Cost should not be the only and primary criterion in choosing the best supplier for printed circuit board (PCB) fabrication and assembly. You can easily find a great number of suppliers that provide cheap PCB fabrication and assembly services in China, but you must consider the kind of quality that the supplier has to offer. Quality is often defined as conformance to a set of standards and in the context of PCB manufacturing and assembly, quality is ensuring that the end-product meets the established visual and functional requirements. Quality check points should be in-place at material incoming, during processing and prior shipping the parts. The PCBs should pass certain criteria like material types, copper thickness, stack up, dimensions, correctness and alignment of components, continuity of circuits and resistance., etc.
Generally speaking, quality control plays the most critical role in PCB fabrication and assembly. No electronic product will work well without sufficient quality control in production and inspection processes. In this article, we will talk about the several commonly used quality inspection methods in PCB fabrication and assembly, and we will see the importance of each quality inspection and testing method.
Commonly Use Quality Inspection and Testing Methods:
- Visual Inspection (Manual and Automated Optical Inspection)
- X-ray Inspection
- In-Circuit Testing (ICT)
- Flying Probe Testing
- Functional Testing
- Burn-In Testing
Visual inspection can be done through manual and/or an Automated Optical Inspection (AOI) machine. Manual inspection is the simplest method to check printed circuit board issues. A qualified operator inspects the boards under some prescribed magnification and checks for any abnormality based on the design requirements and work instructions. This type of inspection does not require a highly specialized machine but poses some disadvantages like limitation due to operator experience and work fatigue.
On the contrary, the AOI machine addresses the issues from manual inspection like adjustable light sources, magnification, and teachable software for image comparison. The AOI machine can detect defects in the surface of the PCBs using various magnifications and can be either 2D or 3D based on machine capability. In PCB assembly, AOI machines use image features to identify the faults in the printed circuit board. Its major parts include a camera, handler, lens, software, and a light source. The following defects can be detected:
- Foreign materials- stray conductive or non-conductive parts that are trapped into the boards
- Etching- insufficient conductive pattern on the circuit board
- Misaligned via- changing of hole position with respect to the center on the bond pads
- Scratch- dents or marks on the PCB surface
Visual inspection has some limitations in PCB fabrication and assembly, as it is not able to accurately identify defects that can cause electrical connection issues. Thus, electrical tests are still required to know the functional performance of the circuit boards.
X-ray inspection is an inspection technique that can be used to check the solder joints in the PCB and identify faults in multi-layer PCB; especially helpful in defecting the errors under BGA components, which cannot be found by visual inspections. The PCB goes inside a vacuum X-ray chamber and X-rays are allowed to penetrate the components to produce sharp images of the parts. The equipment parts include an X-ray source, a detector, and a movable platform. X-ray imaging takes advantage of the different x-ray absorption of various types of materials. Materials with higher atomic weight, density and thickness would absorb more X-rays thus producing well-defined images. Thus, x-ray inspection is helpful in screening soldering issues, missing components, voids, and solder bridging. This non-destructive type of inspection is very essential in PCB quality control as hidden flaws and potential issues are detected early, such as there are BGA components in a high-tech PCB fabrication and assembly.
Some defects can be encountered during PCB assembly even in a highly automatic production line. Common boards faults that may have functionality impact include solder bridging, broken pins, and missing components. Even if incoming materials and components are 100% good, downstream processing may induce defects on the boards. The total cost of analyzing, correcting, and replacing defective boards is high and must be avoided, thus, early and time zero detection of faults is very critical. Before packaging and shipping the printed circuit boards, the board circuitry must go through testing to ensure that the products meet the quality standards. Since printed circuit boards can contain hundreds to thousands of connections, manual testing is a tedious method as it would require connecting and re-connecting each test point to know if the parts are operating properly. Thus, a test fixture called an “In-Circuit Testing” (ICT) machine will be more beneficial for a higher throughput.
ICT machines are used to test the functionality of the circuits on the boards by detecting any faults in the circuit boards. The machine tests the component one-by-one, and the process is being done while the parts are “in-circuit” or while they are connected. If all the tests are successful, then the PCB fabrication and assembly are sorted as good. Knowing which component is causing the failure is easier for ICT since testing is being done per component. Some board faults that ICT can detect are missing components, short/open circuits, capacitance, and resistance abnormality.
To connect to each “node” of the circuit, ICT uses a fixture or test nails to be able to contact to the component leads while vacuum is activated to keep the board attached to the fixture. The tester comes with a monitor that can prompt the test operator for program selection and viewing of test status.
Flying Probe Test
The basic set-up of a Flying Probe Test consists of a work holder to keep the PCB in place and movable test pins (thus termed as “Flying Probe Test”) that can conveniently access test points in the printed circuit board. Though it is versatile and efficient, it is a test process comparably slower than ICT, that’s why it is more commonly used for small-scale and prototyping services. In this method, the Device Under Test (DUT) is conveyed into the tester while the probe moves point to point as dictated by the test program. The probe tests each connection while the receiving sensors measure the signal to detect any shorts, opens and electrical anomaly in the circuit. Flying Probe test requires no fixture and very adaptable to broad range of testing options.
In burn-in testing, the extreme and extended operating conditions of the PCB are being simulated to have an early detection of failures. This type of testing is being applied to printed circuit boards with medical, military and power applications where safety, mechanical and thermal stresses are highly critical. It is typically applied to prototype boards by running a current with high temperature ranges to analyze the boards’ functionality. During burn-in testing, the boards are loaded inside an oven with temperatures up to 125 degrees Celsius and applied with voltage to check any early failures. Engineers will be able to examine the overall performance of the PCBs under extreme parameters, to ensure the PCB fabrication and assembly will work properly in the usage environment.
Functional testing is the final test process and is a crucial step in PCB assembly to validate that the products meet the design and specification requirements. A functional tester mimics the operating environment of a printed circuit board by means of applying load while measuring the functional test items like current, voltage, analog and digital signals. In-circuit testing focuses on the performance of individual components while functional testing verifies the total functionality of the printed circuit board. Simulating the field conditions help determine if the board will be able to operate properly during application.
Any issues encountered during functionality testing help evaluate and improve the design and material selection for current or future development of boards. The test set-up includes a fixture like in ICT: signal sources, measuring apparatus, test program and screen display. A simple and most basic method is to power-up the board and check if the components are working properly as intended. Note that ICT is entirely different with functional testing as the latter is applied to finished boards with the main intent to detect defects and any design issues. Generally speaking, functional testing is the most reliable testing in PCB fabrication and assembly, since most of the functions will be tested before shipping out.
Being at the core of the electronic devices that we use in our daily lives, PCB manufacturers must have a solid and thorough quality controls to minimize and even eliminate defects and non-conformities during the PCB fabrication and assembly process. A reliable supplier should put emphasis and value to quality to ensure that the end product will perform according to requirements. The enumerated inspection techniques and test processes prevent scrappage, rework, customer returns and defects that all entail unwanted costs and non-value adding activities for both the suppliers and customers. Therefore, when trying to find a reliable PCB manufacturing and assembly supplier in China, you need to consider their quality control methods and implementation seriously.