The boundary scan test is quickly becoming a standard in PCB testing. It requires no probes, offers high access, and has various other benefits. Below, we explain how it works and its immense popularity with PCB manufacturers today.
What is a Boundary Scan Test?
Boundary scan is a method of checking circuits without relying on physical test probes for every node. Instead, it utilizes dedicated circuitry located on the pins of IC devices.
The test, which was developed by the Joint Test Action Group (JTAG), is ratified by the IEEE 1149.1. It derives its name from its structure and working, which involves special circuits located on an IC’s peripherals.
Today, PCB designers and manufacturers are finding the technology essential. It offers more benefits than traditional methods and is sometimes the only way to access hidden nodes during a QC process.
Boundary Scan Key Components and Architecture
A boundary scan is a complex structure with various connections.It consists of key architectural elements defined by the IEEE 1149.1 standard. Let’s explore the purpose of each component.
Boundary Scan Cells
These are circuits or flip-flops added to the input and output (I/O) pins of ICs. They contain the logic that determines test and non-test conditions.
The test cells remain transparent in normal device operations, activating only when put in a test mode. That way, the IC functional signals do not affect the cells.
Scan Chain
The chain is a serial connection that links the bidirectional cells together, forming shift registers. The registers enable the streaming of data from one cell to another until it is output on the other end.
The chain is arranged such that the output data of one device forms the input of another. This connection enables the shifting of instructions serially through the daisy chain and back.
Test Access Port (TAP)
The TAP manages the testing process. It comprises the following: TAP controller, data registers, and an instruction register. It uses a set of input/output pins to control the entire process.
How Boundary Scan Testing Works
The boundary scan tester or equipment works by shifting data through the path that connects the cells or shift registers. The following is the sequence after connecting the necessary hardware.
Setup: The operator loads the BSDL (boundary scan description language) files and connects the necessary hardware to the PCB via the TAP pins.
Data Capture: The TMS or TCK pins place the cells/registers in data capture mode to observe pin states.
Data Shift: Data shifts through the registers to check interconnect and soldering faults (external shift) – an internal test checks IC logic. The test pattern is usually unique to the PCB and its assembled chips.
Data Analysis: A computer program compares the captured data to expected results. A fail shows a broken trace or soldering defects, such as bridging.
Why is Boundary Scan Test Essential in PCB Design
The modern PCB is densely populated with many SMT devices, making probing circuits difficult. The boundary scan method solves the problem by eliminating the need for physical probes.
Many PCB designers today add JTAG structures to their plans. It makes checking dense, complex circuit boards easier, quicker, and less costly. These are boards that traditional equipment would struggle to check.
Additionally, the JTAG interface allows other operations, such as low-speed programming and troubleshooting/debugging integrated circuits. As you can see, it’s a technology with many functions.
Boundary Scan Test Benefits
The JTAG boundary scan test has many advantages when compared to methods that use physical probes. The benefits explain why PCB designers and manufacturers prefer it over other techniques. They include the following:
Reduced Fixturing
Custom jigs take time and money to make. The JTAG approach eliminates the need for expensive fixtures and their time-consuming production processes. That, in turn, makes checking circuit boards quicker and less costly.
High Access
Probes cannot physically assess buried vias, inner layers, or pads under BGA packages, and the tight spaces of high-density PCBs.
The JTAG interface solves this problem by using IC connections to send fault-detection signals. The improved access makes the technology necessary for most modern PCB assemblies.
Versatility
The JTAG interface not only verifies interconnects. It can also check the internal logic blocks of ICs or configure programmable chips. Additionally, you can use it to debug digital circuits. These capabilities make it highly versatile.
Standardized Testing
JTAG is a standardized technology, ensuring compatibility across IC and PCB vendors. It also makes designs reusable, reducing product development times and costs.
Other quality control techniques, such as ICT and FCT, are highly customized. Their execution depends on specific PCB designs. They also follow in-house guidelines provided by the manufacturer, which may vary widely.
JTAG applications go beyond verifying IC interconnects
Resource: https://youtu.be/yoMBiYYj4z4?si=gEZfTToTTImuxikK
Boundary Scan Test Application in Modern Electronics
Boundary scan in PCB manufacturing focuses on testing interconnects at the level of chip packages. It identifies open circuits, short circuits, stuck-at faults, and missing components.
That said, modern JTAG equipment does more than just basic continuity checks. Advanced tools can program flash memory, PLDs, and microcontrollers directly on the board (“In-System Programming”).
The technology also helps debug hardware designs by allowing engineers to control pin states manually. Compliant PCBs can have partial or full capability, depending on how many ICs on the board support the IEEE 1149.1 standard.
Compliant PCBs can have a partial or complete capability. Partial capability means that only several ICs are built with test cells. Full capability means all ICs support JTAG and come equipped with the necessary logic.
Boards that benefit from the technology are high-density multilayered types with limited space for pad placement or probe access. They can be PCBs for consumer electronics, telecommunication, medical, automotive, defense, or aerospace.
Conclusion
Understanding boundary scan is an essential part of PCB design requirements today. That’s because circuit boards are getting smaller and more complex, making physical test probes challenging. The test relies on IC pins to place signals, enabling better access than when using mechanical connections.
