PCB Inspection and Testing Techniques
Testing printed circuit boards (PCBs) throughout the design and manufacturing processes is essential in ensuring quality products. It avoids situations where designers and manufacturers realize that the product is faulty at the last minute, while the boards are in full production, or already in the market.
Even after following proper design and manufacturing processes, there is always a risk of defects, bugs, human errors at the prototype stages. Identifying and addressing these issues before the final product is critical in ensuring the performance, functionality, and reliability of the products. A wide range of defects in PCBs may arise due to human error, a wrong manufacturing process, poor design, and other practices.
Benefits of testing include
- Identifying and addressing faults and bugs such as short circuits, opens, poor soldering, functional issues and more.
- It provides an opportunity to address any potential issues early before going into final production, saving time and money. Fixing the problems on finished products is usually more difficult, time-consuming and costly
- Reducing wastage and costs since the testers use the small-scale assemblies and prototypes instead of complete products. This prevents throwing away faulty, full-scale assemblies.
What to test on PCBs
A PCB consists of several different parts and components. Each of these has an impact on the overall performance of the circuit and the electronics assembly as a whole. Ideally, it is important to test everything. This includes, but not limited to verifying the;
- Electrical conductivity
- Mechanical strength
- Soldering quality
- Cleanliness
- Tests for the target environment
- Lamination — peel strength
- Quality of hole wall
- Component placement, alignment, polarity, orientation, etc.
PCB testing techniques
Generally, the testing involves verifying design features in terms of visual, structural, electrical and functionality. In most cases, there are various techniques for testing each of these areas, and the choice depends on factors such as board complexity, application, design, etc. The common methods include;
- In-circuit testing (ICT)
- JTAG boundary-scan
- Automated optical inspection (AOI)
- Automated X-ray inspection (AXI)
Testing teams can use either manual visual inspection (MVI) or automated test equipment (ATE) methods to check the PCBs after the assembly process. However, the automatic test methods, such as the Automated Optical Inspection AOI), and the Automated X-ray Inspection (AXI) are more effective for the assembly level tests, but usually costly.
In addition to the visual and electrical tests on the contacts, some applications such as military, aerospace, mining and similar industries require mechanical testing. This ensures that the BGA and other components will withstand the shocks, vibrations and other rough conditions in the operating environments. In most cases, the tests are destructive and involve subjecting the PCB to shock and shear forces. Measuring the strain helps to establish the mechanical properties of the solder joints.
In-circuit testing (ICT)
The ICT comprises an in-circuit tester, a fixture, and the software and can cover most of the defects that occur during the manufacturing processes. Testers can use it to check for shorts, opens, resistance, capacitance, and inductance, in addition to verifying the polarity or orientation for devices such as diodes, transistors, and ICs.
The in-circuit tests check the components based on a model of the design. Theoretically, it has the potential to detect about 98% of PCB faults. However, this may not be practically possible, especially when it cannot access all the nodes, as well as its inability to measure very low capacitance and inductance values.
Benefits of ICT include simple defect detection, programming, and easy to interpret test reports. However, it has drawbacks such as costly equipment, difficulties updating the test equipment systems since they mechanically fixed, inability to access some nodes in complex circuits, etc.
The two commonly used ICT techniques are the Bed of nails, and Flying probe. Each has its place, benefits, and limitations, and the choice depends on the nature and complexity of the PCB under test.
Bed of nails technique
The bed of nails or universal grid in-circuit testing relies on multiple spring-loaded pogo pins that make contact with several points on the PCB. These pins resemble the bed of nails, hence the name. In the test, each of the pogo pins makes a contact with the circuit node or the point under test. This method can identify, shorts, opens, solder joint bridges, defective components, and other PCB faults.
A typical in-circuit test comprises of multiple pins spread across the board. Applying the multiple pins ensures tens or hundreds of simultaneous connections and tests. Each of these is about 35mm long and usually inserted at the end of a net such as a surface mount pad, a hole, or a test point. With all nets connected, the test takes about 7 seconds.
During the test, the pins introduce some signals and voltages into the circuit after which they measure the resulting values down the line.
Generally, the bed of nails technique is a fast, low-cost test method that is suitable for mass production systems, simple circuits, and analog boards. However, it may be limited when working with complex boards and especially those with small pitch widths, SMDs, BGA and similar components.
Flying probe test
The technique uses an element with a smaller pitch to make contact with the test points such as the SMD pins. This is suitable for small-sized contacts down to a 0.2mm test pitch. In practice, it uses several probes to make contact with the pins, pads and vias and test for opens, shorts and electrical parameters such as the polarity, resistance, and capacitance.
Some test equipment may include a camera to determine if there are missing components and analyze the sizes, shapes, orientation, polarity, and other physical properties of the components.
Automated Optical Inspection (AOI)
The AOI method uses one or multiple cameras to optically analyze the PCB. It uses software to compare the images from the PCB under test with those from a similar reference board. Another option is to compare with ideal design specifications. The optical inspection is usually at the end of the assembly line where it helps to verify the quality of the finished product.
Other than performing tests on the PCB under assembly, the AOI method can monitor the manufacturing process. Using the technology in the pick and place machines enables the manufacturers to track the processes in real-time, and correct assembly defects such as potential component misplacement and misalignment.
In some applications, the optical inspection involves using an endoscope to view the connections between the BGA and the PCB.
AOI method is only useful on PCBs where the points to test are optically visible.
Automated X-ray inspection (AXI)
AXI provides a non-destructive testing technique with the ability to detect solder defects invisible to the human eye or when using the automatic optical inspection. It does not require a physical connection and can find defects under the large IC packages such as the BGA, Micro BGAs, QFN, LGAs, CSPs, etc.
Generally, the x-ray technique is suitable for testing invisible areas located in the center. The method relies on the ability of materials to absorb the x-rays according to their thickness and atomic number. Because the absorption rate is directly proportional to the element’s atomic weight, heavier materials such as solder usually absorb more x-rays and are more visible. The lighter elements such as the integrated circuit package appear more transparent because they absorb fewer x-rays.
A typical x-ray image of a BGA is as shown below. The relatively transparent sections refer to lighter materials while the darker parts reflect heavier parts such as the solder.
As such, the x-rays can penetrate the IC package and inspect the soldering and connections where it identifies structural defects such as shorts, opens, insufficient solder, excess solder, and voiding.
Other capabilities include checking for
- Bad alignment for the BGA and other large chips
- Connections that are not symmetrical
- Consistency of the package standoff height
- Popcorning — which occurs when some balls merge to form irregular shapes
- Soldering analysis where it checks the inside of the solder to identify defects such as bubbles, insufficient filling, etc.
The method is ideal for checking the board, its layers, soldering, component orientation, alignment, and other physical features.
Choosing a PCB testing solution
The techniques vary according to the type of PCB, testing to perform, application, sensitivity, and tolerance. For example, the medical, aerospace, military and similar applications require higher levels of reliability.
Most often, it is easy to check a simple, single or two-layer PCB using traditional test methods. However, as the level of complexity increases due to high component densities, multiple layers, miniaturization, and other factors, testing require advanced techniques such as AOI and AXI.
In-circuit testing will work for most basic circuits, but as the level of complexity and component density increases other techniques such as AOI and AXI become necessary. X-ray is suitable for PCBs with large chips such as the BGAs and others where some connections are invisible even when using the optical method.
Testing the printed circuit boards during the manufacturing stages provides the designers with an opportunity to identify defects and other issues that would affect the performance of the final product. Addressing the defects before the final product is much easier, less costly and is an effective strategy that improves the quality while reducing expensive repairs, recalls and material wastage.
Although a combination of all the techniques will dramatically provide a comprehensive analysis, and identification of almost all the defects, it is important to look at the most cost-effective solution.
