A Board Test Overview

Six years later, PCBs haven’t returned to the rococo splendor of their earliest predecessors— confections of rainbow-banded fixed resistors, multicolored disc and cylindrical capacitors, and geometrically fascinating combinations of diversely shaped coils, discrete transistors, potentiometers, variable capacitors, and connectors. Nevertheless, they have escaped the neoclassical austerity of 1994’s versions to offer a seemingly impenetrable level of complexity. Like a romantic allegory, a board today offers much that’s hidden from the eye.

To penetrate a board’s mysteries, you’ll need a variety of equipment. In 1994, Romanchik reported, in-circuit testers were the choice of many test engineers. Although still fulfilling a valuable role, in-circuit testers now require assistance from inspection equipment as well as from final functional test. Test information from all this equipment must be networked to enable you to quickly pinpoint process conditions that lead to abnormal failure rates, so you can promptly make repairs and adjustments.

Today, you can assume the semiconductor devices mounted on your boards are functioning properly—that they have benefited from the many device testers² and device-test techniques³ now available. Although every application is different, it’s unlikely you will need to test those devices at speed in circuit.

Instead, the role of PCB test is to ensure that the parts have been properly mounted on the board, that proper solder connections have been established, and that devices haven’t been destroyed during manufacturing—for example, by the heat of the solder-reflow operation. You must ensure your devices remain functional after PCB manufacture, but you needn’t test them thoroughly or at speed. There’s no known failure mechanism that would degrade a device’s operating speed as a result of soldering to a PCB.⁴

A typical PCB test process begins with an optical inspection of the solder paste, an approach that ensures that solder paste is distributed in the proper pattern and in proper quantities. Next comes post-place optical inspection—a step that ensures the correct components (verified perhaps by inspection of part numbers stamped on the devices) are properly located on the board. Next comes post-reflow inspection. This step could use optical equipment or x-ray equipment—the latter can examine solder connections beneath components as well as look for voids within solder balls that can indicate potential failure mechanisms.

These inspection steps are followed by electrical in-circuit test—a procedure that’s similar to its 1994 counterpart but with an important difference: Many nodes that could be probed by
a bed-of-nails fixture in 1994 are now hidden, beneath ball-grid-array components or within multilayer boards. In-circuit testers, therefore, need help in the form of boundary-scan devices⁵ that permit access to hidden nodes by means of board testers equipped with boundary-scan-controller hardware.⁶

Finally, PCBs undergo functional test. Functional testers typically interface to a board under test via its edge connector, but some—called emulators—take over for a board’s processor, exercising and monitoring the board through the processor socket.⁷

Figure 1 illustrates all the steps, and indicates the type of equipment used for each one. For a list of the page numbers where you can find manufacturers of the products, see our Buyer's Guide. T&MW

Figure 1.  PCB test is a multistep process. All aspects must be tied together by an information backbone that enables production personnel to continually optimize processes. The figure lists the type of equipment needed for each test or inspection step and also lists the type of software needed for the information backbone; see our Online Buyer’s Guide.