Optical Probes and Electrical Resistivity Measurements of Conductive Die Attach Adhesives
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245 Mat. Res. Soc. Symp. Proc. Vol. 515 ©1998 Materials Research Society
In this study, we have incorporated the surface and chemically specificity of SERS to analyze the chemical nature of a lubricant/flake interface in either the neat form or in a filled conductive adhesive. In particular, our studies focused on the thermally-induced variations in the chemical properties of the Ag flake surface either in an ambient environment (air) or embedded in a polymer host. Numerous studies have shown that this optical technique possesses sufficient chemical specificity and surface sensitivity to monitor interfacial processes such as adsorption and decomposition. In addition to these optical studies, complementary DC electrical conductivity measurements were performed on the filled adhesive under thermal processing conditions that are employed in the curing stage. EXPERIMENTAL PROCEDURE SERS was chosen for the vibrational analysis since previous Raman studies of ellipsoidal and spherical shaped Ag particles and commercially available Ag flake have demonstrated a significant increase in the molecular scattering efficiency via the electromagnetic enhancing nature a cw of the this geometry [4,5]. Raman spectra were obtained by using the 514.5 nm output from Spectra Physics 164 Argon Ion laser. The incident intensities were kept below 0.5 W/cm2 so as to avoid both photo-thermal and photo-chemical modifications in the adsorbate layer on the Ag surface. The collected scattered light was dispersed by a SPEX 1877 Triplemate spectrometer and detected by a Princeton Instruments liquid nitrogen cooled charge coupled device (CCD). Typical integration times for the acquisition of a surface vibrational spectra ranged from 200 to 600 seconds. Under the same fluence conditions, corresponding spectra from a neat fatty acid sample were recorded with 10 to 30 second integration times. As a control sample, initial SERS measurements were performed on Ag flake sample that was milled in a stearic acid/solvent system to produce an ellipsoidal surface area of - 2 gm by 10 gm, a sub-micron thickness, and a residual lubricant adlayer coverage of - 1.5 monolayers. The conductive epoxy was Ablebond 8175A, which was also used in the DC electrical conductivity measurements. The Ag flake that is used in this adhesive was also examined with SERS; however, details regarding flake processing were not available from the manufacturer, but it is presumed that the lubricant used was a fatty acid. All samples were investigated as received with no additional surface treatment or processing. Prior to all measurements, the Ag-filled epoxy was stored at - -40'C to ensure stability. The sample volume resistivity of the commercially available adhesive (Ablebond 8175A) was measured in-situ during the curing process. In the low ohmic range, 0.0001 - 10 ohm, fourterminal contact measurements were employed. A narrow Teflon® cell (4.5 cm length, 0.5 cm height, 0.5-cm width) was constructed to meet the requirement for maintaining isopotential lines during the 4-terminal me
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