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Small-chip Attachment on Copper Leadframe with Sintered Nanosilver Paste
Keywords: sintered nanosilver paste, low temperature die attachment, copper leadframe
Sintered nanoscale silver paste provides a low-temperature alternative to solder for die attachment. Unlike solder, the sintered attachment does not melt upon reaching the original attachment temperature and therefore may be used at higher temperatures. Again, unlike solder, no brittle intermetallic compounds are formed during the bonding process that may affect reliability. Higher electrical and thermal conductivities mean less Joule heating and better heat dissipation characteristics. The porous microstructure imparts low elastic modulus for lower thermomechanical stress and enhanced reliability. The state of the technology has reached a point where it is now possible to obtain die-shear strengths comparable to solder at sintering temperatures between 250C and 280C with little or no applied pressure, depending on the chip size. In addition to attachments on silver or gold-coated surfaces, it is possible to form bonds on bare copper if done under inert or slightly reducing atmosphere. Because attainment of a strong bond depends on the paste being in contact with a clean (oxide-free or untarnished) surface, a study was made to determine if the attachment process will work on copper leadframes with either an anti-tarnish or anti-EBO (epoxy bleed-out) coating. Mechanical silicon devices with silver back metallization measuring less that 3 mm x 3 mm were attached on the leadframes without pressure at temperatures as low as 260C. The heating profile consisted of a drying stage with a peak temperature not exceeding 180C and a sintering stage lasting 10 minutes at peak temperature. The sintering temperature ranged from 260C up to 280C. The total heating time was designed to fit within a 1 hour window. The sintering atmosphere in the chamber was varied from pure nitrogen, to nitrogen + 4% hydrogen and to nitrogen + 1% oxygen. Measurements showed that attachments sintered in pure nitrogen or nitrogen with hydrogen produced die-shear strengths of at least 30 MPa even at temperatures as low as 260C and was just as strong as those bonded on bare copper. Sintering in nitrogen + 1 % oxygen caused the die-shear strength to drop below 30 MPa but still above 20 MPa. In the presence of oxygen, the binder removal is due to oxidative combustion. The low level of oxygen appeared to cause incomplete binder burnout that interfered with the sintering while also causing some oxidation of the copper. In pure nitrogen, the telltale signs of incomplete burning were not observed. In nitrogen+ hydrogen, the die shear strength appeared to have been enhanced slightly along with the sintered microstructure. Sheared attachments that exposed the copper surface showed patches of silver still attached indicating formation of strong bond with the copper. The results also indicate that the type of leadframe coating, either anti-tarnish or anti-EBO coating, does not significantly affect the bonding strength.
Jesus N. Calata, Research Scientist
Virginia Tech
Blacksburg, VA

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