In-Circuit vs. Functional Test

As manufacturing processes improve and circuitry has moved from discrete components to highly-integrated programmable components, effective test strategies must now place more emphasis on functional test rather than in-circuit test (ICT).

In-circuit test performs a “schematic verification” by testing individual components of a printed circuit board (PCBA) one at a time by comparison against a software model of some parameters of the component. It is not done “at speed” and does not verify interoperability but is very effective at finding manufacturing defects.

ICT excels at:

• Locating manufacturing defects such as solder shorts, missing components, wrong components, and open connections.
• Performing many tests without power applied to the device under test (DUT), thus avoiding most conditions that could damage an assembly.
• Minimizing test programming effort as the programming consists of concatenating software component models and switching test instruments onto each part via a bed-of-nails (BON) fixture.

ICT shortcomings are:

• Test fixtures tend to be expensive due to the need for an electrical connection to each electrical node of the circuit. Hundreds of spring action pins (pogo pins) are often necessary.
• Usually ICT does not test continuity through connectors, so connector faults are often not identified. Functional test (FCT) verifies that a PCB assembly functions properly by providing stimulus to an assembly and verifying the response.

Functional tests are designed to assure that circuitry functions within specifications. Testing is usually done “at speed” through DUT connectors and/or BON fixture. The number of pogo pins typically needed for a functional test fixture is significantly less than an ICT fixture.

FCT excels at:

• Identifying functional defects within a printed circuit board assembly
• Assessing functionality while applying marginal power supply voltage and/or current
• Appraising functionality of DUT while applying a range of input stimuli amplitudes or currents
• Determining DUT power consumption during operation
• Uncovering problems with analog circuitry such as:
  • Wrong oscillator frequency
  • Analog signal clipping/distortion
  • Amplifier gain or bandwidth issues
  • Drive currents of power output circuits
  • Potentiometer adjustment issues
• Revealing issues with digital circuitry such as:
  • Signal timing (design or component related)
  • Communications problems (Ethernet, DeviceNet, Serial, etc)

FCT shortcomings are:

• Test programs require a thorough understanding of the DUT performance, thus programming costs are typically higher that ICT. But, graphical programming significantly reduces the cost to implement functional tests.
• FCT often utilizes high speed instrumentation to characterize signals from the DUT. High speed equipment is more expensive than instrumentation intended for low speed measurements.
• Testing through connectors can cause reliability issues as connectors wear.  Bloomy mitigates this with sacrificial interposer cables.