Effect of Thick Film Firing Conditions on the Solderability and Structure of Au-Pt-Pd Conductor for Low-Temperature, Co-Fired Ceramic Substrates

Low-temperature, co-fired ceramics (LTCC) are the substrate material-of-choice for a growing number of multi-chip module (MCM) applications. Unlike the longer-standing hybrid microcircuit technology based upon alumina substrates, the manufacturability and reliability of thick film solder joints on LTCC substrates have not been widely studied. An investigation was undertaken to fully characterize such solder joints. A surface mount test vehicle with Daisy chain electrical connections was designed and built with Dupont{trademark} 951 tape. The Dupont{trademark} 4569 thick film ink (Au76-Pt21 -Pd3 wt.%) was used to establish the surface conductor pattern. The conductor pattern was fired onto the LTCC substrate in a matrix of process conditions that included: (1) double versus triple prints, (2) dielectric frame versus no frame, and (3) three firing temperatures (800 C, 875 C and 950 C). Pads were examined from the test vehicles. The porosity of the thick film layers was measured using quantitative image analysis in both the transverse and short transverse directions. A significant dependence on firing temperature was recorded for porosity. Solder paste comprised of Sn63-Pb37 powder with an RMA flux was screen printed onto the circuit boards. The appropriate components, which included chip capacitors of sizes 0805 up to 2225 and 50 mil pitch, leadless ceramic chip carriers having sizes of 16 I/O to 68 I/O, were then placed on the circuit boards. The test vehicles were oven reflowed under a N{sub 2} atmosphere. The solderability of the thick film pads was also observed to be sensitive to the firing conditions. Solderability appeared to degrade by the added processing steps needed for the triple print and dielectric window depositions. However, the primary factor in solderability was the firing temperature. Solderability was poorer when the firing temperature was higher