Low-temperature co-fired ceramic (LTCC) based system-in-package (SiP) is an emergingmultilayer module technology for wireless communication applications, mainly due to itsexcellent high-frequency material properties. LTCC-SiP modules are typically soldered onto anorganic motherboard, but the lifetime of the 2nd-level solder joints is often poor due to the highstress level of the joints in test/field conditions. Moreover, using lead-free solders in theinterconnections of LTCC modules raised new questions about the feasibility and reliability of thesolder joints in LTCC applications. Therefore, the characteristic features of the 2nd-level solderjoint configuration were determined in this thesis work.
It was proved that collapsible Sn4Ag0.5Cu spheres are not a feasible option in LTCC/PWBassemblies with a large global thermal mismatch; a non-collapsible ball grid array (BGA) jointwith a plastic core solder balls (PCSBs) was required to attain an adequate lifetime for suchassemblies. To enhance the thermal fatigue endurance of the non-collapsible lead-free joints, anovel BGA joint consisting of Sn7In4.1Ag0.5Cu solder and PCSBs was developed. Moreover,this work proved that there is a relationship between the primary failure mechanisms of variousSn-based lead-free solders and thermomechanically induced stress level in the present non-collapsible BGA joint configuration.
The effect of the plating material of the solder lands on the failure mechanism of the BGAjoints in the LTCC/PWB assemblies was studied. The results showed that the adverse phenomenarelated to the sintered Ag-based metallization materials can be avoided using electroless nickelwith immersion gold (ENIG) as a deposit material. On the other hand, this study also demonstratedthat the inadequate adhesion strength of the commercial base metallization in the ENIG-platedmodules resulted in the disadvantageous failure mechanism of the test assemblies. Therefore, thecriteria for material selection and the design aspects of reliable 2nd-level interconnections arediscussed thoroughly in this thesis.
