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Test & Measurement


Testing time for PCB manufacture


Printed circuit board assemblies are central to the operation of virtually all electronic devices so their reliability is a fundamental requirement. But reliability, says Offshore Electronics’ Dave Cleal, technical director, depends on the way assemblies are tested during and after manufacture


R


eliability is like magic: just about every business indicator – from energy efficiency, safety and


environmental performance to staff morale and retention of customers – improves in companies that establish reliability as a core value.


Reliability is essentially the ability of a system or component to perform its required functions under stated conditions for a specified time. In short, it is the consistency of results from a test.


inserted into holes drilled in PCBs). The PCB mechanically supports and electrically connects electronic components or electrical components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non- conductive substrate.


As PCBAs continue to shrink in size,


increase in complexity and rise in operating voltages, reliability becomes a


the subsequent handling and assembly of the boards into the finished product. Reliability also depends on the manner in which assemblies are tested during and after the manufacturing process, as an incorrect test procedure may fail to detect production or component faults. After all, every electronic product is liable


to suffer ‘bug’ and issues, even if it is properly designed and built with utmost care. PCBA testing in the early stages of development can save millions of pounds later in the manufacturing process. But how do you test the PCBA after component population? The main methods for ensuring a PCBA is dependable are functional and in-circuit testing. Each of these has its place in the manufacturing process, but it is important to understand their differences so that effective quality control procedures are followed. The primary variance between the two is that, while a functional test quite simply checks that a device functions


In-circuit testing requires that components are tested independently against a program model containing parameters specific to the component and its functionality. This means that a faulty or missing component is automatically located without needing skilled engineering analysis. In-circuit testing is conducted with automatic test equipment – apparatus that uses automation to quickly perform measurements and evaluate the test results. This requires a ‘bed of nails’ fixture to be built for each project, which adds additional cost to the test process. A bed of nails tester is a traditional


electronic test fixture which has pins inserted into holes in a laminated sheet which are aligned using tooling pins to make contact with test points on a PCB and are also connected to a measuring unit by wires. If the product volumes are high enough, this is seldom a problem. The test


Nowhere is reliability more important than in ‘mission critical’ applications – those that are essential to the smooth operation of the business, or the very survival of the organisation. Printed circuit board assemblies (PCBAs)


are central to the operation of virtually all electronic devices from small lamp controllers to large industrial power generation equipment. In many cases, these devices must perform consistently well to ensure they don’t fail and cause a serious impact on the business. Printed circuit board assembly involves connecting electronic components on printed circuit boards (PCBs). This is a multiple-step process that includes ‘surface mount’ technology (where the components are placed directly onto the surface of PCBs) and ‘through-hole’ technology (a mounting method for electronic components that involves the use of leads on the components that are


36 April 2018


major issue, particularly because of creepage and clearance. Creepage is the separation between two PCB solder ‘eyes’. The correct creepage distance will prevent unwanted tracking – the breakdown of insulation material that makes a path to short across the isolation barrier. Clearance denotes the shortest distance through the air between two conductive parts. The correct clearance will prevent unwanted arcing. However, the reliability of PCBAs will also depend on a number of other factors, all of which will impact on the performance of the assembly in both the short and long term. These factors 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


Components in Electronics


as it should, in-circuit testing looks at each individual component to prove that it is the correct type, has been fitted to the board in the manner specified, and that it works. Functional testing looks at the finished assembly and applies inputs and power to ensure it performs as intended. It does, however, require each assembly to be able to function independently; for example, an interface assembly can’t be tested if it requires a separate control or power assembly to work unless both assemblies are tested simultaneously, or tested on a suitable test rig.


A typical 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.


program is written by engineers, designed around the specification for the board design. In circuit testing is also generally very fast, typically just a few seconds even for a complex PCBA, which results in a lower cost per PCBA tested. Ideally, a board should undergo both a types of test. However, this may not always be cost effective. At Offshore Electronics, we contract manufacture PCBAs on behalf of our customers. We prefer in-circuit testing since it offers a shorter test time and better faulty part diagnosis. However, each test method has its place and that’s why we offer both options. Our expert production and test engineers work closely with customers to determine the most efficient and cost effective methods of validating and testing their PCBA's to their required specification.


www.offshore-electronics.co.uk www.cieonline.co.uk


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