuring leakage currents

• Quality of hole wall

• Mechanical resistance test

• Quality of the welds 

• Copper plating, tested with tensile strength and analyzing the stretching that follows

• Cleanliness checks, accounting for weather resistance, humidity, and corrosion

• Component alignment, polarity, placement, and orientation

• Lamination, which tests the laminate’s resistance to peeling by force or application of heat

• Environmental tests are particularly necessary for PCBs used in humid environments. [1]

PCBs can be tested by engaging the following testing methods:

In-Circuit Testing

In-circuit testing (ICT) is the most potent type of PCB testing available. It is also known as a "bed-of-nails test", used to verify that each electronic component on the PCB is in its proper location and is operating properly. Short circuits, open circuits, resistance, capacitance, and other factors are checked throughout the test.

To carry out this test, fixed probes that are arranged in accordance with the PCB's layout are used. The solder connection's integrity is examined by the probes. To begin the test, the bed of nails tester presses the board against the bed of probes. The board has access points that are already constructed to connect the ICT testing probes to the circuit. They exert a certain amount of pressure on the connection to make sure it is secured. ICT is usually performed on bigger connections and ball grid arrays (BGAs).

Bed-of-nails technique is expensive and not very adaptable since each board requires its own bed-of-nails. Also, it demonstrates some challenges in testing boards with high component density, where pin spacing is decreased. The ICT method offers the advantage of being able to identify some flaws in both the individual components and their connections without having to power the board. Its drawbacks include the high cost, which is exacerbated by the complexity of the bed of nails and the control software. Another demerit is the inability to test the connectors. [2]

Flying Probe Testing

The Flying probe test, also known as the fixtureless in-circuit test (FICT), is a type of in-circuit test that operates without the use of specialized test fixtures, hence lowering the overall cost of the test. With this technique, the board is held in place by a basic fixture while a software-controlled program moves test pins about and checks specific test points on the board. Due to its versatility, the flying probe test is well-liked throughout the electronics manufacturing industry. 

Although flying probe testing is used for the same purposes as conventional ICT, it has significant benefits and drawbacks because of how it is conducted. While the flying probe can rapidly, efficiently, and affordably adapt to different boards with a simple programming change, it is typically slower than the conventional ICT. This characteristic makes it a perfect testing technique for prototype and small-production tests, but less efficient for large-scale production. [3]

Flying Probe Test on PCB

Automated Optical Inspection (AOl)

AOI is a visual inspection for printed circuit boards that require no contact. Cameras and microscopes are used in AOI to take pictures of the PCB. The software then looks for inconsistencies between these photos and the board's schematic. By this procedure, technicians may thoroughly inspect each board and spot any issues right away. The board is not powered up during AOI, and it may not cover all part types. Due to these inadequacies, it should be used in combination with another test technique, such as functional testing, flying probe testing, or ICT.

A PCB's performance can be impacted by a variety of faults that AOI can detect. It can specifically find solder gaps, excess solder, and under- and over-etching. It also covers component defects, such as those that are missing, have the wrong values, as well as components that are positioned or aligned incorrectly. Moreover, AOI can recognize and rectify BGA shorts and open circuit connections and detect lifted lead deficiencies on PCBs.

AOI can be performed at any stage of the manufacturing process. It can be placed right before, during, or after reflow soldering, where most errors occur. This way, any discovered issues can be immediately corrected, ensuring that more products pass the final inspection or board testing methods. [4]

Burn-In Testing

Burn-in testing is an intensive type of testing designed to analyze the load capacity of PCBs and detect early errors. During this process, the printed circuit boards go through electrical or thermal testing. In the first instance, electricity is continually applied to the board for about 40 to 160 hours. To examine the PCBs' performance at high temperatures, another option is to expose them to temperatures up to 125 degrees Celsius for 160 hours or longer. The printed circuit boards may frequently also be put through an environmental stress test. Infant mortality is the term used when a board fails.

The ICs are mounted on the board and put under stress while being tested. The board utilized for the burn-in test is constructed from FR4 flame retardant material, which enables it to endure temperatures of up to 125 degrees Celsius. Boards consisting of ceramic and polyamide are applied when the temperature of the board exceeds 125 degrees Celsius. By using the required voltage, the boards are tested in the burn-in oven. The board is removed when the stress test is finished and examined for flaws. To hold the ICs on the boards, sockets are provided.

It is important to note that burn-in testing can reduce the lifespan of the circuit board, particularly if the test subjects the board to greater stress than it is designed to withstand. To prevent overstressing the PCB, the testing limit may be lowered after a shorter time if few or no flaws are discovered. [5]

X-Ray Inspection

As the name implies, this type of testing is essentially an inspection tool. It is also referred to as AXI. By scrutinizing solder connections, internal traces, and barrels during this test, an X-ray technician can find flaws early on in the production process. Using X-ray inspection, it is possible to inspect components that are often out of sight, like connectors and ball grid array packages with solder joints under the chip package. This check typically requires skilled, knowledgeable operators.

2D PCB X-ray Component Inspection

More precise inspection techniques, such as those based on X-ray technology, were necessary due to the high density provided by SMT technology and the difficulties to locate connections in BGA and CSP (Chip Scale Packages) Welds can be seen clearly in radiographic pictures because they are formed of a material having a higher atomic weight than the other components on the board. The primary benefit of the AXI technique is that it can identify every connection and weld, including those that are concealed by the package. Additionally, solder joints can be examined, highlighting any potential bubbles. There is also a good likelihood of finding any replicas of exclusive electronic components. However, this technique is known to be a time-consuming and expensive process. [6]

Functional Testing

Functional testing, as its name suggests, is aimed at testing a circuit's functionality by simulating the electrical signals capable of stimulating it and observing the results. It is the final stage of PCB inspection and verification. Through the interface connectors, the circuit is accurately powered and electrically stimulated. A software program analyzes the measurements taken at appropriate locations on the PCB and confirms that they match the design requirements.

The functional testing process ensures that the final product works as it should. This test could be as simple as an on/off power test or as in-depth as a test using testing software. The ultimate goal of the testing process is to pinpoint and rectify any issues that may arise after a product has been completed. The procedure tests the board's digital and analog signals to simulate its operation. It may also employ specialized software to detect circuitry issues. Manual inspections of various aspects of the PCB manufacturing process may also be included.

The functional test has the advantage of detecting potential circuit anomalies that only appear when the circuit is powered. In addition, it can measure the power absorption at particular circuit points. The expense and complexity of the test system are the drawbacks. It requires equipment that is extremely complex but not very adaptable, as it is frequently configured just to examine a single card.

Other Functional Tests

Depending on the situation, there are other types of functional tests that can be used to examine a PCB. A PCB functional test confirms how a PCB behaves in the product's application environment. Depending on the PCB and final product, the requirements for a functional test, as well as its development and methods, can differ significantly.

Other functional tests for PCB assembly include:

• Solderability Test: This is done to check whether a component offers the level of wetting required for a strong solder connection. A subpar test result suggests a subpar connection. Before completing assembly, it is crucial to do solderability testing, which is an essential component of PCB functional testing. When the metal doesn't solder during the test, it reveals that the PCB has flaws that will impair its performance. As a result, the PCB could experience solder oxidation and other undesirable issues.

• Contamination Testing: Detects ionic residues that remain from the manufacturing and soldering process. It is necessary to ensure ionic cleanliness before applying the conformal coating to printed circuit boards. Contamination testing also successfully removes most non-ionic contaminants that can degrade the board, causing corrosion and other problems.

• Micro-Sectioning Analysis: This is an effective tool for failure analysis as well as in-process verification. It is a threefold inspection tool for checking the internal makeup of a PCB for defects, opens, shorts, and other failures. Typically, the inspection criteria for the Micro-Section Analysis are included in the product specification designated by the customer. The analysis could, however, also adhere to guidelines like IPC 6012, MIL-PRF55110/31032, or both.

• Time-Domain Reflectometer (TDR): A time-domain reflectometer is a tool employed to locate problems in transmission lines and coaxial cables. The TDR scans the transmission line with a low-voltage pulsed signal to look for any reflections caused by an impedance mismatch.

There won't be any reflections if there is no impedance mismatch over the entire transmission line, but if there is a break in the transmission line at a specific location, a portion of the pulsed signal will be reflected to the TDR. The TDR can determine the precise position of the fault and the type of fault, such as an open circuit, short circuit, or impedance mismatch, by calculating the timing and propagation velocity of the received pulse.

• Peel Testing: is the practice of testing adhesion properties of film bonded to the substrate, usually by tensile. This is done to determine the strength required to peel the laminate from the board.

• Solder Float Testing: Determines the level of thermal stress a PCB's holes can withstand. To carry out this test, extreme temperatures are applied while measuring the resistance of the PCB's holes. [7]

Key Takeaways 

Electronics production requires adequate testing to make sure the final product that goes into the market is functional. Printed circuit boards, as the heart of electronic products, are no different. This is why the industry engages in numerous testing methods and processes to make sure PCBs are properly fabricated, according to desired functions and capacity. The testing procedures discussed in this article are necessary for producing qualitative circuit boards, without giving room for cost-incurring defects.

References

1. Seed Studio. 7 Most Popular PCB Testing Methods During Manufacturing and Assembly. 2021. [Cited 2023 Feb 16] Available from: https://www.seeedstudio.com/blog/2021/06/18/7-most-popular-pcb-testing-methods-during-manufacturing-and-assembly/

2. ConRo Electronics. What is In-Circuit Testing (ICT)? 2022. [Cited 2023 Feb 16] Available from: https://www.conro.com/what-is-in-circuit-testing-ict/

3. Candor Industries. Flying Probes and Testing: Everything You Need to Know. 2021. [Cited 2023 Feb 16] Available from: https://www.candorind.com/flying-probes-and-testing-everything-you-need-to-know/

4. Valley Services Electronics. Using Automated Optical Inspection (AOI) for PCB Manufacturing Validation. 2021. [Cited 2023 Feb 17] Available from: https://www.vse.com/blog/2021/02/11/using-automated-optical-inspection-aoi-for-pcb-manufacturing-validation/

5. EMS Solutions. Importance of Burn-In Testing for PCBs. 2022. [Cited 2023 Feb 17] Available from: https://www.myemssolutions.com/importance-of-burn-in-testing-for-pcbs/

6. Electronics and You. 6 Benefits of PCB X-Ray Quality Control and Inspection. 2022. [Cited 2023 Feb 17] Available from: https://www.electronicsandyou.com/benefits-of-x-ray-inspection-of-pcb-for-quality-control.html

7. Seed Studio. Everything You Need to Know about PCB Assembly Functional Testing. 2021. [Cited 2023 Feb 17] Available from: https://www.seeedstudio.com/blog/2021/06/18/pcba-functional-testing/