PCBs are an essential part of every electronics segment. With differentiate applications; diversified PCBs are available in industry. Specialized PCBs are required for all applications, designed as per given parameters. In order to meet application requirements PCB has to be tested on designing & assembly stages. With the complexity of the applications, process & components involved, testing procedure have became a very crucial & must part of a PCB line. In this article ahead we shall talk about some of the latest trends in PCB testing.
Modern PCBs are composed of a variety of complex electrical elements, more than resistors, capacitors, diodes, transistors and fuses, consists of ICs, MCUs & micro-circuits. In order for such a complex PCB to function properly, each component must play its part: the result should be a PCB consisting of an array of conductive pathways, etched on copper sheets, which works to mechanically support a larger electronic device. If one component fails, the PCB may fail. Therefore, it’s important that each component be monitored and tested to ensure maximum performance. The challenges are added in modern PCBs by miniaturizing sizes, multilayer designs, sensitive components, specified applications (like RF or microwave PCBs) and crucial operational conditions. All these factors have made PCB designing an essential & sophisticated process in PCB manufacturing line. In this article, we shall explore some of the challenges & latest PCB testing methods.
Testing a PCB achieves 50% goal of testing entire furnished product at a very initial stage of designing & manufacturing.
Design – For – Test – A Costly PCB design rework
In the design phase of many printed circuit boards, the engineer is more interested in functional testing; making sure that the system he has developed meets the original specifications. Little thought may be given to incorporating features that allow visibility into the board to make sure it meets specs of the in-house manufacturing associates or the outside manufacturing company. At the design and layout stage the assumption is often made that manufacturing will be flawless, allowing the development team to concentrate on functional tuning of the product. Unfortunately the test engineer at the manufacturing house is seldom given due consideration during design. The result is that when manufacturing personnel test the assembly of that particular board, it is a difficult, if not impossible, task. The critical access or test points that would make the board more easily testable are not there. If DFT requirements had been considered, the probes of a flying probe tester would have incorporated the necessary contact points to make contact on the circuitry for measurement purposes.
Functional Test of PCB
Functional test (FCT) is usually the final step in manufacturing process. It provides a pass/fail determination on finished PCBs before they are shipped. An FCT’s purpose in manufacturing is to validate that product hardware is free of defects that could, otherwise, adversely affect the product’s correct functioning in a system application. FCT verifies a PCB’s functionality and its behavior. It is important to emphasize that the requirements of a functional test, its development, and procedures vary widely from PCB to PCB and system to system. Functional testers typically interface to the PCB under test via its edge connector or a test-probe point. This testing simulates the final electrical environment in which the PCB will be used. Functional test simulates the operating environment for the product under test thereby minimizing the expensive cost for the customer to provide the actual testing equipment. It eliminates the need for expensive system tests in some cases, which saves the OEM lots of time and financial resources. It can check the functionality of the product anywhere from 50% to 100% of the product being shipped thereby minimizing the time and effort on the OEM to check and debug it. Prudent testing engineers can extract the most productivity out of functional test thereby making it the most effective tool short of system test.
In-Circuit Testing, ICT for a furnished PCB
In circuit testing (ICT) uses a bed-of-nails test fixture to access multiple test points on the PCB’s bottom side. With sufficient access points, ICT can transmit test signals into and out of PCBs at high speed to perform evaluation of components and circuits. The bed of nails or in-circuit tester such as 3070 from Agilent is the most common and popular in the contract manufacturing environment. ICT provides OEM customer such benefits as, ICT covers 100% testing so that all power and ground shorts are detected. ICT testing does power up testing and eliminates customer debug needs to almost ZERO.
Automatic Optical Inspection
PCB designs are becoming more complex and components smaller and smaller. Today, the abilities of the human eye are mostly not sufficient for reliable quality control. Also, zero-defect strategy is required now a day in almost all applications. All these factors have essentialate the requirement of a inspection systems with high-performance camera modules. Such Automated Optical Inspection (AOI) Machine allows operator conducts non-contact tests on bare board and assembled printed circuit boards (PCB) at different stages of PCB assembly.
Such AOI units normally consists of a operating system, to enable programming for measurement & comparison functions like, conducting automated checks on bare boards or assembled PCBs and to detect known variations such as scratches, stains, open, shorts, solder thinning, missing, incorrect value or incorrectly placed components. An AOI can perform test for faults like, Print shifts, Scratches, Finger prints, Screen aging, Excess paste, Missing paste, Contamination, Screen defects (holes), Wrong Screen, Substrate shrinking, Wrong position or diameter or non-circularity, not filled Ditches (divots) or elevations.
Solder Paste Inspection System (SPI): Ensures 100% soldering performance
Solder paste deposition is the key process in board assembly operations using SMT techniques. Solder paste printing is a complicated process and many factors can contribute to its accuracy, including composition and rheology of the paste, stencil used and the type of squeegee and process conditions used to lay the paste down. Since the solder paste printing is an important process, it is very important to test & verify the amount & height of solder paste printed, as distraction in microns can cause short or open circuit in later stages.
SPI equipment is used in Printed Circuit Board manufacturing to monitor and control one of the most crucial steps affecting the finished quality of circuit board. SPI systems measure the height and volume of the solder pads before the components are applied and the solder melted, and when used properly, can reduce the incidence of solder-related defects to statistically insignificant amounts. Critical to the SPI measurement is the accuracy of the height measurement because that has a direct correlation with solder volume and defects. The importance of accurate height measurement in SPI, is signified by the fact that that accurate solder height inspection helps not only the finished quality of the product, but also reduces the inspection time required thereby positively impacting productivity and therefore the manufacturing cost.
X-Ray Inspection for Multilayer PCBs
Automated X-ray inspection (AXI) is a technology based on the same principles as automated optical inspection (AOI). It uses X-rays as its source, instead of visible light, to automatically inspect features, which are typically hidden from view. The increasing usage of ICs and BGAs, where the connections are underneath the chip and not visible, have made ordinary optical inspection is impossible. Because the connections are underneath the chip package there is a greater need to ensure that the manufacturing process is able to accommodate these chips correctly. Additionally the chips that use BGA packages tend to be the larger ones with many connections. Therefore it is essential that all the connections are made correct. To meet these increasing complex demands, X-Ray inspection systems, came to picture, which can inspect, Test & measure Inner layers of PCBs & Underneath solder joints.
In addition, X-ray inspection systems are used wherever defects need to be detected by non-destructive means. The spectrum of use is broad and includes a wide range of different sectors and inspection tasks, from testing of materials for cracks and air inclusions to foreign matter inclusions and shape deviations. In the electronics industry, too, the increased use of miniature housings and the trend toward moving components inside the electronic assembly require high quality inspection that catches hidden defects cost-effectively and with certainty. The X-Ray & CT inspection system can be used to detect following defects:
- Broken wedge bonds
- Lifted ball bonds
- Wire sweep
- Die attach
- Populated and unpopulated PCBs
- View surface mount defects i.e. misaligned devices, solder joint porosity, bridging
- Detailed inspection of vias, through hole plating and multi-layer alignment
- BGA and CSP inspection
- Non-lead solders inspection.
JTAG, Joint Test Action Group Standard
Joint Test Action Group, also known as JTAG, is the common name for IEEE std 1149.1. This standard defines a particular method for testing board-level interconnects, which is also called Boundary Scan. The JTAG hardware interprets information from five different signals: TDI (Test Data In), TDO (Test Data Out), TMS (Test Mode Select), TCK (Test Clock), and TRST (Test Report-optional).
The primary advantage of boundary-scan technology is the ability to observe data at the device inputs and control the data at the outputs independently of the application logic. Simple tests can find manufacturing defects such as unconnected pins, a missing device, an incorrect or rotated device on a circuit board, and even a failed or dead device. JTAG defines several different configurations with a few of these being an 8 position connector, a 14 position connector, and a 20 position connector.
Conclusion
“Prevention is better than Correction”, signifies the requirement of a testing stage in PCB designing & assembly line, especially when the case if of sophisticated & costly PCBs like tablets or aerospace. With the advanced PCB testing and measurement devices we can achieve “Zero Defect” in furnished PCBs.
