SMTA International Conference Proceedings


Authors: H.-J. Albrecht, M. Heimann, Ch. Götze, and M. Löffler
Company: Siemens AG
Date Published: 10/11/2007   Conference: SMTA International

Abstract: Based on experimental analysis of different lead-free solder materials for conventional and advanced components the quality of Pb-free interconnections, specially the interface characteristic will be discussed followed by reliability results. The leading lead free alloy choices are the near ternary eutectic SnAgCu alloys. The ternary eutectic is close to SnAg3.8Cu0.7 with a melting temperature of 217°C. The near eutectic, commercially available alloys are SnAg3.8Cu0.7 and SnAg4.0Cu0.5 compositions, furthermore SnAg3.0Cu0.5 as a under-eutectic material. Solder materials demonstrate evolving microstructure and mechanical behavior that changes significantly with environmental exposures such as thermal cycling and high temperature storage. Solder joints in electronic assemblies can fail by a variety of modes. One common mode is the interface failure, the separation between the solder material and the package and the pcb pad. The main crack path is usually in or very near the intermetallic layer formed during reflow and increased by accelerated aging conditions.

The damage mechanism for the components on different board finishes at elevated operating conditions is not sufficient understood in terms of the dependency of leadfree composition, the microstructure itself and the IMC formation related to all material and process parameter. One of the reason is the extended growth of binary, ternary and qua-ternary intermetallic layers and the related mechanical properties.

One area of particular interest is solder joint reliability under temperature cycling. A substantial platform of knowledge exists regarding the performance of the eutectic Pb-Sn alloy in this area, including analyses of the impact of various different pad surface finishes (Imm Sn, Imm Ag Ni-Au, Cu OSP, HAL finishes etc.), components, upon joint integrity. Similarly, due to the intensive testing of the SnAgCu (SAC) alloys, much data have been generated to characterize their performance under comparable test conditions. However, it has been found by several experiments that the SnAgCu alloys exhibit increased sensitivity to high strain rate failure (drop testing depend on board finishes and phase formation), that the crack growth is different after TCT as well as the property, under extended temperature and time conditions, to develop Kirkendall void-induced joint failures (Cu OSP board finish). The rigidity of SAC solders is mainly imparted by the Ag3Sn and Cu6Sn5 (Fig. 1) intermetallic particles and be modified by changes in the original composition (Ag content).

Temperature cycling test conditions should be consistent with the IPC 9701 specification. Here are different test conditions applied to study the performance of lead-free interconnects. Mostly the TCT conditions were linked to compare results with SnPb based experiences and to guarantee the equivalence by choosing lead free solders. Thermal cycling accelerated life testing is an established technique for thermo-mechanical evaluation of electronic assemblies. The goal of such experiments is to gage the field life and identify failure mechanisms of solder joints by subjecting it to temperature cycling conditions that are harsher (larger temperatures and/or more rapid transitions) than expected in the real application. The knowledge about the influences are not complete, therefore tests have to be continued to collect the physics of failure.

To predict the lifetime of lead-free solder joints different accelerated test procedures are applied in terms of the study of the main fatigue mechanism. The degradation of lead-free solder joints is different to the conventional lead-containing solder materials. Therefore the lifetime prediction based on the collection of failure free cycles or hours in the test must be collected with the study of the different failure modes depend on the acceleration method.

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