Boundary Scan

What is Boundary-Scan, What is it used for? Boundary Scan is used to test, program and debug printed circuit boards (PCBs).

Let’s begin with a bit of history on why Boundary Scan came about.

Why is Boundary Scan Needed

Back in the late 80’s and early 90’s, several developments in Circuit Board technology came together to make board test more difficult, namely:

1. The move to surface mounted devices
2. The use of Multilayer Circuit boards
3. The lack of test points on circuit boards, due to these issues

As a result of these changes, traditional in-circuit test (ICT) tools, so-called bed of nails testers were no longer able to fully test Printed circuit boards (PCBs). To address this vexing problem, a consortium of companies–known as the Joint Test Access Group or JTAG was formed.

What Can You Do With Boundary Scan?

There are several levels of testing supported by Boundary Scan: Chip Level and Board Level.

in addition, Boundary-Scan tools can be leveraged at various phases of the development lifecycle, namely: The Design phase, The Prototype Debug Phase, The Production Phase and the Field Repair Phase.

The following sections will introduce these functional areas of test.

Chip Level Boundary Scan Testing

Boundary Scan allows you to do the following types of chip level testing:

• Presence of the device – Is the device on the board; did it get soldered on.
• Orientation of the device – Is it oriented correctly; is it rotated, shifted, the wrong package…
• Is it bonded to the board – Is it soldered properly or are their issues with the solder joint, is the internal pin to amplifier interconnect damaged?
• Read the devices ID register (get chip revision level information), Verify Basic operation

Board Level Testing Boundary Scan Testing

Testing at the board level adds inter-device and board-level testing such as:

• Ability to verify the presence and integrity of the entire scan chain and each device on it.
• Device interconnect tests
• Open, Short and Stuck At (0,1) failures
• Cluster (non-jtag) device testing such as: DDR2 RAM, Ethernet MAC/PHY, FLASH, CPLD, FPGA, etc..

The Value of Boundary Scan Throughout the Development Life Cycle

This section will detail the value of Boundary Scan at the various phases of product development.

PCB Design Phase: Design for Test (DFT) Factors

Test coverage and overall product quality are crucial to a products success. If a product needs to be recalled or repaired in the field (so-called “truck rolls), the effects can be devistating to the bottom line.

When designing printed circuit boards, one of the key things you can do to improve overall quality is to adhere to design for test (DFT) guidelines. Basically, DFT guidelines help you created systems with a high degree of test coverage so that you can catch as many defects as possible before you ship your product.

The key way to increase overall test coverage with boundary scan is the use of IEEE 1149.1 compliant devices. These devices should be placed strategically in your design to provide access to as many devices and nets as possible.

If you are working on products that require long, labor intensive operations to change or even systems that cannot be changed at all due to testing constraints like FDA, etc, you will also want to consider embedded hardware diagnostics to compliment your over all test picture.

Additional DFT information can be found in the post entitled: Design for Test.

PCB Prototype Debug and Bring Up Phase

When bringing up and debugging your new hardware, Boundary Scan comes to the rescue in several important ways:

Testing Partially Populated hardware

When you get your initial boards, not all devices may be fitted. That’s fine, with Boundary Scan, you only need good power, ground and at least one part on the JTAG Chain to begin testing. You should be able to ID the part on the chain and then test for opens and shorts for any board area that is touched by this device.

Initilizing and Programming Devices

You may also be able to do initial device programming. For example, if the device on the chain is a microprocessor or DSP, most likely you will have access to RAM and FLASH memory via the address, data and control bus. This can allow you to ID and Program and test these so-called (cluster, or non-jtag) devices. FPGAs, CPLDs (XILINX and ALTERA, for example) may also fall into this category.

Finding Assembly Defects

Prototypes are often rushed through assembly in order to make engineering deadlines. As a result, assembly and manufacturing problems will exist. Boundary Scan is prefect for testing for common problems like unfitted or ill fitted devices, solder issues (cold or hot Joints), as well as open, shorts, stuck at and device functional failures.

Improving Debug Productivity

As initial firmware or diagnostics are written for your new hardware, Boundary Scan can be used to rule out “bad hardware” by providing a “golden” test that validates that a board is good. With this type of testing, you will be able to focus your debug efforts on the new release of firmware, knowing full well that your hardware is good.

Production Test Phase

Manufacturing test is the clearly sweet spot for Boundary Scan. It was designed to compliment existing test methods and to over come the problems of evolving board technology.

Quality Control

Boundary Scan can be provided to external Contract Manufacturers on in-house manufacturing sites as a way to control and audit they types of testing that they do as well as a way to increase overall quality.

Part of Comprehensive Tests

Boundary Scan can be used as a part of a comprehensive test environment including ICT, Visual and XRAY inspection and thermal test.

Sub-Assembly Support

As part of the production process sub-assemblies from the CM can be tested in-house to avoid infant failures and the need for rebuilds in production.

Device Programming

As previously mentioned devices can be programmed via JTAG boundary Scan. CPLD and FPGA images can be loaded as well as boot loader code and diagnostics software. Boundary Scan also makes a perfect tool for production test repair stations that allow test engineers to troubleshoot complex and expensice boards in the “bone yard”.

Field Service and Repair

Modern Boundary Scan tools can be quite portable and low cost allowing them to be used by the field service staff for board level troubleshooting and repair and device configuration.

Boundary Scan: Where to Get the Specification

If you would like to get the “official” Boundary-Scan spec, please follow the link below:

The IEEE 1149.1 Boundary Scan Spec