As 5G evolves toward 6G, the design and manufacturing of communication PCBs are facing unprecedented challenges. The application of millimeter-wave frequency bands, ultra-high-density interconnect (HDI), and the relentless pursuit of low insertion loss have exponentially increased the complexity of circuit boards. In this context, traditional testing methods can no longer meet verification requirements, while Boundary-Scan/JTAG (IEEE 1149.1 standard) technology has emerged as a critical pillar for ensuring the quality and reliability of these complex systems throughout the entire process-from design and assembly to final testing. A comprehensive DFM/DFT/DFA review process must incorporate JTAG test strategies as a core element to address the challenges posed by BGA, LGA, and other packaging technologies that make physical probing inaccessible.

As baseband and fronthaul engineers, we understand that even a single bit error in eCPRI/O-RAN RU interfaces can degrade overall system performance. Therefore, during manufacturing, it is crucial to ensure flawless electrical connections at every solder joint. Boundary-Scan/JTAG provides an efficient, non-invasive, and structured testing method capable of precisely locating manufacturing defects such as open circuits, short circuits, and missing components, thereby ensuring the successful delivery of high-performance HDI PCBs.

The Core Role of Boundary-Scan/JTAG in 5G/6G Complex Interconnect Testing

The essence of Boundary-Scan/JTAG technology lies in embedding a boundary scan cell (Boundary-Scan Cell) between each I/O pin and the internal logic of a chip. These cells are connected via a serial scan chain, forming a complete test pathway that can be controlled through a standard Test Access Port (TAP). For 5G/6G communication PCBs, this means we can verify the integrity of thousands of network connections without physically accessing every test point.

For O-RAN RU boards packed with BGAs, FPGAs, and high-speed processors, JTAG offers the following advantages:

1. Comprehensive Fault Coverage: Detects pin-level open circuits, short circuits, bridging, and incorrect components.
2. Simplified Test Interface: Requires only 4 to 5 pins (TCK, TMS, TDI, TDO, TRST) to access the entire scan chain, significantly reducing the complexity of test fixture design.
3. In-System Programming and Debugging: Beyond structural testing, JTAG can also be used for in-system programming (ISP) of FPGAs, CPLDs, and flash memory, streamlining firmware updates.

Addressing High-Density SMT Assembly Challenges: How JTAG Validates BGA and High-Q Component Connections

Modern 5G/6G radio frequency front-end (RFFE) modules integrate numerous high-Q filters, duplexers, and multiplexers, which are highly sensitive to soldering quality. During complex SMT assembly, even minor soldering defects-such as cold solder joints or solder balls-can introduce parasitic parameters, severely impacting RF performance and leading to degraded out-of-band rejection or group delay distortion. Boundary-Scan/JTAG testing plays the role of a "quality inspector" in this stage. By verifying the digital control lines connecting the RF transceiver in BGA packaging with peripheral high-Q components, JTAG indirectly ensures these critical devices are correctly installed and connected. For high-frequency PCBs employing mixed technologies, their complex SMT assembly process may also include selective wave soldering techniques. JTAG testing can cover connection issues potentially introduced by these processes, ensuring the electrical integrity of the entire board. This lays a reliable foundation for subsequent expensive network analyzer testing (S-parameter measurements).

From DFM/DFT/DFA Review to Production: How JTAG Ensures S-Parameter Consistency

S-parameters are the gold standard for measuring RF component performance, but the stability and consistency of their measurements heavily depend on the reliability of the device-under-test (DUT) physical connections. If insufficient DFM/DFT/DFA review is conducted during the design phase, neglecting JTAG test pathway design, production may face risks of batch-to-batch S-parameter performance drift.

Boundary-Scan/JTAG ensures PCB physical layer consistency by performing a thorough structural check before functional testing. Once JTAG tests pass, engineers can proceed with S-parameter measurements with greater confidence, as connection issues caused by soldering defects have been ruled out. This "structure-first, function-later" testing strategy not only improves efficiency but also guarantees reproducibility of S-parameter measurements from prototypes to mass production-critical for maintaining communication module insertion loss and out-of-band suppression specifications.

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DFT for JTAG Design Key Points (5G/6G Adaptation)

Beyond Traditional ICT/FCT: JTAG Advantages in Power Network & High-Speed Interface Validation

Traditional In-Circuit Test (ICT) and Functional Test (FCT) rely on physical probe contact with test points, known as "bed-of-nails." As 5G/6G PCB densities increase, reserved space for test points becomes extremely limited, making conventional Fixture design (ICT/FCT) exceptionally challenging and costly.

Boundary-Scan/JTAG elegantly solves this dilemma. It utilizes the chip's own pins as virtual test points, significantly reducing reliance on physical test points. This not only minimizes the need for complex Fixture design (ICT/FCT) but also enables testing of high-density areas on both sides of the board. Additionally, JTAG technology can extend to testing interconnects between non-JTAG devices (by driving/sensing adjacent JTAG device pins) and validating Power Delivery Network (PDN) integrity-ensuring stable power supply for critical chips, which is vital for maintaining signal integrity in high-speed eCPRI interfaces.

Integrating Traceability/MES: How JTAG Test Data Drives End-to-End Quality Control

In the era of smart manufacturing, data is the cornerstone of quality and efficiency improvement. Every Boundary-Scan/JTAG test generates detailed diagnostic reports, which can be seamlessly integrated into the factory's Traceability/MES (Manufacturing Execution System). This creates a comprehensive "digital record" for each PCB, tracking its journey from birth to shipment.

By correlating JTAG test data with SMT assembly equipment data, manufacturers can quickly identify systemic issues in production, such as placement deviations from specific pick-and-place machines or abnormal temperature profiles in reflow ovens. This data-driven closed-loop control not only improves first-pass yield but also provides robust support for root cause analysis. At HILPCB, we incorporate JTAG testing as a standard procedure in our SMT Assembly and Small Batch Assembly services, ensuring every delivered circuit board undergoes rigorous structural validation and achieves full lifecycle quality traceability through the Traceability/MES system.

Test Coverage Matrix (Objects × Methods)

Object/Defect	AOI	X-Ray	JTAG	FPT	ICT	FCT
BGA Solder Joint Bridging/Voids	△	✓	-	-	△	△
Digital Interconnect Opens/Shorts (BGA↔Device/Connector)	△	-	✓	✓	✓	△
AC Coupled Differential Pair (eCPRI/SerDes) Interconnect	-	-	✓（1149.6）	△	△	Functional
Power Delivery Network (PDN) Switch/Pull-up Pull-down	-	-	✓ (Drive/Sample)	-	✓	Function

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Conclusion

In the rapid development wave of 5G/6G communication technologies, the increasing complexity of PCBs has raised higher demands for testing and verification. Boundary-Scan/JTAG, with its unique advantages, has become an indispensable tool to address these challenges. It is not merely a testing technology but also a bridge connecting design, manufacturing, and quality control, spanning the entire process from DFM/DFT/DFA review, SMT assembly, to final validation. By effectively leveraging Boundary-Scan/JTAG (including 1149.6) in synergy with SPI/AOI/X-Ray, FPT, ICT/FCT, and combining it with Traceability/MES data-driven closed-loop systems, we can ensure the reliability of millimeter-wave and low-loss interconnects, laying a solid foundation for building high-performance, high-stability next-generation communication networks.

Frequently Asked Questions (FAQ)

• Can JTAG verify RF performance?
  JTAG is a structural test and does not measure analog RF metrics; RF performance relies on VNA/S-parameters and functional validation.
• How to test AC-coupled differential pairs?
  Use devices supporting IEEE 1149.6 to perform structural tests on AC-coupled differential interconnects.
• What about devices without JTAG?
  Leverage adjacent JTAG devices to drive/sample their pins or cover them via FPT/ICT.
• When to consider ICT?
  Introduce ICT for throughput improvement when versions stabilize and production volumes increase, while JTAG continues handling structural regression and ISP.

Related links

• HDI PCBs
• high-frequency PCBs
• SMT Assembly
• Small Batch Assembly