Chemical Interactions of Au and Ag Conductors in Ceramic Based Electronic Packages
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Here we discuss the interaction between Au and Ag thick film conductors on separate layers of
a Au/glass-alumina composite/Ag heterostructure used in multichip module packaging [1]. These structures, know as low temperature cofired ceramics (LTCC) are formed by screen printing conductor patterns from metallic inks onto 80-90 ýim thick "green" dielectric tape. Layers of tape are screen printed with conductor patterns and possibly with vias and then are laminated together and fired to oxidatively burn-out the organic materials and to sinter the metal particles and the glass-alumina dielectric into a monolithic structure. In addition to conductors, simple circuit components such as resistors, capacitors and inductors can be produced [2]. To optimize corrosion resistance and wire bondability, the top or surface layer conductors are often printed using Au [3]. To minimize cost and maximize conductivity, buried conductors (those sandwiched between dielectric layers and after firing, hermetically sealed from the atmosphere) are often printed using Ag [3]. After firing structures containing Au and Ag conductors on separate but overlying layers, we and others have observed the formation of blisters at the Au conductordielectric interface [3-6]. A photograph of a parallel plate structure containing a surface Au conductor and a one dielectric layer deep Ag conductor is shown in Figure 1. Chemical analysis of the Au conductor indicates that after firing, the Au is alloyed with -2% Ag. In contrast, if the Au and Ag conductors are electrically connected then no blistering occurs, although, in this case after one firing the Au is alloyed with -10% Ag. We extend here these initial observations and more thoroughly examine the interactions leading to blistering and alloying in terms of the thermodynamic and kinetic stability of the Au/dielectric/Ag heterostructures.
275 Mat. Res. Soc. Symp. Proc. Vol. 390 ©1995 Materials Research Society
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I Ag+
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(a)
(b)
Figure 1. Blisters in surface Au conductor after firing. Au/Ag parallel plate structure (a). Schematic of cross section (b).
EXPERIMENTAL PROCEDURES The structures studied consisted of 8 layers of DuPont 951 dielectric tape printed with - 6x6 thick parallel capacitors. The capacitors were screen printed on the top (surface) mm2 and 10 W.m layer and buried one or two layers down giving capacitors with dielectric separations of 80 or 160 pim. The remaining 5 or 6 layers were unprinted and served to strengthen the structure. Surface conductors were Au (DuPont 5734 ink) and buried conductors were Ag (DuPont 6142 ink). Structures were also prepared with the Au and Ag conductor plates electrically connected using a Ag (DuPont 6141) or Ru (DuPont 5740) via through the dielectric tape layer(s). Laminated structures were fired in air using the following profile: ramp from room temperature to 2500C at 50C/min, 0.5 hr. dwell at 250 0C, ramp to 4500C at 30C/min, dwell at 4500C for 2 hrs. to insure complete removal of the organic binder, ramp to 850 0C at 4*C/min, 0.5 hr. dw
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