Board approval: the benefits of PCBA testing
03 September 2018
The majority of electronic devices rely on printed circuit board assemblies (PCBAs) – and it is critical that these are properly tested to identify any production or component faults, as this piece explains…
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PCBAs are crucial to the operation of electronic products – from small consumer electronics devices to large-scale industrial equipment. PCBA reliability is critical, and depends on a variety of important factors, including: the original board design; the specification and quality of components and materials used; the quality of the assembly process; production conditions; and the skills of machine operators.
However, one factor that is often overlooked is the way in which assemblies are tested – both during and after the manufacturing process – because an incorrect test procedure may miss production or component faults.
There are various test methods to prove the integrity of PCBAs. The two most commonly used are functional testing and in-circuit testing. While each has its place in the manufacturing process, it is important to understand the differences between them in order to implement effective quality control procedures.
In essence, a functional test does exactly what it says: it checks that a PCBA functions as expected. In-circuit testing, by comparison, examines each individual component on a PCBA – checking that it is the correct component, has been fitted correctly, and that it works. Exploring these two approaches highlights significant differences between them.
Functional testing looks at the complete, finished assembly. It applies inputs and power to ensure that the PCBA performs its desired function to a certain specification. However, the test requires each assembly to function independently: so, if an interface assembly requires a separate control or power assembly to function, both assemblies must be tested simultaneously – or a suitable test rig will need to be constructed. The test may also depend on additional data regarding the specification of associated boards, or of the finished device.
Usually, a functional test only proves that the assembly basically operates. It does not generally detect the presence of faults in parts used as protection elements, which are inactive under normal operating conditions.
At the same time, an assembly that fails a functional test will often need skilled analysis to identify and rectify the root cause of the failure. Functional testing is also good at detecting problems in analogue circuits – such as incorrect oscillator frequencies, or potentiometer adjustment issues – and determining a device’s power consumption during operation.
Although in-circuit testing (ICT) might include a degree of functional testing, its main purpose is to validate each component. It is more comprehensive, as components are tested
independently against a program model: one that contains parameters specific to the component and its functionality. This is carried out in a structured manner – initially without power – to test for short or open circuits, followed by testing passive components and simple semiconductors. Afterwards, power is applied to allow the more complex semiconductors to be tested.
The strength of this approach is that faulty or missing components are automatically located without needing skilled engineering analysis.
In-circuit testing is performed using automatic test equipment (ATE). This means building a ‘bed of nails’ fixture for each project, which adds cost to the test process. However, if product volumes are high enough, this is seldom a problem. The test program is written by test engineers, designed around the specification for the board design.
In-circuit testing is also generally very fast, typically just a few seconds for even a complex PCBA, which results in a lower cost per PCBA tested.ICT is generally excellent at detecting manufacturing defects, such as missing components or open connections – and carrying out tests without power, thereby avoiding the chance of damaging an assembly.
In addition to these testing methods, PCBA quality can also be assessed just with a closer look: for example, at Offshore Electronics, we recently began using two Blundell Inspex HD 1080p full digital microscopes to further enhance the job of assessing quality. Prior to this, we used a Mantis static fixed bench magnifier to assess the PCBAs. It had two magnification zoom options (x4 and x10) and allowed one user to see the PCBA being assessed. The new digital microscopes link to HD screens, so their images can be seen by the whole team – making the process more collaborative. Magnification can now also be adjusted anywhere between x1.8 and x98.
The microscopes have a removable USB storage image capture facility, so analysed images can be saved and uploaded. Quality assessors can add the images to the build information on the tablets used by production staff – who can immediately see any issues that need to be corrected on the production floor. All the images are also added to the company’s compendium of inspection knowledge, so that any faults can be recorded, meaning lessons learned for future production. Adding the digital microscopes means that more PCBAs can be checked at any one time – boosting efficiency in the process.
Both functional and in-circuit testing have their place. Ideally, the two are used together to confirm the complete integrity and functionality of each PCBA. However, this is not always cost-effective – and a decision should be taken as to which method is most appropriate.
At Offshore Electronics, we tend to advise using in-circuit testing, because it offers a shorter test time and better diagnosis of faulty parts. Once a fault is found, the ATE quickly identifies the problem – and the fault can easily be repaired. For functional test failures, faults must be diagnosed using engineering resources.
In practice, we offer both options – with production and test engineers working closely with customers to determine the best, most cost-effective way of validating and testing PCBAs to the required specification.
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