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.
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
With sponsorship from Motorola, MICOM, Ford Electronics, IBM, DEC and AT&T through MARC, Ume developed a special oven with a glass top through which the PCB placed inside is visible. A white light shines on the PCB through the glass grating, and an inexpensive, compact, charge-coupled device (CCD) camera captures warpage digitally as it occurs.
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. |
