Types of testing

The different types of test and test technologies require different test equipment. Some may require stacks of proprietary test equipment, whereas others require specific test systems designed for the particular test technique. Accordingly decisions about the type of test that need to be performed need to include elements including the optimum type of test for the given circuit assembly as well as the investment required in the test equipment. It may be that a trade-off needs to be made between where the faults are found and the cost in production against the investment in test equipment and test systems

Choice criteria for the different test types

There are many different types of test and test techniques that can be used in a production environment. The choice of technique and test technology depends upon the requirements and aspects including the complexity of the board, the density of components on the board, the level of test needed at a given stage in production, whether any tests have already been undertaken on elements of the assembly, the investment acceptable in test equipment and test systems, and any other relevant criteria for the particular test environment. In addition to this a balance between manual labour and the cost of the test equipment or test system needs to be made.

Test types

There are many different types of test and test technology from which the choice can be made. These vary in the cost of the test equipment, level of manual intervention required, the test approach and many other aspects:

• Manual Inspection:   This is very costly and not as effective as most automatic forms as people quickly become accustomed to what is presented and cannot be expected to operate either as quickly or as accurately as a computer. They are good at looking for the unusual or for resolving problems that occur.
  
  
• Automatic Optical Inspection (AOI):   This is widely used in many manufacturing environments, and 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 and the process can be altered in real time to correct the process problem. It takes time to set up and learn the board, but 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.
  
  
• X-Ray inspection:   With the advent of BGA packages this type of inspection can look through IC packages and examine the solder joints underneath to evaluate whether they are good. It does however have the drawback that only one view is used (AOI generally has more than one camera to gain a better picture of the solder joints and the board). This means that it is not as accurate as AOI.
  
  
• In Circuit Test (ICT):   In-Circuit Test, ICT has been in use for many years and is a particularly effective form of 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.
  
  
• Manufacturing Defect Analyser (MDA):   A Manufacturing Defect Analyser, MDA is effectively a simplified form of ICT. It 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.
  
  
• Functional Automatic Test Equipment, FATE:   This term usually refers to the large functional test systems in a specially designed console. They are generally used for testing digital boards but not as widely used as they used to be. 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.
  
  
• Boundary Scan:   With access to nodes on boards becoming considerably more difficult, forms of test like ICT are not always viable. To overcome the problem a type of test known as boundary scan has been developed. In very basic terms boundary scan a long data word is passed into ICs on the board that are able to be put into a boundary scan mode. The boundary scan ICs have a shift register latch on the cell adjacent to the I/O pin and in this way serial data can be passed into and out of the device, allowing a tester to control and observe the behaviour of the device and circuitry around it. Many large ICs such as microprocessors and the like are available in boundary scan versions, and by ensuring that these devices are used, access can be gained either directly or indirectly to many of the nodes on the board and a test implemented. In view of the limited access available to the nodes on many boards, boundary scan is becoming more widely used.
  
  
• "Rack and Stack" Instrumentation:   One way in which boards, or units themselves can be tested is using a stack of remotely controlled test equipment. The most widely method of controlling the test equipment is to use the General Purpose Interface Bus (GPIB). There may also be a test interface adapter required to control and interface to the item under test. Whilst the GPIB is relatively slow and has been in existence for over 30 years it is still widely used. It provides a very flexible method of test. Laboratory test equipment can often be used as most items of lab test equipment have a GPIB port. The main drawback of GPIB is its speed and the cost of writing the programmes although 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 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.
  
  The first system to be developed was named VXI (VME eXtension for Instrumentation). The VXI system is based around the VME bus it enables instruments to be designed on a single card that can be plugged into a standard chassis. As the VXI chassis are designed for mounting a 19 inch rack a considerable amount of space can be saved. A variety of instruments are available in VXI format, although there is not the selection that is available in bench mount format.
  
  Another system in widespread us is named PXI.
  
  
• Combinational test:   Often a single ICT, Functional tester, or boundary scan tester cannot provide all the functionality that is required to properly test a board. Instead elements of all three are required. As a result combinational testers that enable more than one type of test to be executed can be seen. These enable a wide variety of boards to be tested, although they may not be as comprehensive as a dedicated single type tester.
  
  

Summary

The choice of the correct type of test or test technology is particularly important when developing a new product. By choosing the correct type of testing for the product it is possible to maximise the level of test coverage while reducing the production costs.