Journal of SMT Article


Author: Jinlin Wang
Company: Intel Corporation
Date Published: 10/1/2001   Volume: 14-4

Abstract: Dynamic contact angle (DCA) systems are used to provide information on wetting, adhesion, and surface cleanliness via contact angle measurements and surface energy characterization. The measurements are used both for materials and process characterization as well as for failure analysis. Traditionally, a manually operated goniometer is used for contact angle measurement, which is not very repeatable due to operator to operator variations and is also very tedious. The recent advances in contact angle measurement instrumentation have made contact angle measurement much easier and more reproducible. New contact angle measurement systems often employ a CCD camera to capture the video image of a droplet on a sample, which is digitized and analyzed, using image analysis software. The system can also provide dynamic measurement to capture time dependence of interfacial properties and measure surface tension for liquids, using a pendant drop technique.

In this paper, we will discuss some of the applications of a video contact angle system to the organic land grid array (OLGA) packages. The interfacial properties between underfill, die, and substrate; and thermal interface material, die, and heat spreader; are important to the integrity of the package. The consistency of substrate surface is critical for the quality of the OLGA package. The contamination of substrate surfaces will increase the contact angle and cause a dewetting problem. The effect of substrate cleanliness on the contact angle was investigated. Cleaned surfaces had much smaller contact angles than uncleaned surfaces. The difference in surface energy has a great impact on the underfill processing. Substrates with high surface energy typically showed fewer voids and bubbles in cured underfill material than low surface energy substrates. Three probing liquids, DI water, methylene iodide, and glycerol were used in contact angle measurements for two OLGA substrates from different suppliers. The contact angles for the three liquids were consistently larger for one substrate than the other. The surface energies were calculated, using different methods, such as geometric mean, harmonic mean, and acid-base methods. The substrate with low contact angles had high surface energy. The contact angles of an underfill on these substrates were measured at the dispensing temperature of the underfill to validate the room temperature results. These results qualitatively agreed with the room temperature results. The contact angle of underfill on OLGA substrates decreased with time due to viscous resistance of the underfill. The wettability of different underfills was studied. The contact angle of two underfill materials under both uncured and cured conditions were measured on silicon dice at room temperature. Significant differences were noticed in the contact angles measured for these two underfills, which also exhibited very different flow behavior and curing properties. The surface energy of a silicon wafer and solder coupons were also measured.

The surface energies for several heat spreaders with different surface treatments were calculated, using different methods. The surface energy of the heat spreaders correlated well with adhesion results of thermal interface material on heat spreader. The heat spreaders with low surface energy typically showed delimination at the interface of heat spreader and thermal interface material.

Our results showed that contact angle measurements could provide useful information about interfacial tension and are a good tool for quality control and product development where interfacial properties are important.

Keywords: OLGA, wettability, contact angle, surface energy.

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