PCB Assembly

PCB Assembly Test: What is PCBA Testing, Types and Advantages

Circuit card assembly testing

The electronic devices that we use almost every day and everywhere consist of printed circuit boards (PCBs) that contain the components acting together to accomplish different functionalities. The design and manufacturing of these PCBs must go through processes to ensure a defect-free product that consumers can conveniently use. This requires the PCB to go through the last step, that is, testing.

What are PCBA and PCBA testing?

Printed Circuit Board Assembly, also called ‘PCBA’ for short, involves a series of processing steps to mount or place the electronic components on the printed circuit boards. PCBA starts by stencil printing a solder paste into the boards, and then placing the parts of different types, functions, and sizes on the pre-printed paste. The next step is to subject the boards with components to reflow to completely cure the paste and achieve a reliable joint. The assembled boards are then inspected using a high-speed machine with a vision system which is automatically operated.

PCBA does not end after optical inspection. A PCB can be visually good but electrically defective. This is where the PCBA Testing comes into play. PCBA Testing is the process of electrically testing the assembled boards to screen for any manufacturing flaws, material defects or board design issues.

Types of PCBA Testing

1. In-Circuit Testing (ICT)

The first type of PCB assembly testing that is most used for large-volume PCBA production is In-Circuit Testing (ICT). In-circuit testing is a bed-of-nails type of PCB assembly testing that can perform simultaneous testing of the PCB contact points through a customized fixture. Both the diagnostic and testing times are relatively faster for in-circuit circuit card assembly testing. The weakness of ICT is that it has less flexibility since it can only cater to a single type of PCB and is high-priced due to its complex software program and costly tooling. An in-circuit tester can be stand-alone or integrated with a functional test system for more efficient fault coverage and throughput.

2. Flying Probe Testing

The more flexible type of testing is called Flying Probe Testing which uses a probe that can move or fly to test the PCB. It can test fine-pitched devices but is relatively slower than ICT making it a more suitable technique for prototyping and small-scale production. It has a short development time and can easily be revised according to PCB layout changes unlike with ICT wherein expensive revision of the testers will be required.

3. Functional Testing

The third type of circuit card assembly testing is called Functional Testing. This circuit card assembly testing method takes a longer time since the boards are being tested for all their functionalities, ensuring that the boards will work according to requirements using introducing stimuli and measuring the response of the boards. Functional circuit card assembly testing also has expensive design and development.

Take note, however, that 100% test coverage of all components is not possible due to considerable factors like stray impedance, cost, and processing time. To increase confidence in test coverage, PCBA companies would sometimes implement both ICT and functional testing. 

4. Other PCBA testing

PCBA testing

Other in-process and offline controls during assembly must also be implemented such as stencil print inspection, automated optical inspection (AOI) and X-ray inspection to further strengthen the quality of the PCBs. Below are details explaining each inspection process:

  • Stencil Printing Inspection (SPI)

After stencil printing the solder paste on the PCBs, the position, volume, and formation of the paste are inspected by a stencil printing inspection machine. The volume of the solder paste is measured relative to the aperture volume. The alignment of the solder paste is also checked during this inspection process which is typically measured in terms of width and length.

  • Automated Optical Inspection (AOI)

Automated Optical Inspection (AOI) is a rapid inspection technique that can check for missing components, misalignment, tombstoning, and other visual defects that break the supposed image of the PCB. Using a vision system software, a camera scans and hovers over the board and with a predefined program. The captured images are compared with a reference image to determine visual faults on the PCB. Some vision parameters like resolution, brightness and lighting need to be controlled to balance both the accuracy and speed of inspection.

  • X-ray Inspection

Since AOI is limited only to the cameras’ line of sight, another inspection method needs to be done to “see through’’ the printed circuit boards. X-ray inspection is an inspection process that detects issues on the component and PCB interfaces. Soldering quality is best checked using an x-ray inspection as it can scan through the solder material. Shorting caused by solder bridging and bridging are both easily seen in an X-ray image. X-ray technique utilizes the different absorption capabilities of materials, generating unique images and patterns for voids/air gaps and components.

  • Reliability testing 

Reliability testing is also done such as environmental and physical tests which mimics the long-term application of the printed circuit boards. Examples of reliability circuit card assembly testing are vibration, bending, burn-in and thermal cycling. The ability of the boards to withstand the stresses at a defined temperature, duration and stress level defines the reliability performance of the boards. Output responses like delamination, test readings and warpage measurements are some of the important items that need to be monitored during these reliability tests.

 

Advantages of PCBA Testing

PCBA test

  • Economic Benefits

Circuit card assembly testing provides financial returns by being able to detect manufacturing defects before the parts leave the assembly floor. If the PCBA undergoes testing, the parts that have issues will no longer be shipped to the end-user, avoiding revenue losses resulting from customer returns, tedious analysis, and worst, accidents from unsafe parts. Despite the high cost of test development, hardware, software, and tester maintenance, circuit card assembly testing is beneficial in ensuring that only boards that comply with the specifications are shipped to the customers.

  • Quality Control

In every stage of PCBA, quality control is a must-have. The manufacturing errors incurred from reflow, component placement and stencil printing will be detected during testing. 100% circuit card assembly testing after assembly will also be able to determine if there are escapees from the preceding controls. With such, assembly processes can be further improved to lessen the test rejects.

  • Early Feedback System

Another main advantage of circuit card assembly testing is its immediate detection of issues right after assembly processes. Without testing, the defects may be detected in the latter electronics assembly processes which may make it harder to analyze the issue. The cost implications of late detection are also worse since more units are produced without immediate correction of the process. Early detection will give the printed circuit board assembly team sufficient time to rectify the issue and contain the affected products.

  • Customer Satisfaction

Customers anticipate a high-quality PCB upon shipment. Zero field failures and zero customer complaints are the primary quality key performance indicators (KPI) which can only be possible if rigorous screening post-assembly is implemented. Circuit card assembly testing effectively yields rejects that fail the electrical and functional requirements of the circuit boards. If zero complaints are met, supplier ratings will be higher, opening opportunities for more orders and new product introduction.

 

Conclusion

The increasing difficulty in testing arises due to the higher density and miniaturization of printed circuit boards. It has become more inefficient to access the test points due to the narrow paths and very limited space. These challenges can be mitigated through careful design, testing and prototyping. The key to effective circuit card assembly testing is to strategically partner with a PCBA assembly and testing company with sufficient expertise in test development and processing.

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