Selecting the right ATE
08 April 2010
The ongoing reduction in size of PCBs and the components that are mounted on them, coupled with the increasing complexity of the circuits, poses a growing challenge for the engineer who needs to select an ATE system that will last.
There is a wide choice of ATE vendors and even more system integrators who can provide test solutions for assembled PCBs and end of line testing (EOLT). But choosing the best Automated Test Equipment (ATE) solution has never been a simple task, and the trend towards fitting more functionality into smaller boxes leads to miniaturisation and greater complexity of PCB designs.
This, combined with shorter product lifecycles, is now making it tougher than ever before. Rapidly changing technology makes it difficult to define test requirements even for six months into the future, but the ATE system will be expected to last for several years. The key question that needs to be answered in order to make the right system choice is, ‘Which test system is best suited to my production process, and will it still be suitable in the future?’
In order to reach at least a partial answer to this question there should be some understanding of any current manufacturing issues. This can be from personal knowledge and experience, or ideally through examination of the reports produced by a data management system that is capable of recording not only product failures, but also the associated repair or rework that was needed. When the precise nature of the problems have been established, and they have been categorised, a suitable test strategy or range of strategies can be decided.
If this historic data is not available then some basic criteria can be applied. These include:
• How many types of PCB need to be tested?
• What volume of these will be produced?
• How complex are the PCBs?
• How much electrical test access is there on the boards?
The answers to these questions can provide an initial pointer to the most suitable test strategy, and from that the most suitable test platform can be deduced.
The types of PCB and unit test systems available can be broken down into three categories:
• Inspection Method
• Automated Optical Inspection (AOI)
• Automated X-Ray Inspection (AXI)
• Structural Method
• In-Circuit Test (ICT)
• Flying Probe Test (FPT)
• Boundary Scan Test (BST)
• Functional Method
• Functional Test (FCT)
• Functional Unit Test (FUT)
• End of Line Test (EOLT)
Any one of these systems has its own merits, and there is considerable overlap between them. To maximise test coverage, each system typically has a predetermined order in any given electronic product assembly and test line, so different types may need to be deployed at different stages of manufacture.
Inspection method
The inspection types of system mentioned above are applicable to the paste and reflow stages of PCB assembly, but are unable to apply electrical stimuli to the boards under test, and for that reason have been excluded from discussion.
In every case, whether a structural or functional test strategy is to be adopted, the target system will need to be programmed with the test instructions. In the case of ICT or functional test, it will be necessary to build varying levels of ‘bed of nails’ fixturing to allow connection of the test system to the UUT (Unit Under Test).
Structural Method
The programming process for ICT normally makes use of the electrical connectivity information held in the CAD file(s) and the Bill of Materials (BoM) file. A number of processes can then be used to combine the two, and automatically produce the test programme together with the fixture build information. The next phase is then to debug the UUT against a known good test subject.
ICT systems have been the backbone of PCB test worldwide since the early 1980s. All of the ICT systems currently on the market operate by applying an electrical stimulus to the component under test and measuring its effects. The parameters that can be determined include connectivity, presence, value, orientation, operation and the detection of short circuits, so they ensure that the component has been assembled correctly and has a higher probability of overall functionality. However, due to the complexity of circuit configurations and PCB architectures, the achievable test coverage with any type of structural test system is likely to be less than 100% due to the limitations of test probe access.
FPT uses similar techniques to those of ICT, but instead of using a fixture to connect to the UUT, a number of movable probes are used to make contact with the PCB. Because of the need to move each probe physically to a specific X, Y, Z location, the test times are significantly greater when using a FPT than when testing the same UUT on an ICT system.
However, because no fixture is required, a FPT is ideal for small batch testing when quick turnaround times are key; for example with a new product introduction. Most of the FPT systems currently available also incorporate a limited degree of optical inspection, which is useful in assuring the correct build of mechanical components.
BST is a digital test technique that allows boards to be tested without the need for a full ‘bed of nails’ test fixture. Each boundary scan device has a 4 or 5 wire Test Access Port (TAP) in which the contacts are inter-connected to form a chain, with each end of the chain being connected to the test system. Each device that supports boundary scan will have a BSDL description that defines the devices’ capabilities and implementation in accordance with the IEEE 1149.1 standard
Test data can be clocked around the chain and by using special registers within each device, data can be written or read back from each device pin. This allows the interconnections between devices to be tested without requiring physical test point access.
A typical boundary scan test will include an ‘infrastructure’ test, to ensure that the chain formed by the devices is functioning correctly and the correct ID code can be read for each device. An ‘interconnect’ test will then check the integrity of the connections between the boundary scan devices; and this test will also detect any open or stuck pins. Finally, a cluster test may be performed to test non-boundary scan devices such as simple logic or memory. These tests are applied using the Boundary Scan devices as so called ‘silicon nails’. Diagnostic software is then used to analyse the data emerging from the chain, providing a user-friendly fault report.
Functional Method
Functional test in this context is the testing of a complete PCB assembly (FCT), product (FUT) or integrated system (EOLT) to evaluate its compliance against a set of specified parameters. This can be achieved in many ways, ranging from simply fitting the UUT into the final product and checking the overall functionality, to having a dedicated test platform that applies the necessary stimuli to simulate real life operating conditions and monitors the response from the UUT.
A functional test platform is normally assembled from a number of specific modules that communicate via a common bus or buses and run under a common software platform. These units have historically been 19in rack instruments controlled via GPIB from a PC. More recently, these systems are likely to be PXI instruments with soft front panels, with the addition of external instrumentation controlled over GPIB, USB, CAN, or LXI.
With the miniaturisation of electronic assemblies, and from that, the lack of testpoint access, the pendulum has swung from ICT to Functional Test where access is gained via board or unit connectors. However, this method of testing can have its downsides when reporting a faulty unit. This is due to the whole assembly being tested as a single unit or functional block (cluster), and as such the diagnostics cannot always pin point a failing component in the same way as ICT, therefore potentially making the repair a more skilled task.
Open architecture test
This leaves the PCB manufacturer with the problem that, while for their current range of products the preferred method of test may be in-circuit or flying probe, within the lifetime of the ATE system they may also need to consider functional test or system test. A machine purchased today may not meet all future needs. To justify purchasing other machines might be costly in terms of factory space, operator training, and support, as well as financially.
A tester with an open architecture, such as that used in the Aeroflex 5800 Series, which allows integration of instrumentation from third party suppliers solves this problem by allowing a single machine to be configured cost effectively for a range of different test environments. Furthermore an open approach to software will allow the simple integration of third-party drivers and other software.
The Aeroflex 5800 Series consists of PXI-based testers in three different body styles – bench-top, rack-mounted, and floor-standing – which offer a scalable, cost-effective and future-proof test system. Software is .NET compliant, enabling the use of third party software such as Teststand, Labview, C++, and Visual Basic. Aeroflex modules are available to perform analogue in-circuit and functional test, featuring up to 3456 analogue test points and up to 1152 digital test channels. The small footprint gives the system a very high pin to volume ratio, while remaining highly configurable and allowing program development to be as rapid as possible.
There are a large number of considerations when purchasing a new ATE system, and the main benefits the purchaser should particularly look for include quick application development times, minimal fixturing costs, the flexibility to perform in-circuit and functional test, boundary scan capability, minimal test time (and higher throughput), and low cost, leading to a short return on investment.
Aspects that are frequently overlooked by the purchaser are the quality of after sales support and the longevity of the product. Nevertheless, these can be the key differences between an adequate system choice and a really successful one.
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