Best practices in functional/factory floor testing and inspection
For a printed circuit board (PCB) manufacturer, testing presents a wide variety of both problems and opportunities. Test too much, and you waste capital, and effort and lose production efficiency. Test too little, or in the wrong ways, and you ship bad products, or lose time isolating problems in your processes. Further complicate the testing balancing act with changing trends in design and manufacturing and new test technologies, and it becomes clear that finding the right regime is harder than ever.
PCB testing once was a relatively simple matter. The boards had components arrayed on one side and affixed with through-hole mounts. A relatively simple In-Circuit Test (ICT) using a so-called bed-of-nails fixture could hit the contacts on the back of the board and signal if an assembly was good or bad. As devices shrink in size and increase in complexity, effective testing faces a host of problems. Boards are double-sided or multi-layered, and pack more components onto less and less real estate. Higher circuit operation speeds limit the use of built-in test pads for ICT access.
“A major issue is that surrounding limited-access testing issues, says Jack Rozwat, general manager of the America’s SST Field Operations for Santa Clara, Calif.-based based Agilent Technologies, a manufacturer of optical and X-ray inspection devices and in-circuit testers. “Every year [boards] are getting more dense, and more complex.”
Modern manufacturing processes influence testing as well. Ball grid array packaging limits or eliminates access to mounted components, and lead-free soldering alters the type and distribution of faults present in assembled boards.
Today’s test engineer has a deep toolbox when it comes to testing. In the early stages of the production, Automatic Optical Inspection (AOI) can assist with solder-paste inspection and pre- and post-solder inspection. After soldering, X-ray technology can peer through mounted components to inspect connections.
“We’ve seen a lot of growth in our optical and X-ray businesses,” says Rozwat. “X-ray, by definition, is almost like the ultimate limited-access solution.”
Another technology that addresses the problem of limited access is known as boundary scan, or JTAG (after the Joint Test Action Group), a technology that was developed in the late 1980’s. As physical access for in-circuit testing went away with the adoption of ball grid arrays and increases in complexity, boundary scan technology offered a way to build testing capacity into board and component architecture to improve test coverage. Utilizing IEEE standard 1149.1, designers could now build in circuitry on a device or board level to assist in testing, maintenance and support of printed circuit boards. At its simplest, boundary scan can provide a shorts-and-opens structural test, either alone, or to supplement traditional in circuit testing. As the technology matures and more board and component designs incorporate IEEE 1149.1 standards, boundary scan technology has begun to offer more testing and development options that have helped spur adoption.
“The traditional way was only ICT, then they added X-ray and AOI, and they added boundary scan into the mix in the early 2000’s,” says Glenn Woppman, president and CEO of Dallas-based ASSET InterTech, a producer of boundary scan testing equipment and software. “Now we’re seeing people say, ‘Maybe we get off the nails all together and use only non intrusive technologies.’ That’s AOI, the X-rays and us through the JTAG port with application of a shorts-and-opens structural test, functional integrity testing with emulation and the flexibility of in-system programming.”
Even in circuit testing is developing in new ways to keep it relevant. As testing options increase, interoperability, cost efficiency and programming capacity become important, according to John VanNewkirk, president and CEO of Arlington, Wash.-based CheckSum, a manufacturer of low-cost in-circuit testing and in-system programming equipment.
“Our view is if we make our platform very easy to use for in-circuit, get the job done at low cost, work well with boundary scan providers and be the best at programming,” says VanNewkirk.
In spite of the new tools and technologies, proper test strategies are still elusive. “There is no recipe for how much optical inspection you need, and how much X-ray inspection you need, how much boundary scan, and how much electrical testing you need,” says Rick Nelson, chief editor of Test & Measurement World. “This is all a matter of art.”
“Test engineers have to go out and be responsible for the alphabet soup of technologies to figure out the best test strategy,” says VanNewkirk.
