FEATURE TEST & MEASUREMENT
TESTING PRINTED CIRCUIT BOARD ASSEMBLIES: WHAT'S BEST FOR YOU?
Offshore Electronics' Dave Cleal, technical director, describes the two principal methods of printed circuit board assembly testing and explains their respective advantages and disadvantages
W
hether it's a small item of consumer electronics or a large-
scale industrial controller, it will inevitably contain one or more printed circuit board assemblies (PCBAs), the reliability of which will be critical - not just for the end-user whose application may be fault intolerant, but also to the reputation of the PCBA supplier and the future health of his business. While the reliability of PCBAs is a prerequisite, this will depend upon a number of factors, all of which will have an impact on the performance of the assembly in the short and long term. These include the original board design; the specification and quality of the components populating the board and any protective coating applied to them; the care with which each assembly is manufactured; the production conditions and skills of surface mount and wave soldering machine operators, and the subsequent handling and assembly of the boards into the finished product. Reliability also depends on the manner
in which assemblies are tested, both during and after the manufacturing process; an incorrect or inappropriate test procedure may not detect those production or component faults that ultimately lead to failure in service. There are various test methods to prove the integrity of PCBAs; however, the two most commonly used are functional testing and in-circuit testing. Although each has its place in the manufacturing process, it is important to understand their differences so that effective quality control procedures are followed. Before we look at each in more detail, the primary difference is that while a functional test quite simply checks that a device functions as it should, in-circuit testing looks at each individual component to prove that the correct type, has been fitted to the board in the manner specified, and that it works. Exploring these two approaches further, highlights significant differences.
FUNCTIONAL TESTING
Functional testing looks at the complete, finished assembly and applies inputs and power to ensure the assembly performs
16 MAY 2018 | ELECTRONICS Offshore Electronics
www.offshore-electronics.co.uk T: 01481 712721
/ ELECTRONICS
components are tested independently against a program model containing parameters specific to the component and its functionality. This is performed in a structured
Figure 1: In-circuit testing
as intended. The test does, however, require each assembly to be able to function independently; for example, it is not possible to test an interface assembly if it requires a separate control or power assembly in order to function unless both assemblies are tested simultaneously, or tested on a suitable test rig. The test may also depend on additional data relating to the specification of associated boards or of the finished device. A functional test only proves the basic operation of the assembly and is unlikely to detect the presence of faults in parts, such as circuit protection elements, which are inactive under normal operating conditions. Furthermore, an assembly failing a functional test will often require skilled analysis to identify and rectify the root cause of failure.
Fig. 2: PCB
IN-CIRCUIT TESTING Although in-circuit testing can include a degree of functional testing, its primary purpose is to validate each component. In some respects it is more comprehensive than functional testing as
manner, initially without power, to test for short or open circuits, followed by the testing of passive components and simple semiconductors. Power may then be applied to facilitate the testing of more complex semiconductors. The advantage of this approach is that the faulty or missing component is automatically located without needing skilled engineering analysis. In-circuit testing is performed using automatic test equipment (ATE) that usually includes a ‘bed-of-nails’ fixture that must be built for each project. While this adds cost to the test process, providing that the product volumes are high enough this is seldom a problem. The ATE is loaded with a test program compatible with the board design. In circuit testing is rapid, typically taking just a few seconds even for a complex PCBA, which reduces the cost per PCBA tested.
BEST OF BOTH WORLDS? In an ideal world, a board should undergo both a functional and in-circuit test. However, this may not always be cost effective and a decision then has to be made as to which method is most appropriate. Offshore Electronics, claims to prefer
in-circuit testing since it offers a shorter test time and better faulty part diagnosis. If a fault is detected the ATE quickly identifies the problem and a repair can be undertaken; remember: for functional test failures, faults need to be diagnosed using engineering resources. In practice, the company offers both
options, its production and test engineers work closely with all its customers to determine the most efficient and cost effective methods of validating and testing their PCBA's to their required specification.
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