What is In-Circuit Testing

Rush PCB Inc makes all types of PCBs, starting from simple ones, to complex multi-layered ones, and HDI types. Apart from multiple layers, a complex HDI PCB can have hundreds of components with thousands of solder joints and traces covering almost the entire PCB surface.

Printed circuit board users expect each component on the board to perform according to the designer’s intentions. However, even a minor defect in the board could defeat the performance and overall quality. In-circuit testing or ICT is one way to test the PCB assembly to ensure it performs flawlessly. With ICT, it is possible to check each component on the board and verify its characteristics for any imperfections. This is the main reason why the electronics industry relies upon ICT methods.

What is In-Circuit Testing?

ICT or in-circuit testing is a method of checking a PCB assembly for the functioning of individual components, thereby certifying their collective performance. The operator places electrical probes at individual points to check component characteristics. With ICT, it is possible to check for basic characteristics of passive components like resistances, capacitors, inductors, and switches. It can detect shorts and opens very easily.

ICT can be a fully automated method of testing, flagging any discrepancy to the operator, who can correct it by replacing the faulty component. Typically, the operator conducts an ICT process in two phases—Power-off testing and Power-on testing. As suggested by the names, a power-off test involves testing components before the application of power, while in a power-on test, the operator tests the board after they have supplied it with the necessary power.

Defects Identified with ICT

ICT is popular as it can uncover numerous defects in boards. The thorough test coverage of ICT makes it a very useful testing method to deploy. While there are other test methods to check the functioning of a board, ICT offers the advantage of checking individual components. In fact, with an ICT, it is possible to check for:

• Open Circuits
• Short Circuits
• Solder Bridging
• Missing Components—active and passive
• Wrong Components
• Wrong Orientation of Active and Passive Components
• Soldering and Process Issues
• Resistor Values
• Capacitor Values
• Inductor Values
• Position of Jumpers

Defects Not Identified by ICT

ICT has its shortcomings. There are some cases where ICT fails to detect the defect. These include:

• Missing Mechanical Fixings
• Missing Decoupling Capacitors
• Redundant Power Connections
• Missing Multi-Power Connections
• Surface Contaminants on the Board

Types of In-Circuit Testers

Two types of basic in-circuit testers are in popular use—the bed-of-nails testers, and the flying lead testers.

Bed-of-Nails Tester

This is a fixture-based tester with multiple pins. While designing the PCB, the designer defines a set of points where the probes of the tester will engage. These points carry a testing pad, and the designer provides the x-y coordinates of these test pads, usually set to a grid.

The fabricator of the test jig uses these coordinates to locate the testing pins. The testing pins are usually spring-loaded and with multiple prongs to make a proper electrical connection. Each contact pin connects to a microcontoller-based testing circuit that runs a program for testing various parameters.

Although the bed-of-nails testing jig can test many boards in a short time, it is a dedicated jig for a specific circuit board. This makes it rather expensive, and manufacturers use such a testing jig only for testing boards in large volume production.

Flying-Lead Tester

This type of in-circuit testing offers a more flexible method. It involves a fixture with two to four electrical leads that are flexible enough to connect to various testing points on the PCB. As flying-lead testers are not dedicated to any specific board design, operators can deploy this versatile testing method easily and quickly when a production line changes to manufacturing a different printed circuit board. As testing with flying leads requires sequential process steps, this method is not as fast as the bed-of-nails tester.

Whatever type of in-circuit tester is in use, certain elements are common to such testers:

• A fixture—bed-of-nails or flying leads
• A controller board to which the fixtures connect
• An interface between the controller board and the fixture
• Analog to Digital converters
• Software for programming the controller

ICT provides reliable and high-quality testing for the high-volume production of PCBs. It ensures that the board assembly will perform as intended by its designer.

Advantages and Disadvantages of Using ICT

Using ICT for testing PCB assemblies have their advantages and disadvantages. Operators and OEMs desirous of using ICT methods must understand the scope and applicability of these versatile testers.

Advantages of ICT

ICT methods are popular on account of their simplicity, speed, and accuracy in locating defects in boards. Most PCB manufacturers employ this method of testing because ICT:

• Is easy to program.
• Detect defects in components, shorts, opens, and other faults.
• Has lower probability of operator error.
• Can test in high speed, good for high throughput assembly lines.
• Is comprehensible and reliable compared to other testing methods.
• Is a thorough process covering manufacturing defects, assuring high fault coverage.
• Is suitable for both through-hole components and surface mount components.
• Has low maintenance requirements.

Disadvantages of ICT

Despite the above advantages, ICT methods have a few shortcomings as well:

• Test fixtures can be expensive to develop and manufacture.
• Probe spacing can be a limitation, especially for high-density SMT components, like connectors.
• Inconsistencies can result from improper contact of test pins due to the presence of flux on test pads.
• It is necessary to keep the test pins clean and replace them frequently if they wear out and result in false calls.

However, these flaws of the ICT method are easily overcome, and the advantages outweigh the disadvantages by a large margin.

Conclusion

Testing a PCB assembly is crucial in establishing high quality in the manufacturing process. Rush PCB Inc recommends a proper understanding of ICT methods to take the best advantage of this technology. Our staff has extensive knowledge not only of ICT methods but also of other testing methods. Feel free to reach out to us for your next project, and we will happily offer our best advice.