PCB Assembly Testing

In-Circuit Testing

One of the most powerful and comprehensive tools for printed circuit board test is called, In-Circuit Test (ICT). ICT test equipment uses a bed-of-nails (test probes) to access circuit noes on a board assembly and measure components on an isolated basis. That is, one component at a time, regardless of any other components electrically connected to them. Resistance, capacitance, inductance, operation of analog components, and also some function of digital circuits can be measured. Complexity of digital circuits may make full test prohibitively expensive, but otherwise ICT can be a key tool to verify printed circuit boards are manufactured correctly and therefore, will have a high probability of performing as specified.

Overview

ICT equipment measures each component one at a time to verify it’s in the proper location and of the correct value. Since most assembly defects arise from the manufacturing process consisting of shorts, opens, or wrong parts, ICT can catch most, if not all, these types of defects. When ICs fail, one major cause is static damage. These failures can be detected by ICT as well. Some testers can even test functionality of ICs to provide a higher confidence.

ICT does not functionally test the circuit assembly, so does not guarantee the assembly’s operation. Rather once the design is proved correct, it can be used to ensure the assembly has been performed correctly.

ICT equipment

• Tester-system of a matrix of drivers and sensors that are used to set up and perform measurements. This can be used for a variety of board designs.
• Fixture-the ICT system connector interfaces with a fixture. The fixture is a custom designed interface between the ICT and the individual unit to be tested. It takes the connections for the driver sensor points and routes them to the specific points on the unit under test via spring-loaded test pins or a “bed-of-nails.” This is a unique unit for each assembly tested.
• Software-is written for each board type to be tested. It controls the test system, defines the points to test, the value ranges for pass / fail criteria. The software is also unique for each assembly design to be tested.

ICT systems are relatively expensive to purchase and have a high cost of use (due to the custom fixtures and programming). So are typically used on high volume and high value assemblies.

Test coverage

Practically speaking, 100% test coverage is not possible due to:

• Physical access to all circuit nodes on the assembly.
• Low value capacitor or inductors as internal capacitance or inductance of the test system may mask accurate testing.
• System limitations for the total number of nodes vs capacity of the system. However, “implied testing” may be used to gain some level of confidence when capacity is an issue. This technique is where large sections of circuit containing multiple components are tested as a single entity.

Types of IC Testers

Several different types of testers are commonly available. Selection depends on the manufacturing / test process, production volume, and product design

• Standard - machines capable of basic resistance, continuity, capacitance, and some device functionality.
• Flying probe –simple fixture which holds the unit under test with contact made via a few probes that can move around the board and make contact as required. Movements are under software control so any board updates can be accommodated in software rather than to a physical “bed-of-nails” test fixture.
• Manufacturing Defect Analyzer (MDA) - This offers basic In-circuit test of resistance, continuity and insulation. But is limited to detection of manufacturing defects like short circuits across tracks and open circuit connections.

While ICT has many advantages and can be an ideal form of printed circuit board test, as electronic component sizes continue to decrease and densities increase, difficulties accessing all nodes becomes more and more difficult so consideration of other test techniques may be necessary.