Fixture Design (ICT/FCT): Tackling the Challenges of AI Chip Interconnection and Carrier PCB Packaging with High-Speed Interconnects

Under the wave of artificial intelligence (AI) and high-performance computing (HPC), the design complexity of AI chip substrates and PCBs has grown exponentially. High-density interconnects, tens of thousands of BGA pins, and transient current demands of hundreds of amperes pose unprecedented challenges to product quality and reliability. In this demanding context, exceptional Fixture design (ICT/FCT) is no longer just a final step in the production process but a core pillar throughout the entire lifecycle of design, manufacturing, and validation, directly determining whether AI hardware can achieve successful mass production and stable operation.

As power integrity engineers, we understand that every millivolt of voltage drop or picosecond-level timing deviation can lead to catastrophic system failures. Therefore, the importance of a test fixture capable of accurately simulating real-world operating conditions while not interfering with the performance of the device under test (DUT) is self-evident. This article delves into Fixture design (ICT/FCT) in the context of AI chip substrates, analyzing its key challenges and solutions in signal integrity, power distribution, mechanical precision, and integration with advanced manufacturing processes.

Why Is AI Substrate Testing So Complex and Critical?

Traditional PCB testing methods fall short when applied to modern AI substrates. These substrates typically integrate 2.5D/3D-packaged AI accelerators, high-bandwidth memory (HBM), and numerous high-speed I/O interfaces. Their complexity manifests in the following aspects:

Ultra-High Density and Fine Pitch: The BGA solder ball pitch of AI chips has shrunk to 0.4mm or even smaller, with tens of thousands of densely distributed connection points. This demands test probes with extremely high positioning accuracy and stability, as even the slightest deviation can lead to poor contact or damage to expensive chips.
Stringent Electrical Performance Requirements: High-speed buses like PCIe 5.0/6.0 and CXL are highly sensitive to impedance matching and signal attenuation. The test fixture itself must not become a bottleneck for signals and must be co-designed as part of the entire high-speed channel.
Massive Power Consumption and Thermal Challenges: AI chips can consume hundreds of watts under full load, with rapid transient current fluctuations. The FCT (Functional Test) fixture must provide stable, clean high-current power and effectively manage heat generated during testing to prevent the DUT from throttling or failing due to overheating.
Tight Coupling of Manufacturing and Assembly: From IC substrate manufacturing to final SMT Assembly, minor tolerances at each stage accumulate and impact testability. Thus, design for testability (DFT) must be implemented during the design phase, and test strategies must align with manufacturing capabilities.

In this context, First Article Inspection (FAI) becomes the first critical checkpoint to verify whether the design and manufacturing processes are aligned. Through comprehensive inspection of the first article, FAI can identify systemic issues early, avoiding risks in mass production. The foundation of effective FAI execution lies in meticulously designed test plans and fixtures.

Core Differences and Synergies Between ICT and FCT Fixture Design

When discussing the complexities of Fixture design (ICT/FCT), it is essential to first clarify the roles and distinctions between the two core testing strategies:

In-Circuit Test (ICT): The primary goal is to verify component-level correctness. It accesses test points on the PCB to check each component's parameters (e.g., resistance, capacitance), pin soldering accuracy (open/short circuits), and basic functionality (e.g., diode polarity). ICT fixtures are often referred to as "bed-of-nails," with a design focus on achieving physical access to all critical network nodes.
Functional Test (FCT): The goal is to simulate the end-use environment and validate whether the entire PCBA functions as intended as a system. FCT fixtures must provide power, input signals, and clocks to the DUT while capturing and analyzing its output signals. It focuses on system-level behavior rather than individual components. For AI carrier boards, the synergy between ICT and FCT is crucial. ICT can quickly screen out manufacturing defects such as soldering and component errors, while FCT ensures the complex functionality of chips under high-speed and high-load conditions. An excellent Fixture design (ICT/FCT) strategy leverages Boundary-Scan/JTAG technology to compensate for the inability of physical probes to access high-density BGA pins, thereby reducing fixture complexity while improving test coverage.

Comparison of ICT and FCT Fixture Design Considerations

Consideration Dimension	ICT Fixture Design Focus	FCT Fixture Design Focus
Probing Target	Maximize test point coverage, contact all network nodes	Focus on I/O interfaces, power inputs, critical signals, and JTAG ports
Signal Integrity	Primarily focus on DC and low-frequency signal connectivity	Critical. Requires coaxial probes, short-path routing, and impedance control
Power Supply	Only requires low current for partial components during testing	Requires stable, high-current, low-noise power to simulate real loads
Mechanical Complexity	High, with a large number of probes (thousands) and demanding alignment accuracy requirements	Moderate, fewer probes but may include complex connectors and heat sinks
Integration Technology	Often combined with Boundary-Scan/JTAG to reduce physical probes	Requires integration of high-speed instruments, power supplies, loads, and data acquisition systems

Challenges of High-Speed Signal Integrity in Test Fixture Design

When signal rates reach GHz levels, every physical structure in the test fixture-from probes to connecting wires-becomes a potential signal killer. Probes and their leads form a "stub," introducing impedance discontinuities that cause signal reflection and attenuation, which can severely close the high-speed eye diagram in extreme cases.

To address this challenge, advanced Fixture design (ICT/FCT) must adhere to the following principles:

Use High-Frequency Probes: Select coaxial or RF probes specifically designed for high-bandwidth applications, with internal structures that maintain characteristic impedance of 50 ohms or 100 ohms.
Minimize Lead Length: The physical path from the probe tip to the test instrument must be as short as possible. Microstrip or stripline structures should be used for internal wiring, with precise impedance control.
Optimize Grounding Loops: Provide a closely adjacent, low-inductance ground return path for each high-speed signal. Probe layouts should adopt patterns like G-S-S-G (Ground-Signal-Signal-Ground) to reduce crosstalk.
De-embedding Technology: For the most demanding applications, the S-parameters of the test fixture itself can be measured using a network analyzer. The fixture's impact on the signal can then be algorithmically "removed" from the final test results to reveal the DUT's true performance.

Highleap PCB Factory (HILPCB) has extensive experience in manufacturing high-speed PCBs. We deeply understand the impact of material selection and stack-up design on signal integrity and extend this expertise to provide design-for-testability recommendations, ensuring that testing does not become a bottleneck for product performance.

How to Address AI Chip Power Integrity Requirements in Fixture Design?

As a Power Integrity (PI) engineer, I consider this the most overlooked yet critical aspect of Fixture design (ICT/FCT). AI chips impose extremely stringent demands on the Power Delivery Network (PDN): impedance as low as milliohms, peak currents reaching hundreds of amperes, and transient responses in nanoseconds.

An unqualified FCT fixture PDN can lead to two major issues:

Excessive IR Drop: The power path within the fixture (from the power interface to the probe and then to the DUT) inevitably has resistance. If this path is poorly designed, high operating currents can cause significant voltage drops, preventing the DUT from operating at its rated voltage and leading to FCT misjudgments.
Introducing Noise and Oscillation: Excessively long power traces introduce additional inductance, which increases the PDN impedance in the mid-to-high frequency range, degrades transient response, and may even resonate with the decoupling capacitors on the DUT, leading to system instability.

To ensure the validity of testing, the power section of the fixture design must:

Adopt Multi-point, Large-area Power/Ground Probes: Use multiple parallel, high-current-capacity probes to contact the DUT's power and ground planes, minimizing contact resistance and inductance.
Implement Kelvin Sensing: Use independent "Sense" probes to measure voltage directly near the DUT's power pins and feed this back to the test power supply. This allows the power supply to compensate for voltage drops along the fixture path, ensuring the DUT receives precise voltage.
Integrate Decoupling Capacitors on the Fixture: Place bulk capacitors with high capacitance and low-ESL ceramic capacitors near the DUT probe locations on the fixture to serve as local charge reservoirs, meeting the chip's transient current demands.

High-Performance Test Fixture Design Implementation Process

1CAD/Gerber Analysis

2Test Point and Strategy Definition

3Probes and Hardware Selection

4Mechanical Structure and Thermal Design

5Signal Integrity/Power Integrity Simulation

6Fixture Manufacturing and Assembly

7Verification, Calibration, and Deployment

The Critical Role of Mechanical Precision and Probe Technology in AI Substrate Testing

The assurance of electrical performance relies on reliable physical contact. For AI substrates, especially those using advanced packaging like IC substrates, mechanical precision is the cornerstone of Fixture design (ICT/FCT).

Key challenges include:

Alignment Accuracy: The fixture's locating pins must precisely match the positioning holes on the PCB to ensure thousands of probes land accurately on the center of tiny test pads (typically less than 200 microns in diameter).
Warpage Control: Due to complex layering and uneven copper thickness, AI substrates are prone to deformation after reflow soldering. Test fixtures must incorporate effective clamping or vacuum adsorption mechanisms to flatten the PCB, ensuring uniform contact for all probes.
Probe Selection: Different probe tip types (e.g., spear, crown, or star-shaped) and spring forces are required for various test point surfaces (such as OSP, ENIG, or bare copper) to ensure good conductivity while avoiding pad damage. The quality control of the entire SMT assembly process directly determines the reliability of test points.

HILPCB's manufacturing capabilities ensure the PCB itself exhibits excellent flatness and dimensional accuracy, providing a solid foundation for high-precision test fixture integration.

Integrating Boundary-Scan/JTAG Technology to Optimize Test Coverage

As BGA package pins are hidden beneath the chip, traditional physical probe testing methods face limitations. Boundary-Scan/JTAG (IEEE 1149.1 standard) technology offers an elegant solution. Through the chip's built-in Test Access Port (TAP), it serially accesses each functional pin, enabling:

Interconnect Testing: Checks for opens and shorts between chip pins and connectors without physical probes.
In-System Programming (ISP): Programs and configures devices like Flash and FPGA.
Auxiliary Functional Testing: During FCT, controls the chip to enter specific test modes or reads internal states via the JTAG port.

Integrating Boundary-Scan/JTAG into Fixture design (ICT/FCT) significantly reduces the need for physical probes, especially in high-density areas. This not only lowers fixture manufacturing costs and complexity but also avoids signal integrity issues caused by physical probes. A modern testing strategy must combine ICT, FCT, and Boundary-Scan/JTAG organically.

HILPCB One-Stop Service Value

DFM for Testability

Early intervention in design to optimize test point layout, ensuring manufacturability and testability.

Advanced PCB Manufacturing

Reliable manufacturing services for complex PCBs, including high-layer counts, HDI, and IC substrates.

Precision SMT Assembly

Equipped with advanced placement and soldering capabilities to ensure high-quality component assembly.

Integrated Testing Solutions

Comprehensive solutions from ICT, FCT to aging tests to ensure product quality.

Complete Traceability System

Full quality traceability from materials to finished products through the Traceability/MES system.

Integration of Traceability/MES with Test Data

In mass production, quality control is not just about "pass/fail" binary judgments but more importantly about data collection and analysis. Traceability/MES (Manufacturing Execution System) is the core to achieving this goal. Each PCBA that passes testing must have its unique serial number associated with detailed test data, including ICT resistance and capacitance measurements, FCT voltage and current power consumption, and any fault codes.

An intelligent Fixture design (ICT/FCT) should include automated barcode or QR code scanning functionality, automatically reading the PCBA's serial number at the start of testing. After testing, all data is automatically uploaded to the Traceability/MES database. This integration brings significant value:

Rapid Root Cause Analysis: When batch issues are detected, data can be quickly traced back to identify specific batches of materials, equipment, or process parameters related to the failure.
Process Capability Monitoring: By statistically analyzing test data (such as fluctuations in a specific voltage), the stability of manufacturing processes like SMT assembly can be monitored, enabling predictive maintenance.
Quality Compliance and Customer Reporting: Providing customers with complete production and test data reports to meet stringent traceability requirements in high-end industries (e.g., automotive, medical).

Impact of Conformal Coating on Test Fixture Design

To enhance product reliability in harsh environments, many AI hardware products apply a layer of Conformal coating (protective coating) on the PCBA surface. While this protective film can prevent moisture, dust, and corrosion, it also complicates testing by covering test points, preventing probes from making good electrical contact.

Strategies to address this issue must be considered during the design stage, with three main approaches:

Post-Test Coating: This is the most straightforward method but may increase PCBA handling steps in the process.
Selective Masking: Before applying Conformal coating, mask all test points with peelable tape or adhesive dots. This places higher precision requirements on automated production lines.
Using Piercing Probes: When designing Fixture design (ICT/FCT), employ probes with sharp tips (such as spear or star heads), which are designed to penetrate thin layers of Conformal coating and make direct contact with the underlying pads. This method requires precise control of probe pressure and coating thickness to ensure reliable contact without damaging the pads.

The choice of strategy depends on a comprehensive consideration of cost, production volume, reliability requirements, and coating type.

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Conclusion

In the field of AI hardware, competition is essentially a race of performance, reliability, and time-to-market. A seemingly backend production step like Fixture design (ICT/FCT) profoundly impacts these three core elements. From addressing electrical challenges like high-speed signals and transient power to meeting micron-level mechanical precision requirements, and seamlessly integrating with advanced manufacturing systems such as Boundary-Scan/JTAG and Traceability/MES, test fixture design has become a precision engineering discipline that combines multidisciplinary expertise.

Overlooking the complexity of test fixtures is akin to building a giant ship without sea trials-the potential risks are immeasurable. Successful AI product development requires embedding testability from the design phase and selecting partners with deep expertise across the entire process, from HDI PCB manufacturing to Turnkey Assembly. As a leading PCB solutions provider, HILPCB leverages its extensive technical expertise and one-stop service capabilities to help customers overcome every challenge from design to mass production, ensuring your innovative ideas are delivered to the market with the highest quality standards and reliability.