Process Tracks Circuit Board Warpage To Improve Quality

November 15, 1996

Printed circuit boards (PCBs) power everything from portable radios to refrigerator-sized supercomputers, but they remain vulnerable to a simple, heat-induced threat: warpage. A warped PCB may cause a device to stop working. Boards that warp during manufacturing after expensive components are added can mean thousands of dollars in losses.

Now manufacturers have a new weapon against warpage: Thermoiré®, a novel experimental technique developed at the Georgia Institute of Technology and licensed by Electronic Packaging Services (EPS) Ltd., Co. The patented process provides real-time data about PCB warpage, helping manufacturers avoid design problems and save money, said Thermoiré® developer Dr. Charles Ume.

"Electronic packaging companies can use the warpage information to make changes in their PCB design early," said Ume, an associate professor in the School of Mechanical Engineering. "That way, there's no mass production of a product that has a problem."

The heat that can warp PCBs is generated each time we turn on computers, camcorders or other PCB-run devices. Also, temperatures up to 230 degrees Celsius are an integral part of PCB processing.

"In addition, if the PCB is small, thin and also densely populated with components, as is the current industry trend, that is an invitation for warpage-related reliability problems," Ume said.

Noted EPS manager Dirk Zwemer: "It's not uncommon to see losses of one to three percent in a mature product, and it can be much higher for some designs."

Research sponsors, who were brought together through Georgia Tech's Manufacturing Research Center (MARC), include Motorola, MICOM, Ford Electronics, IBM, DEC and AT&T.

For this new process, Ume developed a special oven with a glass grating top, through which the PCB placed inside is visible. A white light shines through the glass grating onto the PCB, and an inexpensive, compact, charge-coupled device camera captures warpage digitally as it occurs.

The flat glass grating is etched with equally spaced parallel lines. It is placed above and parallel to the PCB. A beam of white light is directed onto the glass at a specific angle, causing the etched lines to create a shadow on the surface of the PCB. When the surface of the PCB curves due to warpage, 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 that 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.

Developed in MARC's Advanced Electronic Packaging Lab, the Thermoiré® technique can be used to simulate the three major kinds of soldering processes -- infrared reflow, convective reflow and wave.

The automated oven system can reproduce any given soldering temperature history used in producing a board, while measuring PCB warpage at any specified time interval or temperature. That means 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 the types of materials used in the construction of the PCB.

The ability to measure thermally induced warpage also enables manufacturers to validate their numerical warpage predictions, which are created using finite element modeling techniques.

If a certain amount of warpage is allowable, the new technique lets manufacturers measure initial warpage, rather than assuming the board is flat before transistors and other items are added. Manufacturers can then determine how much additional warpage develops during further processing or attachment of components.

Ume's technique also allows 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 said. "Savings in scrap PCBs, rework, down time and loss in market share can run into the millions of dollars."

The research sponsors asked Ume to commercialize Thermoiré®. With help from Georgia Tech's Advanced Technology Development Center, the EPS Ltd., Co. licensed the technology and began offering help to the electronics industry in August 1994. Georgia Tech is a partner in the company.

Although EPS was formed to help the electronics industry, future Thermoiré® users could include manufacturers of foil, tape, resin, glass fibers and aviation equipment, said manager Dirk Zwemer. Other possibilities include putting Thermoiré® on an assembly line and making it more automated for the average worker.

"The technical applications are larger than the original sponsors knew it would be when they started," he said. "We want to be a good example of technology transfer."
-end-
RESEARCH NEWS AND PUBLICATIONS OFFICE
430 Tenth St. N.W., Suite N-112
Georgia Institute of Technology
Atlanta, Georgia 30318
MEDIA RELATIONS CONTACTS:
John Toon (404-894-6986);
Internet: john.toon@edi.gatech.edu;
FAX: (404-894-6983)

TECHNICAL:
Dr. Charles Ume (404-894-7411)
Internet: (charles.ume@me.gatech.edu);
Dirk Zwemer or Patrick Hassell, Electronic Packaging Systems Ltd. Co. (404-881-1114);
Internet: (EpsEpsEps@aol.com).

WRITER: Lea McLees
-end-


Georgia Institute of Technology

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