Gary L. Zender

Pull strength evaluation of Sn-Pb solder joints made to Au-Pt-Pd conductor on low-temperature co-fired ceramic

Cracking was observed in the side walls of gold (Au) filled vias in low-temperature, co-fired ceramic (LTCC) substrates. Further analysis indicated the likely source as the constituents of the glassy phase component of the gold platinum-palladium (Au-Pt-Pd) thick film used for the conductor traces and pads. The successful approach toward mitigating the cracking phenomenon was to place a Au thick film layer between the Au-Pt-Pd layer and the LTCC substrate, which significantly curtailed the diffusion of glassy phase components into the latter. However, it was necessary to determine the effects of the additional thick film layer on the microstructure and overall mechanical strength of tin-lead (Sn-Pb) solder joints made to device pads. Acceptable pull strengths were measured in the range of 3.5 – 4.0 lbs. The solder joint pull strength was sensitive to the number of firing steps as defined by the thick film layer construction. Both the solder/thick film and thick film/LTCC interface strengths had roles in this trend, thereby affirming the synergism between material, interfaces, and the firing processes The pull strength was optimized when the pad length ratio, 4596:5742, was 1.0:0.5, which was characterized by a reduced occurrence of the thick film/LTCC failure mode.

Pull strength evaluation of Sn-Pb solder joints made to Au-Pt-Pd and Au thick film structures on low-temperature co-fired ceramic -final report for the MC4652 crypto-coded switch (W80).

A study was performed that examined the microstructure and mechanical properties of 63Sn-37Pb (wt.%, Sn-Pb) solder joints made to thick film layers on low-temperature co-fired (LTCC) substrates. The thick film layers were combinations of the Dupont{trademark} 4596 (Au-Pt-Pd) conductor and Dupont{trademark} 5742 (Au) conductor, the latter having been deposited between the 4596 layer and LTCC substrate. Single (1x) and triple (3x) thicknesses of the 4596 layer were evaluated. Three footprint sizes were evaluated of the 5742 thick film. The solder joints exhibited excellent solderability of both the copper (Cu) lead and thick film surface. In all test sample configurations, the 5742 thick film prevented side wall cracking of the vias. The pull strengths were in the range of 3.4-4.0 lbs, which were only slightly lower than historical values for alumina (Al{sub 2}O{sub 3}) substrates. General (qualitative) observations: (a) The pull strength was maximized when the total number of thick film layers was between two and three. Fewer that two layers did not develop as strong of a bond at the thick film/LTCC interface; more than three layers and of increased footprint area, developed higher residual stresses at the thick film/LTCC interface and in the underlying LTCC material that weakened the joint. (b) Minimizing the area of the weaker 4596/LTCC interface (e.g., larger 5742 area) improved pull strength. Specific observations: (a) In the presence of vias and the need for the 3x 4596 thick film, the preferred 4596:5742 ratio was 1.0:0.5. (b) For those LTCC components that require the 3x 4596 layer, but do not have vias, it is preferred to refrain from using the 5742 layer. (c) In the absence of vias, the highest strength was realized with a 1x thick 5742 layer, a 1x thick 4596 layer, and a footprint ratio of 1.0:1.0.