Underfill Material Permits Flip Chip Rework
01/01/2000
Rocky Hill, Conn.
At the culmination of more than a year`s research and development, Loctite Corp. has introduced Loctite 3567 for flip chip, ball grid array (BGA) and chip scale packaging (CSP) assemblies. The material is said to provide the processing and reliability of conventional thermoset underfills with the added advantage of reworkability. Developed jointly with Cornell University, it relies on a patented epoxy monomer added to the formulation to permit thermal rework. The project is part of the National Institute of Standards` Advanced Technology Program in microelectronics packaging and is expected to be crucial to the development of other products, including wafer-applied fluxing reworkable underfills.
Until recently, flip chip technology has been seen only on high-reliability, high-cost devices because of known good die issues, the high cost of hybrid multilayer printed wiring boards, and off-line testing and processing requirements. Such assemblies also carried an inability to rework underfill adhesives. By contrast, this new product reportedly permits a flip chip to be replaced when testing determines its defectiveness, thus minimizing incidents compelling the entire board to be discarded. For this reason, reworkable materials are especially important for complex, expensive multichip modules wherein the entire device is rendered unusable if only one chip is defective.
In using the new thermoset, a defective package is removed from the printed circuit board (PCB) by heating the package and underfill to standard rework temperatures. Next, a gentle high-speed brushing, requiring no solvents or acids, prepares the site for a replacement device, followed by an examination of the boards by infrared to verify the cleanup. Finally, thermal cycling tests conducted on the second device confirm that the rework process has not diminished its performance.
"We see reworkable underfills as crucial to the increased usage of flip chip technology," says Dr. Larry Crane, manager of product development at Loctite. "The ability to rework flip chips will make this technology as commonly accepted as other packaging methods, such as CSPs and BGAs, which have already been tested at the package level prior to board level assembly."
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Figure 1. Heating CSP and BGA assemblies is possible with reworkable underfill adhesives, thus removing the need to scrap the entire PCB.
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Figure 2. After a defective flip chip is removed, gentle high-speed brushing eliminates any remaining residue. No solvents or acids are needed.