Enhanced thermal fatigue endurance and lifetime prediction of lead-free LGA joints in LTCC modules

Purpose - The purpose of this paper is to describe the effect of indium alloying on the thermal fatigue endurance of Sn3.8Ag0.7Cu solder in low-temperature co-fired ceramic (LTCC) modules with land grid array (LGA) joints and the feasibility of using a recalibrated Engelmaier model to predict the lifetime of LGA joints as determined with a test assembly. Design/methodology/approach - Test assemblies were fabricated and exposed to a temperature cycling test over a temperature range of -40-125 degrees C. Organic printed wiring board (PWB) material with a low coefficient of thermal expansion was used to reduce the global thermal mismatch of the assembly. The characteristic lifetime, theta, of the test assemblies was determined using direct current resistance measurements. The metallurgy and failure mechanisms of the interconnections were verified using scanning acoustic microscopy, an optical microscope with polarized light, and scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) investigations. Lifetime predictions of the test assemblies were calculated using the recalibrated Engelmaier model. Findings - This work showed that indium alloying increased the characteristic lifetime of LGA joints by 15 percent compared with Sn3.8Ag0.7Cu joints. SEM/EDS analysis showed that alloying changed the composition, size, and distribution of intermetallic compounds within the solder matrix. It was also observed that a solid-state phase transformation (Cu,Ni)(6)Sn-5(-> (Ni,Cu)(3)Sn-4 occurred at the Ni/(Cu,Ni)(6)Sn-5 interface. Moreover, the results pointed out that individual recalibration curves for ceramic package/PWB assemblies with high (>= 10 ppm/degrees C) and low (approximate to 3-4 ppm/degrees C) global thermal mismatches and different package thicknesses should be determined before the lifetime of LGA-type assemblies can be predicted accurately using the recalibrated Engelmaier model. Originality/value - The results proved that indium alloying of LGA joints can be done using In-containing solder on pre-tinned pads of an LTCC module, despite the different liquidus temperatures of the In-containing and Sn3.8Ag0.7Cu solders. The characteristic metallurgical features and enhanced thermal fatigue endurance of the In-alloyed SnAgCu joints were also determined. Finally, this work demonstrated the problems that exist in predicting the lifetime of ceramic packages with LGA joints using analytical modeling, and proposals for developing the recalibrated Engelmaier model to achieve more accurate results with different ceramic packages/PWB assemblies are given.