What is PCB Electrical Testing?

PCB electrical testing is the process of verifying that a printed circuit board functions as intended from an electrical standpoint. It involves checking for short circuits, open circuits, incorrect component placement, and other manufacturing defects through various test methods. The goal is to ensure the PCB meets all design specifications before it is installed in the final product.

Electrical testing is a critical step in PCB manufacturing, as defective boards can lead to malfunctions or complete failure of the end device. Thorough testing helps catch issues early, reducing scrap and rework costs.

Types of PCB Electrical Tests

There are several types of electrical tests commonly performed on PCBs:

In-Circuit Test (ICT)

In-circuit testing verifies the proper assembly of PCB components using a bed-of-nails fixture to access individual component leads. It measures resistance, capacitance, and other parameters to check components and ensure the absence of shorts, opens, and incorrect parts.

Flying Probe Test

Flying probe testers use movable test probes to access test points on a PCB, performing similar checks as an ICT. Flying probe is often used for prototypes or low-volume production where a bed-of-nails fixture is cost-prohibitive.

Boundary Scan Test

Boundary scan, defined by the IEEE 1149.1 standard, uses a built-in test architecture to test connectivity and functionality of digital ICs. Test patterns are sent to the IC’s boundary scan cells to verify interconnects between chips.

Functional Test

Functional testing powers up the PCB and runs it through operational sequences to verify proper functionality. It may involve connecting the board to external equipment to simulate inputs and measure outputs.

PCB Electrical Test Procedures

The typical flow for PCB electrical testing is:

1. Visual inspection
2. Automated optical inspection (pre-test)
3. Electrical test (ICT, flying probe, and/or boundary scan)
4. Functional test
5. Burn-in (for some applications)
6. Final inspection and packaging

Visual Inspection

Before electrical testing, boards go through visual inspection to check for obvious defects like incorrect component placement, solder bridges, lifted pads, etc. Automated optical inspection (AOI) equipment may be used to quickly scan boards and identify visible issues.

Electrical Test

Next, the appropriate electrical test methods (ICT, flying probe, boundary scan) are used based on board complexity, test coverage requirements, and production volume. These tests comprehensively verify board connectivity and component correctness.

For an ICT, the PCB is placed on the tester’s bed-of-nails fixture, which has spring-loaded pogo pins that make contact with test points on the board. The tester then runs through a preprogrammed sequence of resistance, capacitance, inductance, and other measurements to check for defects.

Flying probe and boundary scan tests operate similarly, probing the board and running test sequences to check for issues. Boundary scan requires chips with boundary scan architecture and focuses on interconnect testing between ICs.

Functional Test

After electrical testing verifies the PCB itself is assembled correctly, functional testing checks that the complete board operates as intended. The specifics of functional test vary widely based on the board’s end application.

Typically, the PCB is powered on and connected to equipment that provides inputs and monitors outputs to simulate operation. For example, a computer motherboard would be connected to a power supply, hard drive, display, etc. and run through POST and other diagnostics.

Functional failures can stem from design issues, incorrect programming, or component defects not caught by previous testing. Comprehensive functional testing is important to verify the board is ready for installation in the final product.

Burn-In

For high-reliability applications like aerospace, defense, and medical devices, PCBs may undergo a burn-in period to stress the board and identify early component failures. Burn-in involves operating the board at elevated temperature and voltage conditions for an extended time (up to several days).

Final Inspection

Finally, boards go through a last visual check for any handling damage from testing and are packaged for shipment. Passing boards are typically sealed in ESD bags to prevent electrostatic discharge damage.

Electrical Test Fixtures

For volume PCB production, custom test fixtures are usually required to interface the board with the tester. Common fixtures include:

Bed-of-Nails ICT Fixture

A bed-of-nails fixture has an array of spring-loaded pogo pins that align with test points on the PCB. When the board is placed on the fixture, the pins make contact and allow the tester to access the test points.

Fixtures are CNC machined from materials like G10/FR4 and have top and bottom plates to secure the PCB. The bottom plate has holes drilled for each pogo pin, which are press-fit into place. Alignment pins ensure the PCB is correctly positioned.

Flying Probe Fixture

While flying probe testing doesn’t require a full bed-of-nails, a fixture is still used to secure the board and provide a consistent reference for the probes. Flying probe fixtures are typically simpler, with just alignment pins and a top plate to hold the PCB.

Functional Test Fixture

Functional test fixtures are highly application-specific and provide any necessary interface connections for power and I/O. They may have an enclosure to contain the PCB, connectors/cables for external device hookup, and a base plate for mounting.

Careful fixture design is important for repeatable, reliable electrical testing. Key considerations include:

• Choosing durable, precision-machined materials
• Designing for sufficient probe/pin contact force and stroke length
• Accommodating PCB size and component placement variations
• Providing ESD protection
• Allowing easy loading/unloading of boards

Test Coverage and Yields

The level of test coverage required depends on the end application and reliability needs. More mission-critical products demand extensive testing to ensure a very low defect rate.

Typical targets for different tests include:

Test Type	Coverage Target
AOI	80-90%
ICT	85-95%
Flying Probe	70-80%
Boundary Scan	80-90%
Functional Test	90-100%

Achieving high first-pass yields is the goal – this means a high percentage of boards pass all required tests the first time through, without any debug or rework. Good design for testability (DFT) practices, like including test points and JTAG interfaces, help enable high-coverage, high-yield testing.

Overall yields in the 90-95%+ range are considered very good. Mature, optimized SMT assembly lines can consistently achieve 98-99% first-pass yield with a comprehensive test strategy.

PCB Electrical Test Equipment

A variety of equipment is used for PCB electrical testing, from all-in-one automated systems to handheld multimeters. Common types include:

In-Circuit Testers

Bench-top or floor-standing systems for testing PCBs on bed-of-nails fixtures. Offer high speed, high coverage testing. Examples: Teradyne Spectrum, Keysight i3070.

Flying Probe Testers

Automated systems that use movable test probes to access test points on the PCB. Slower than ICT but don’t require custom fixtures. Examples: Takaya APT, SPEA 4060.

Boundary Scan Testers

Systems that interface with boundary scan-enabled ICs for interconnect testing. May be standalone or combined with ICT/flying probe. Example: JTAG Technologies JT 5705.

Multimeters

Handheld or bench-top instruments that measure voltage, current, resistance, and other parameters. Used for quick checks and debug. Example: Fluke 87V.

Oscilloscopes

Bench-top instruments that display signal waveforms to check timing, noise, and other parameters. Important for functional testing and debug. Example: Tektronix MSO58.

Hipot Testers

Used to verify PCB insulation and dielectric strength at high voltages. Important for safety testing. Example: Associated Research 8106.

When selecting equipment, key considerations include test coverage capabilities, speed, accuracy, and ease of programming. Compatibility with existing fixtures and other equipment is also important.

Investing in high-quality, versatile electrical test equipment enables thorough PCB verification and can significantly improve overall manufacturing quality and efficiency.

Design for Testability

To enable efficient, high-coverage PCB electrical testing, it’s important to incorporate testability features during the design process. This design for testability (DFT) approach includes:

Test Points

Adding test points to the PCB layout provides access for testers to probe the board. Test points are typically plated through-holes or surface mount pads placed on a grid to match the tester’s probes.

For ICT, test points are needed to access component leads and check for shorts/opens. Boundary scan reduces the number of test points required but still needs access to power, ground, and scan chain signals.

JTAG/Boundary Scan

Including JTAG (Joint Test Action Group) interfaces and boundary scan architecture on the PCB enables efficient testing of IC interconnects. JTAG, defined by the IEEE 1149.1 standard, uses a simple 4/5-wire interface to access boundary scan registers on compliant chips.

Boundary scan cells at each device pin can capture and control pin states, allowing verification of connections between ICs without physical access. This is especially useful for high-density PCBs with fine-pitch components that are hard to probe directly.

Built-In Self-Test

Some ICs include built-in self-test (BIST) features that allow them to test themselves and report results. This can reduce the need for external test hardware and improve coverage.

Common BIST functions include internal scan chains, memory BIST to test embedded RAM, and analog BIST for mixed-signal circuits. Standards like IEEE 1500 define methods for embedding BIST in ICs.

Functional Test Hooks

Designing in test hooks for functional testing can greatly simplify test development and debug. Examples include:

• Test modes that put the board in a known state or provide access to internal nodes
• UART/JTAG headers for boundary scan and debug
• Connectors for attaching external test equipment
• Light pipes or other indicators to show board status

Considering functional test needs early in the design process helps ensure the PCB can be efficiently tested in production.

Design Reviews

Holding testability design reviews with the test engineering team is important to catch any issues and optimize the design for testing. Reviews should look at:

• Test point placement and accessibility
• Signal integrity and crosstalk concerns
• Power/ground stability and isolation
• JTAG and boundary scan implementation
• Clearance for test fixtures
• Functional test requirements

Addressing testability upfront avoids costly design revisions later and enables a smooth transition to production testing.

FAQ

What is PCB electrical testing?

PCB electrical testing is the process of verifying the electrical functionality and integrity of a printed circuit board assembly. It checks for manufacturing defects like shorts, opens, and incorrect components to ensure the PCB operates as intended.

Why is PCB Testing important?

Electrical testing is critical to ensure PCB Quality and reliability. Defects can lead to malfunctions or failures in the end product, so catching them early in manufacturing avoids costly scrap and rework. Testing also helps validate the PCB design and component choices.

What are common PCB electrical tests?

Common tests include in-circuit test (ICT), flying probe test, boundary scan test, and functional test. ICT uses a bed-of-nails fixture to probe the PCB, while flying probe uses movable probes. Boundary scan checks IC interconnects via JTAG, and functional test verifies overall operation.

What is design for testability?

Design for testability (DFT) is the practice of designing PCBs with features to enable and simplify testing. Examples include adding test points, JTAG interfaces, and built-in self-test capabilities. DFT helps achieve high test coverage and reduce test costs.

What equipment is used for PCB electrical testing?

A variety of equipment is used, including in-circuit testers, flying probe testers, boundary scan testers, multimeters, oscilloscopes, and hipot testers. The specific equipment needed depends on the PCB complexity, test requirements, and production volume.