Automatic test equipment ATE

- key essentials about automatic test equipment, and automated testing including: optical inspection; in-circuit test; boundary scan; functional test; etc.

ATE automatic test equipment is a vital part of the electronics test scene today. Automatic test equipment enables printed circuit board test, and equipment test to be undertaken very swiftly - far faster than if it were done manually. As time of production staff forms a major element of the overall production cost of an item of electronics equipment, it is necessary to reduce the production times as possible. This can be achieved with the use of ATE, automatic test equipment.

Automatic test equipment can be expensive, and therefore it is necessary to ensure that the correct philosophy and the correct type or types automatic test equipment are used. Only by applying the use of automatic test equipment correctly can the maximum benefits be gained.

There is a variety of different approaches that can be used for automatic test equipment. Each type has its own advantages and disadvantages, and can be used to great effect in certain circumstances. When choosing ATE systems it is necessary to understand the different types of systems and to be able to apply them correctly.

Types of ATE automatic test systems

There is a good variety of types of ATE systems that can be used. As they approach electronics test in slightly different ways they are normally suited to different stages in the production test cycle. The most widely used forms of ATE, automatic test equipment used today are listed below:

  • PCB inspection systems:   PCB inspection is a key element in any production process and particularly important where pick and place machines are involved. Manual inspection was used many years ago, but was always unreliable and inconsistent. Now with printed circuit boards that are considerably more complicated manual inspection is not a viable option. Accordingly automated systems are used:

    • AOI, Automatic Optical Inspection:   is widely used in many manufacturing environments. It is essentially a form of inspection, but achieved automatically. This provides a much greater degree of repeatability and speed when compared to manual inspection. AOI, automatic optical inspection it is particularly useful when situated at the end of a line producing soldered boards. Here it can quickly locate production problems including solder defects as well as whether the correct components and fitted and also whether their orientation is correct. As AOI systems are generally located immediately after the PCB solder process, any solder process problems can be resolved quickly and before too many printed circuit boards are affected.

      AOI automatic optical inspection takes time to set up and for the test equipment to learn the board. Once set it can process boards very quickly and easily. It is ideal for high volume production. Although the level of manual intervention is low, it takes time to set up correctly, and there is a significant investment in the test system itself.
    • Automated X-Ray inspection, AXI:   Automated X-Ray inspection has many similarities to AOI. However with the advent of BGA packages it was necessary to be able to use a form of inspection that could view items not visible optically. Automated X-Ray inspection, AXI systems can look through IC packages and examine the solder joints underneath the package to evaluate the solder joints.
    Read more about the PCB inspection systems and their technology.
  • ICT In circuit test:   In-Circuit Test, ICT is a form of ATE that has been in use for many years and is a particularly effective form of printed circuit board test. This test technique not only looks at short circuits, open circuits, component values, but it also checks the operation of ICs.

    Although In Circuit Test, ICT is a very powerful tool, it is limited these days by lack of access to boards as a result of the high density of tracks and components in most designs. Pins for contact with the nodes have to be very accurately placed in view of the very fine pitches and may not always make good contact. In view of this and the increasing number of nodes being found on many boards today it is being used less than in previous years, although it is still widely used.

    A Manufacturing Defect Analyzer, MDA is another form of printed circuit board test and it is effectively a simplified form of ICT. However this form of printed circuit board test only tests for manufacturing defects looking at short circuits, open circuits and looks at some component values. As a result, the cost of these test systems is much lower than that of a full ICT, but the fault coverage is less. Read more about In-Circuit Test systems & technology.
  • JTAG Boundary scan testing:   Boundary scan is a form of testing that has come to the fore in recent years. Also known as JTAG, Joint Test Action Group, or by its standard IEEE 1149.1, boundary scan offers significant advantages over more traditional forms of testing and as such has become one of the major tools in automatic testing.

    The main reason that boundary scan testing was developed was to overcome the problems of lack of access to boards and integrated circuits for testing. Boundary scan overcomes this by having specific boundary scan registers in large integrated circuits. With the board set to a boundary scan mode, serial data registers in the integrated circuits have data passed into them. The response and hence data passing out of the serial data chain enables the tester to detect any failures. As a result of its ability to test boards and even ICs with very limited physical test access, Boundary Scan / JTAG has become very widely used. Read more about Boundary scan / JTAG test systems and technology.
  • Functional testing:   Functional test can be considered as any form of electronics testing that exercises the function of a circuit. There are a number of different approaches that can be adopted dependent upon the type of circuit (RF, digital, analogue, etc), the degree of testing required. The main approaches are outlined below:

    • Functional Automatic Test Equipment, FATE:   This term usually refers to the large functional automatic test equipment in a specially designed console. These automatic test equipment systems are generally used for testing digital boards but these days these large testers are not widely used. The increasing speeds at which many boards run these days cannot be accommodated on these testers where leads between the board under test and the tester measurement or stimulus point can result in large capacitances that slow the rate of operation down. In addition to this fixtures are expensive as is the programme development. Despite these drawbacks these testers may still be used in areas where production volumes are high and speeds not particularly high. They are generally used for testing digital boards.

    • Rack and stack test equipment using GPIB:   One way in which boards, or units themselves can be tested is using a stack of remotely controlled test equipment.

      Note on GPIB:

      The General Purpose Interface Bus, GPIB was initially referred to as the Hewlett Packard Interface Bus, HPIB, and is also known as the IEEE 488 bus. It was develoepd as a means of controlling test equipment for ATE systems, but its use spread to other applications including providing a means of serial data transmission to printers and other peripherals. Although developed in the 1970s, it still finds applications in many test systems.

      Click for a GPIB tutorial

      Despite its age, many items of rack mounted or bench test equipment still have a GPIB capability. Despite the fact that GPIB is relatively slow and has been in existence for over 30 years it is still widely used as it provides a very flexible method of test. The main drawback of GPIB is its speed and the cost of writing the programmes although test executive packages like LabView can be used to aid programme generation and execution in the test environment. Fixtures or test interfaces can also be expensive.

    • Chassis or rack based test equipment:   One of the major drawbacks of the GPIB rack and stack automatic test equipment approach is that it occupies a large amount of space, and the operating speed is limited by the speed of the GPIB. To overcome these problems a variety of standards for systems contained within a chassis have been developed.

      Note on Chassis based test equipment:

      The idea of containing test instruments within a chassis was first developed under the VXI (VME eXtensions for Instrumentation) guise. The system uses test instruments on a card that can be slotted into a standard chassis. This saves both space and cost when compared to the stand-alone instruments as well as proving an increase in communications speed when compared to other technologies such as GPIB. Later introductions of PXI and then PXI Express provided for lower cost instrumentation while still retaining the advantages of the chassis approach.

      Click on the link for further information about VXI Instrumentation

      Click on the link for further information about PXI and PXI Express

    Although there are a variety of ATE, automatic test equipment approaches that can be used, these are some of the more popular systems in use. They can all use test management software such as LabView to assist in the running of the individual tests. This enables facilities such as the ordering of tests, results collection and printout as well as results logging, etc. Read more about Functional test systems and technology.
  • Combinational test:   No single method of testing is able to provide a complete solution these days. To help overcome this various ATE automatic test equipment systems incorporate a variety of test approaches. These combinational testers are generally used for printed circuit board testing. By doing this, a single electronics test is able to gain a much greater level of access for the printed circuit board test, and the test coverage is much higher. Additionally a combinational tester is able to undertake a variety of different types of test without the need to mover the board from one tester to another. In this way a single suite of tests may include In-circuit testing as well as some functional tests and then some JTAG boundary scan testing.

Each type of automatic test philosophy has its strengths, and accordingly it is necessary to choose the correct type of test approach for the testing that is envisaged.

By utilising all the different test techniques appropriately, it is possible to ATE automatic test equipment to be used to its fullest advantage. This will enable tests to be executed swiftly, while still providing a high level of coverage. Inspection techniques including AOI and X-ray inspection can be used along with In-circuit test, and JTAG boundary scan testing. Functional testing can also be used. While it is possible to use different types of test, it is necessary to ensure that products are not over tested as this wastes time. For example if AOI or X-Ray inspection is used, it may not be appropriate to use In-circuit testing. The place of JTAG boundary scan testing should also be considered. In this way the most effective test strategy can be defined.

By Ian Poole

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