It is vital for PCB designers to consider and incorporate in-circuit testing during the PCB design process to check and confirm connectivity.
Test Styles for In-Circuit Testing and Failure Modes
The two main types of ICT equipment are a bed of nails and a flying probe. Although they perform essentially the same function, their mechanisms are vastly different. Both require that a conductor contact a test pad placed on the board during the PCB design phase, which corresponds with some network. The flying probe uses a series of probes that rush across the board in response to programmed instructions.
The bed-of-nails, instead, is a constructed fixture where spring-loaded pins can contact the board and disconnect as needed.
Choosing which method will be used involves the consideration of time versus cost. The flying probe has an advantage: it is less expensive per design and the best choice for prototyping. The high speed of the bed of nails allows for significant time savings when working with high volumes. A fixture's cost and time would delay turnaround.
This article will concentrate on the bed-of-nails configuration, but the ICT characteristics that are less specific will still be applicable.
What can an in-circuit test detect In-circuit testing is helpful because it can see many standard failure modes.
- A short soldering error can cause a bridge to form between two traces.
- An open gap is a space in the trace's path caused by deletion or incomplete formation.
- The component orientation or placement includes an incorrect part or component.
This detection is crucial for quality reasons and has a financial motivation. The sooner you detect these defects, then the less money will be spent on rework or, in the worst cases, shipping and handling after the product reaches the store shelves.
The Test Types, Challenges, and Considerations
In-circuit tests include a variety of smaller measurements and evaluations. These subtests are grouped into pre- and post-power applications. Power-off criteria are based on resistance measurements in the system. While the power-on criterion is more diverse, it tends to measure performance rather than passive values.
Power-off
- Shorts (trace-to-trace and lead-to-lead).
- Opens
- Resistor values
- Analog component testing (both passive & active)
- Check that the jumpers/switches are in the correct location.
- Component values such as resistance
Power-on
- Check for digital components
- Orientation (analog or digital) of the components
- Verify the correct identification of components placed
- Digital signal processing (bus, clocks, etc.)
- Capacitance, inductance, and their values
The test protocol is linear and reflects the chronological order of manufacturing processes and the increasing complexity of the analysis. The first test is an open/short test that validates both the soldering and fabrication processes. The next step is testing the essential components, including measuring resistors, charging capacitors, and diode biases. ICT concludes with active analog and digital component tests.
A fixture for ICT requires access to test points on a board, as well as being able to isolate components to track individual parameters. To understand how these tests work, it is essential to comprehend ICT's structure. The first is limited only by the accuracy of the nail bed and the layout design to create boundaries between parts and surrounding pads. Electrical isolation can be a little more complicated.
Analog components can be measured easily and, at worst, only require guarding networks with shunt paths to stop current flowing through the tested component. Digital components are another matter. The system must cycle through every possible input (for example, for a logic gate with two inputs, four individual logic states would be required). The issue is further complicated because devices in the logic stream may pre-determine the inputs of a network's digital components. Back driving is a technique that can temporarily force IC inputs into the desired state. There are also similar techniques for interrupting feedback cycles and testing components that branch off of a shared bus, where it is impossible to pinpoint a failure without isolation.
Conclusion
Automation is the only way to handle in-circuit testing, a complex and exhausting process. It can quickly determine whether a PCBA can pass a list of standard failure modes in manufacturing and devices. For a bed nail, for example, it can soon be ramped up according to lot size requirements. It is an art to program an ICT and to trace any failures back to earlier points in production. An experienced assembler can use the system to diagnose and improve quality.
