PRINTED CIRCUIT BOARDS (PCBs) and the technologies they support are among the crowning achievements of the Information Age. Encrusted with tiny transistors and wires and gilded with solder, the boards run everything from portable radios to refrigerator-sized supercomputers.

PHOTO BY GARY MEEK Dr. Charles Ume (right) and former grad student Michael Stiteler examine moiré fringes generated when a PCB is heated in oven Ume developed.

Despite their widespread use and success, the boards remain vulnerable to a simple, heat-induced threat: warpage. A board that warps while in use may cause a computer to stop working correctly. Even a slight twist or bend in a board during manufacturing can make adding components devices difficult, or may cause previously mounted electronics to break off. And that can cost manufacturers a bundle, says Dirk Zwemer, manager of Electronic Packaging Services Ltd., Co.

"It's not uncommon to see yield losses of one to three percent in a mature product, and it can be much higher for some designs," he says. "In some cases, to lose a board at a stage where lots of expensive components have been added can cost a company thousands of dollars per board."

But Zwemer's company offers a way of detecting PCB warpage -- patent-pending technology developed at and licensed from the Georgia Institute of Technology. Dr. Charles Ume of Georgia Tech's School of Mechanical Engineering developed the novel experimental technique for observing and recording PCB warpage.

Heat Sources

To understand Ume's techniques, developed in the Advanced Electronic Packaging Lab in Georgia Tech's Manufacturing Research Center (MARC), one must understand the sources of the heat that can warp PCBs. Heat is an integral part of processing the boards, and increasingly, of using them. Warmer temperatures are generated every time we turn on our computers, camcorders, radios and other PCB-reliant devices. The more often we turn electronic equipment on, the more often the PCBs inside the equipment are subjected to high levels of heating and cooling.

"In addition, the current trend in the industry is making the boards smaller, thinner and more densely populated," Ume explains. "If the PCB is small, thin and also densely populated with components, that is an invitation for warpage-related reliability problems."

Under operating conditions, these components give off a lot of heat in a small area. The heat, in turn, causes the PCB to warm. The degree of warpage will depend on how thin the PCB is.

Additionally, the boards are heated in ovens at about 135 degrees Centigrade during the solder masking process, when a coating that repels solder from certain areas of the boards is applied. The PCBs return to the oven, and to temperatures of about 220 degrees Centigrade, when chips are soldered to them.

Ume's Techniques

The flat glass substrate etched with equally spaced parallel lines is placed parallel to the PCB. A beam of white light is directed onto the glass at a specific angle, and the etched lines on the glass create a shadow on the surface of the PCB. When the surface of the PCB is curved, a moiré pattern is produced by the geometric interference between the etched lines on the glass and the shadow of those lines on the PCB's surface. The more the PCB warps, the greater number of moiré fringes appear.

Ume counts the number of fringes, puts them into an equation, and a computer determines how much warpage has occurred. The warpage process is displayed in real time on a television screen and recorded on video and on computer.

Chart of Young's Modulus for FR-4 (116/116) vs. Temperature

"Electronic packaging companies can use the warpage information to make changes in their PCB design early in the design phase," Ume says. "They may choose to make some design or process adjustments and send the board to us again to be retested. That way, there's no mass production of a product that has a problem."

A patent is pending on Ume's process, which is commercially known as TherMoiré(TM).

The TherMoiré(TM) technique can be used to simulate the three major kinds of soldering processes -- infrared reflow, convective reflow and wave. The automated oven system measures warpage in real time. It can reproduce any given soldering temperature history used in producing a board, while measuring PCB warpage at any specified interval or temperature. As a result, the system can pinpoint which processes or designs may cause the most warping.

Companies can use the results to make design or process changes before production, such as changing soldering temperature profiles, reducing or extending processing times, relocating key components, and changing types of materials used in the construction of the PCB.

In addition to measuring thermally induced warpage, the technique can be used to validate manufacturers' numerical warpage predictions using the finite element technique.

"Most of the companies that manufacture PCBs have experts who can predict when a board will warp using finite element analysis," Ume says. "They can use the techniques we've developed here to check their results. We can also do numerical predictions for them."

If a certain amount of warpage is allowable, the new techniques allow manufacturers to measure initial warpage, rather than assuming that the board is flat before transistors and other items are added to it. Manufacturers can then determine how much additional warpage is added with further processing or addition of components.

Ume's techniques also allow warpage measurement of the different materials that are sandwiched together to make a wiring board -- FR-4 laminates, fiber (prepreg), several varieties of copper foil and newly developed materials.

"These are unique measurement techniques, and the electronic packaging industry is very excited about them," Ume says. "These systems and prediction capabilities will help PCB designers and process engineers to understand how a PCB will warp when it goes through different manufacturing cycles -- even before it is built.

"Savings in scrap PCBs, rework, down time and loss in market share can run into the millions of dollars," Ume says.

Going Commercial

The sponsors of Ume's work asked him to turn his findings into a commercial venture. With help from Georgia Tech's Advanced Technology Development Center, the Electronic Packaging Services Ltd., Co. licensed the technology and began offering help to the electronics industry in August 1994.

Georgia Tech is a partner in the company.

"Industrial sponsors were a driving force behind getting this technology to the marketplace," says business development manager Patrick Hassell. "Current trends in the industry, with respect to outsourcing and dowsizing of internal R&D efforts, make EPS a valuable contract services provider of fast, reliable analysis."

Although the company is aimed at helping the electronics industry, future users of the technology could include manufacturers of aviation equipment, foil, tape, resin and glass fibers, says manager Dirk Zwemer. Future possibilities include putting the technology on an assembly line and making it more automated so the average worker could use it.

"The technical applications are larger than the original sponsors knew it would be when they started," he says. "We want to be a good example of technology transfer."

Figures 1 and 2 -- 26°C before simulated soldering process
Figures 3 and 4 -- 216°C peak temperature
Figures 5 and 6 -- 28°C after simulated soldering process

Further information is available from Dr. Charles Ume, School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405. (Phone: 404/894-7411) (E-mail: charles.ume@me.gatech.edu)

Also contact Dirk Zwemer or Patrick Hassell at Electronic Packaging Systems Ltd. Co., 430 10th Street, Ste. S-003, Atlanta, GA 30318. (Phone: 404/881-1114) (Fax: 404/881-1614) (E-mail: EpsEpsEps@aol.com)

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Last updated: 27 Nov. 1996