In the wave of artificial intelligence (AI) and high-performance computing (HPC), chip design is advancing toward the era of heterogeneous integration with Chiplet and 2.5D/3D packaging. The SoCs, HBMs, and I/O modules within these advanced packages are interconnected through tens of thousands of micro-bumps and high-density RDL (redistribution layers), posing unprecedented challenges to the manufacturing precision and assembly reliability of carrier PCBs. When physical probes cannot access these deeply embedded connection points, traditional in-circuit testing (ICT) methods become ineffective. At this point, Boundary-Scan/JTAG (IEEE 1149.1 standard) is no longer just a testing technique but a core pillar throughout the entire lifecycle of AI chip carrier boards, from design and verification to mass production.

As a system architect specializing in advanced interconnects, I deeply understand that in densely packed copper pillar interconnects and microvia stacking structures, even the slightest defect can lead to the failure of an entire expensive module. Boundary-Scan/JTAG provides an elegant, non-invasive electrical testing method capable of precisely verifying the soldering quality of high-density packaged components like BGAs and LGAs, ensuring seamless interconnections between Chiplets. It has become critical for ensuring the successful validation of complex HDI PCBs and IC carrier boards during rigorous NPI EVT/DVT/PVT phases. Leading manufacturers like Highleap PCB Factory (HILPCB) achieve high-reliability, one-stop solutions by deeply integrating JTAG testing with advanced manufacturing processes.

What Makes Boundary-Scan/JTAG the Cornerstone of AI Chiplet Architectures?

Modern AI accelerators often employ multi-die designs, integrating multiple computing, memory, and I/O Chiplets on the same silicon interposer or organic carrier board. Communication between these Chiplets relies on tens of thousands of high-density, short-distance interconnect traces. Since the pins of BGA (ball grid array) and LGA (land grid array) packages are entirely hidden beneath the chip, traditional physical probe testing methods cannot access these solder joints.

Boundary-Scan/JTAG technology addresses this challenge by integrating a tiny "test cell" (Boundary Scan Cell) into each chip's I/O pins. These cells are connected via a serial path (i.e., a "scan chain") to form a complete test bus. Through a standard Test Access Port (TAP), engineers can:

1. Control Pin States: Set any I/O pin to high, low, or high-impedance.
2. Capture Pin States: Read the signal level on any I/O pin.

This "virtual probe" capability allows us to precisely inspect every connection between chips and between chips and carrier boards without physical contact. For complex SMT assembly processes, JTAG is the most effective means of verifying thousands of BGA solder joints (for shorts, opens, or cold solders), and its importance is irreplaceable.

How Does JTAG Accelerate the NPI EVT/DVT/PVT Product Introduction Process?

During the various stages of New Product Introduction (NPI), time is of the essence. Boundary-Scan/JTAG plays a vital role in accelerating hardware debugging and validation, significantly shortening the cycle from prototyping to mass production.

• EVT (Engineering Validation Test) Phase: When the first prototype boards arrive, JTAG is the first debugging tool used. Engineers can quickly identify shorts and opens caused by design errors or early manufacturing defects through JTAG interconnect testing, thereby verifying the board's basic electrical integrity. This avoids time-consuming power-up and functional testing on uncertain hardware.
• DVT (Design Verification Testing) Phase: At this stage, JTAG is utilized more extensively. It is not only used for interconnect testing but also for In-System Programming (ISP), such as firmware flashing for on-board FPGAs, CPLDs, or flash memory. Additionally, the JTAG port can access the chip's internal debugging modules, assisting software engineers in low-level hardware debugging to ensure proper hardware-software collaboration.
• PVT (Production Verification Testing) Phase: When the design enters mass production, JTAG test scripts are integrated into Automated Test Equipment (ATE), becoming a standard procedure in the production line. It ensures that every manufactured board undergoes rigorous structural testing, laying a solid foundation for subsequent functional testing. It can be said that a robust JTAG test strategy is the key guarantee for the success of the NPI EVT/DVT/PVT process.

Can JTAG detect defects beyond simple shorts and opens?

The answer is yes. While basic interconnect testing is the most well-known application of Boundary-Scan/JTAG, its capabilities extend far beyond this. Modern JTAG tools and techniques have expanded into broader areas:

• Logic and Functional Interaction: By controlling chip pins, specific logic states can be simulated to test whether peripheral circuits (such as pull-up/pull-down resistors and bus transceivers) are functioning correctly.
• In-System Programming (ISP): JTAG is the most commonly used interface for programming and updating on-board non-volatile memory (e.g., Flash, EEPROM) and programmable logic devices (FPGA/CPLD).
• Access to On-Chip Debug Modules: Many complex SoCs (such as ARM or RISC-V core processors) integrate powerful debug modules (e.g., ARM CoreSight). The JTAG port is the standard interface for accessing these modules, allowing developers to set breakpoints, step through code, inspect registers and memory, and perform in-depth software and firmware debugging.
• Post-Silicon Validation: During the chip design phase, JTAG is also used for verification and debugging of internal chip logic, serving as a critical method to ensure functional correctness.

These advanced capabilities have transformed JTAG from a simple production testing tool into a powerful platform that spans the entire product lifecycle, including design, development, manufacturing, and maintenance.

How does Boundary-Scan collaborate with other inspection methods?

In modern electronics manufacturing, no single testing technique is a one-size-fits-all solution. A comprehensive quality assurance system requires combining multiple inspection methods to form complementary testing strategies. Boundary-Scan/JTAG plays a central role in electrical verification, working closely with physical inspection methods.

• SPI (Solder Paste Inspection): At the beginning of the SMT assembly process, SPI is used to inspect the volume, shape, and placement of solder paste printed on PCB pads. This is the first line of defense against soldering defects.
• AOI (Automated Optical Inspection): After component placement and reflow soldering, AOI uses high-resolution cameras to check component position, orientation, polarity, and the presence of obvious visual defects such as solder bridges or solder beads.
• X-Ray Inspection: For bottom-termination components like BGA and LGA, AOI cannot inspect their solder joints. In such cases, X-Ray technology from SPI/AOI/X-Ray inspection is required to penetrate the chip and examine hidden defects such as solder ball shape, size, voids, shorts, or Head-in-Pillow effects.

However, even if SPI/AOI/X-Ray inspection all pass, it does not guarantee 100% electrical connectivity reliability. For example, microscopic cracks undetectable by X-Ray (such as open circuits caused by Black Pad effects) or functional failures cannot be identified through physical inspection. This is where Boundary-Scan/JTAG demonstrates its unique advantage by conducting electrical tests to confirm the conductivity of every connection, ensuring final product quality. HILPCB's SMT Assembly Service combines these advanced inspection technologies with JTAG testing to deliver the highest-quality PCBA products to customers.

What DFT rules are required for JTAG implementation on high-density substrates?

To ensure Boundary-Scan/JTAG works reliably, strict Design-for-Testability (DFT) principles must be followed during the design phase. These rules are particularly critical for IC Substrate PCBs carrying AI chips:

1. Complete Scan Chain: Ensure all JTAG-supported devices are connected in series within one or more scan chains. Chain integrity is the foundation of testing, and any breakpoint will cause the entire chain to fail.
2. Signal Integrity: The JTAG clock signal (TCK) is highly sensitive to signal quality. During PCB layout, TCK traces should be as short as possible, kept away from noise sources, and may require termination resistors to suppress reflections.
3. Clear TAP Access: The pins of the Test Access Port (TAP) (TCK, TMS, TDI, TDO, TRST) should be routed to easily accessible test points or standard connectors for convenient debugging and production testing.
4. Level Shifting: When a scan chain contains chips with different I/O voltages, appropriate level shifters must be used between them to ensure reliable signal transmission.
5. Correct BSDL Files: Each JTAG-supported chip has a corresponding Boundary Scan Description Language (BSDL) file that describes its JTAG structure. Design and test engineers must obtain and use the correct BSDL files from the chip supplier; otherwise, the test tool will fail to recognize the chip.

As an experienced PCB manufacturer, HILPCB's engineering team provides professional DFM/DFT recommendations during the design review phase to ensure robust JTAG design and avoid costly rework later.

How Does JTAG Validate the Effectiveness of Low-void BGA Reflow Process?

Low-void BGA reflow is a core process goal in high-reliability electronics manufacturing. Voids in BGA solder joints reduce mechanical strength and thermal conductivity and may even lead to failures over time. While X-ray inspection is the primary method for detecting voids, Boundary-Scan/JTAG plays the ultimate "judge" role in this process.

An optimized Low-void BGA reflow profile (including preheat, soak, peak temperature, and cooling rate) aims to maximize the expulsion of flux volatiles, thereby minimizing voids. After the process, X-ray can quantitatively analyze whether the void rate meets specifications. However, some latent defects, such as head-in-pillow (incomplete fusion between solder balls and paste) or micro-cracks, may be difficult to discern in X-ray images but can cause electrical opens or unstable connections.

JTAG testing can precisely capture these electrical faults. If JTAG reports numerous interconnect failures on boards that passed X-ray inspection, this strongly suggests systemic issues in the reflow process. By analyzing the failure locations reported by JTAG, process engineers can refine reflow parameters to achieve truly high-reliability Low-void BGA reflow.

What Role Does JTAG Play in Traceability and MES System Integration?

In smart, automated modern factories, Traceability/MES (Manufacturing Execution System) is the core for ensuring quality and process control. The system needs to record all critical data of each circuit board during production, from material batches to process parameters and test results. Boundary-Scan/JTAG test results are a critical data source for Traceability/MES systems. After each board undergoes JTAG testing, its unique serial number is linked to detailed test logs and uploaded to the MES database. These logs not only include simple "pass/fail" results but may also contain:

• Specific failed pins and net names.
• Time taken for the test.
• Information about the testing equipment and operator.
• Software and firmware versions used during testing.

When quality issues arise, this data becomes a goldmine. For example, if the MES system analysis reveals an abnormally high JTAG failure rate on the same net for a specific batch of boards, engineers can quickly trace the potential cause—whether it's a defective batch of components or parameter drift in a pick-and-place machine. This data-driven root cause analysis capability is essential for continuously improving manufacturing processes and increasing product yield. Without the precise electrical fault data provided by JTAG, the value of the Traceability/MES system would be significantly diminished.

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Conclusion

In the complex world of AI chip interconnects and substrate PCBs, Boundary-Scan/JTAG has evolved from a mere testing method into a critical technical platform supporting the entire product lifecycle. It addresses the physical testing challenges posed by high-density packaging, accelerates the transition from NPI EVT/DVT/PVT to mass production, and works synergistically with inspection methods like SPI/AOI/X-Ray inspection to provide comprehensive quality assurance for complex SMT assembly. Furthermore, through deep integration with Traceability/MES systems, JTAG delivers invaluable electrical test data for smart manufacturing.

Successfully navigating the challenges of AI hardware requires not only advanced design but also a partner with deep expertise in DFT, advanced manufacturing processes, and comprehensive testing strategies. Leveraging its extensive experience in IC substrates and high-density interconnects, as well as its one-stop service capabilities from PCB manufacturing to Turnkey Assembly, HILPCB is committed to helping customers transform complex AI designs into highly reliable products. We believe that through close collaboration, we can tackle challenges together and leverage core technologies like Boundary-Scan/JTAG to ensure the success of your next-generation AI products.

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• HDI PCBs
• SMT Assembly Service
• IC Substrate PCBs
• Turnkey Assembly