Name almost any manufactured product with a long track record of success, and you'll find one essential ingredient - a reputation for quality and reliability. This is achieved mainly by the critical manufacturing process, viz., Product Testing. The need for testing is a requirement of all market segments. Regardless of the end market, all products have to be tested before being shipped to the end-customer. This dynamic drives the pervasive nature of the Test and Measurement segment. This article provides an insight into the test & measurement trends driving the electronic assembled printed circuit board (PCBA) manufacturing and repair industry.

A printed circuit board (PCB) is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated on to a non-conductive substrate. A PCB populated with electronic components is a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA). Printed circuit boards are used in virtually all but the simplest commercially produced electronic devices.

Test and measurement plays a major role in the PCBA manufacturing, repair and rework industry. Different methodologies are being practiced today in testing and troubleshooting PCBAs. In the past, PCBAs were tested, troubleshot and repaired in the manual way using a series of bench top equipment like the digital multimeter, logic analyser, oscilloscope, signal generator and analyser and many more. Obviously, many problems still exist - thorough technical knowledge of the PCB under test, T&M equipment usage, servicing expertise supported by detailed documentation, and above all, considerable test and repair time - to name a few. Naturally, as production volumes were increasing things were rapidly getting out of control and the concept of automated test and troubleshooting took shape, and has attained considerable momentum in the last few decades.

Art of PCBA Testing

During the initial stages of automated testing in the early 1980s, the entire bench top test and measurement equipments where interconnected and controlled by a PC via General Purpose Interface Bus (GPIB) or some serial port communication. As time progressed, the demand for high speed testing led to the birth of Automated Test Equipments that where equipped with instrumentations targeting the test industry requirements.

A few decades ago, when PCBs were less complex and assembled with simple electronic components with standard DIP packages, In-Circuit Test (ICT) was the favourite or best option for automated testing. ICT tests a populated PCB in a power off condition, checking for open, shorts, resistance, capacitance, and other basic quantities, which will show whether the assembly has been correctly fabricated using bed of nails fixtures, which make contact with every test points or nodes on the PCB and are also connected to the measuring unit by wires and techniques like continuity test, nodal impedance test/VI curve trace, in-circuit active guarding and measuring where employed. To enhance the test coverage on populated PCBs In-circuit Functional Testing, popularly known as clip-on testing was introduced, where every component on the board is tested for its functionality using test clips and a technique called back driving to force the desired state to the input pins of the device under test (DUT) and measure the output state. Here, the inference is that the overall board functionality can be verified if each and every component on the board is tested and found to be good. Also card edge functional testing was employed, in areas where clip on was not feasible, or a combination of both, to increase the test coverage. As time progressed, the introduction of more complex and high functional integration in electronic devices, coupled with shrinking product sizes, lack of space on PCBs for test pads, demand for lower repair cycle time and pressure for cost effective PCBA manufacturing led to new techniques like:

• Built-in Self Test (BIST), targeting mainly the embedded and memory device, where the circuit is capable of testing itself with minimum external stimulus
• Boundary Scan (BS) for programmable devices that are compatible to IEEE 1149 standard like the FPGA, CPLD, Flash memories. The IC level access capabilities of boundary scan is utilised to perform interconnect test between BS and non-BS devices as well functional testing and In System Programming (ISP) of devices
• Bus Cycle Signature System (BCSS) for cost effective testing of processor based boards where a sampled set of signals forming the processor signature is captured and compared with similarly obtained signal from a known good board to detect faults in the board under test
• Automatic Optical Inspection - AOI is an automated visual inspection of PCBs where a camera autonomously scans the device under test (DUT) for variety of surface feature defects such as scratches and stains, open circuits, short circuits, thinning of the solder as well as missing components, incorrect components, wrong marking/labelling, component temperature and incorrectly placed components. Low costs and programming efforts make AOI a practical and powerful quality tool for both prototypes and high-volume assembles, and
• Automatic X-ray Inspection - AXI is a technology based on the same principles as AOI. It uses X-rays, instead of visible light to automatically inspect PCB assembly faults and especially ones with ball grid array, chip scale packages or flip chips where the solder connections are hidden and orthogonal optical inspection cannot reveal these solder faults.

Future PCBA Testing

The challenges in production and repair of electronic PCBAs are particularly strongly determined by developments in the component fields. The trend is towards even tighter packaging density, complex design and smaller component types coupled with largely hidden or hard to access contact points or connections, shielding plates, heat sinks, leading towards rising demands on innovative and cost effective test techniques for better fault coverage.

Looking into future automated test equipments equipped with flying probes coupled with AOI and AXI would be the next great step towards high speed and cost effective production testing. Today's flying probe system uses programmable rover probes and is capable of performing fixtureless ICT, chip on functional and boundary scan test. All of these trends fundamentally tie back to a need to increase efficiency - reducing test costs while improving productivity to keep up with increasing design complexity.

Conclusion

Each method discussed above has its own strengths and weakness, and an ATE with combination of many of these features is definitely a boon to the PCB assembly and manufacturing industry.

But which one provides the best option for a high volume low mix or a low volume high mix with high quality and value to money service is based on the strategy "As much effective as possible, as much expensive as needed". Use the effective method as much as possible and the expensive one as much as needed.

(Qmax Test Equipments Pvt Ltd, Chennai. Tel: 044-24509627. Mobile: 098402-19230. Fax: 91-44-24509632. Email: chennai@qmaxtest.com, lrkbabu@qmaxtest.com)

About the Author

V K Rajiv, works with Qmax Test Equipments Private Limited, Chennai as Product Manager - Semicon ATE Division for the past 3 plus years and leads the Design and Development of Qmax Low Cost ATE systems for the semiconductor industry. Prior to joining Qmax, he worked for Agilent Technologies, Singapore as Principal R&D Engineer and brings 8 years of experience in the semiconductor ATE industry. He completed his B.E (Electronics & Communications) from Bangalore University and M.S (Signal Processing) from Nanyang Technological University, Singapore. His areas of interest include semiconductor ATE development and market watch on technology growth in the PCB and semiconductor testing.