Adhesion of Screen Printed Conductors on Laser Reduced AlN
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ABSTRACT
Aluminum Nitride (AlN) has not attained the type of success alumina has had with thick film materials. This project is aimed at increasing the number of thick film inks compatible with AIN. Excimer laser reduced AIN is metallic, but not sufficiently conductive to act as an interconnection for microelectronic circuits. The reduced surface acts as a bonding layer to promote adhesion of screen printed metal films. Design of Experiments (DOE) plays a critical role in determining the success of a research project. Completely Randomized Nested Classification design strategy is used in these experiments. The Ag-Pd ink formulated for alumina is screen printed on AIN and alumina substrates in a random order and fired. Wires are soldered to the pads and a pull test is done. Analysis of Variance (ANOVA) is used to compare the adhesion strength of Ag-Pd ink formulated for alumina screen printed on A120 3 and laser reduced AIN substrates.
INTRODUCTION
Background
AIN due to its non-toxic as well as good electrical and thermal properties has great potential as a substrate for both thick and thin film technologies. AIN ceramics are suitable as a substrate material for electronic circuits because of their high thermal conductivity ( 90-170 W/mK), high electrical resistivity ( 1011-1014 _-cm ) and low thermal expansion coefficient (4.34.5 x 10 -6/oC ). AIN, however, has not attained the type of success alumina has had with thick film materials. Although some new commercial inks have been developed for deposition on AIN, the selection of inks for alumina substrates is much greater and offers options for firing temperature, solder compatability and cost. A new technique under development uses an ultraviolet excimer laser to reduce the surface of AIN to an aluminum metal rich layer followed by screen printing to deposit metal ink on the reduced areas [1]. UV laser wavelengths are highly absorbed by AIN causing the irradiated region to heated rapidly. When heated, AMN does not melt, it decomposes to aluminium metal and nitrogen gas. Thick film inks fired on AMN have produced porous bonds due to reaction of PbO with AIN to produce nitrogen bubbles [2]. This new technique of laser reducing the AMN substrate to Al metal should allow the PbO from the glass based thick films to oxidize the metal without the formation of nitrogen bubbles. Preventing bubble formation will produce higher 263 Mat. Res. Soc. Symp. Proc. Vol. 390 0 1995 Materials Research Society
density films. The objectives of the technique are: to increase the number of inks that are compatible with AMN and allow AIN substrates to be processed in the printing and firing facilities designed for alumina substrates. Ag-Pd (4:1) ink developed by FERRO for alumina is used for screen printing. A Du Pont test pattern and test procedure for testing adhesion strength has been used. Screen printing is done on the substrates in a random sequence and fired in an infrared furnace. Tinned copper bus bar wires are soldered to the pads and a pull test is done.
Excimer Lasers La
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