Drop test reliability of wafer level chip scale packages

Abstract

The reliability of the two different types of WL-CSP components being reflow-soldered with a near eutectic Sn3.8Ag0.7Cu and eutectic SnPb solder pastes on Ni(P)/Auand OSP-coated multilayer printed wiring boards have been investigated by employing the standard drop test, statistical failure analyses, fractography and microstructural characterization methods. A significant difference in the reliability performance of the components was observed: the components with the (Al)Ni(V)/Cu metallization (UBM) were more reliable than those with the electroless Ni(P)/Au metallization -independently of the bump, solder paste, surface finish materials or the pad structure of the boards used. The failure analyses revealed that the primary failure mode in the component side is the cracking of interconnections along a brittle NiSnP layer between the electroless Ni(P) of high P-content and the solder alloy, while components with (Al)Ni(V)/Cu UBM fail by cracking along the [Cu,Ni]6Sn5 intermetallic layer. On the board side the cracking occurs in the porous NiSnP layer formed between the electroless Ni(P) metallization and the (Cu,Ni) 6 Sn 5 intermetallic layer. The fact that the cracking occurs predominantly at the components side reaction layer is due to three factors: higher normal stresses in the component side, brittleness of the reaction layer(s) and the strain-rate hardening of the bulk solder interconnections. It is to be noted that the primary failure mode differs from that typically observed in thermally cycled test assemblies, where the nucleation and propagation of cracks are strongly enhanced by the recrystallization of the solder inter-connections.

Improved Drop Reliability Performance with Lead Free Solders of Low Ag Content and Their Failure Modes

“…[1] Extensive studies on improving drop reliability with lead free solder have been focused on two directions, namely investigations on the effect of surface finish methods of substrate [2,3,4] and different composition of lead free solders, or combination of both factors [1,5]. For the effect of lead free solder composition, improved drop resistance on board level reliability had been reported with low Ag content solder, such as Sn-lAg-0,5Cu (SAC 105) [1,5].…”

Effect of Pd Addition in ENIG Surface Finish on Drop Reliability of Sn-Ag-Cu Solder Joint

“…The number of falls that the specimen can withstand before failure is regarded as the lifetime of the specimen. [8][9][10][11][12][13] In the case of an ENIG surface finish, a P-rich Ni layer is formed with a relative increase in the P content in the Ni-P layer according to the reaction between Sn and Ni in the solder after reflow or thermal aging, and it is known that such a P-rich Ni layer is sensitive to external shocks, because of the Kirkendall voids distributed within this layer. 14) Accordingly, in this study, the drop test with or without Pd is performed by jointing a representative lead-free solder consisting of Sn-Ag-Cu on boards with ENIG (Electroless Nickel Immersion Gold) and ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) surface finishes.…”

Wettability study of lead free solder paste and its effect towards multiple reflow

“…Based on the results of many investigations, 96.5Sn-3.0Ag-0.5Cu solder alloy had been selected since it possesses properties which could replace lead solder [6][7][8]. Though, this type of solder alloy has some disadvantages such as higher melting point (216-218 o C) and poor wettability as compared to tin-lead solder.…”