What is a Boundary Scan?

Lakshmi Sandhana

A boundary scan is a method to test the all interconnects on printed circuit boards (PCBs) using boundary scan cells instead of physical probes. It is a standard widely adopted by electronic companies. Prototype debugging and product design can also benefit from boundary scans.

Man holding computer
Man holding computer

In the early 1980s, manufacturers of PCBs relied on in-circuit testers and physical bed-of-nail fixtures to test components. With the advent of increasingly miniaturized components, greater density of devices, multilayer boards, and surface-mounted packaging, it became increasingly difficult to physically access all the interconnects on a PCB. In-circuit testing is vital to examine manufacturing defects such as open and short circuits and damaged or missing components. It became necessary to develop a different methodology to test PCBs without requiring physical access to all the components on the board.

The solution, developed by the Joint Test Action Group (JTAG), was to build physical access to all the components within the device itself. This group of engineers created the process for boundary scan testing in the 1980s. In 1990, it was standardized as the IEEE Std. 1149.1-1990.

While the JTAG didn't invent the concept itself, they were instrumental in converting the basic idea into an international standard. Currently, a boundary scan is also known as JTAG. A revision to the IEEE Std. 1149.1 was introduced in 1993, and this was called 1149.1a. This particular revision consisted of certain enhancements and clarifications. Later, a supplement describing the Boundary-Scan Description Language (BSDL) was added in 1994.

Physical access was inbuilt into the device by including internal serial shift registers at its boundaries. These registers are called boundary scan registers and can be thought of as virtual nails. They can be used to test all the interconnects on the PCB. Boundary scan registers are found at the start and end of the areas most likely to be damaged during board assembly. This is also called the interconnect region.

These boundary scan registers, or cells, can force and capture data from the pins on a device. The data obtained this way is compared with expected results to test the board for faults. This is a much easier way to test components for proper bonding, working functionality, and alignment. Boundary scans were initially utilized in the production phase of a product's life cycle, but due to the establishment of the IEEE-1149.1 standard, they are currently used over the entire product life cycle.

The advantage of using boundary scans to test PCBs are lower equipment costs, which speed up development; short test times; better test coverage; and higher product quality. Electronics manufacturers worldwide rely on boundary scans to effectively and inexpensively test PCBs.

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