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The influence of the coating layer of the soldered device to the reliability of their interconnection with Ni coated Cu wires
Keywords: intermetallic compounds, reliability, diffusion
Goal:Figure out the influnce of the soldered device pad on the microsture evolution and reliability of the solder bump. Especially, the exist of Cu and the thickness of the solder bump. Method: Three kinds of devices with different coating layers were soldered with Ni coated Cu wires. Device #I had Ni pad coated with 10μm Ag; device #II had Cu pins coated with 3μm Ag; device #III coated the ceramic base with 10μm Pd and the thickness of the solder changed from 10μm of #I and # II to 80μm. All of them were soldered with Ni coated Cu wires with SnAgCu305 and then aged at 175℃ for 5 weeks. Cross section and pull test are done every other week. Optical microscope were used to measure the thickness of the IMCs, SEM equipped with EDX were applied to analyze the evolution of the IMCs’ microstructure and the facture interface. The fracture path and reason were also investigated. Results: For device #I, right after soldering, only Ag3Sn and Ni3Sn4 were found at the interfaces at the device side and Cu wire side respectively. 3 weeks later, the Ni3Sn4 were changed into (Ni,Cu)3Sn4 owing to the diffusion of Cu atom in the solder into Ni3Sn4. The activation energy of this Ni3Sn4 based IMC was 63.0KJ/mol, according to Arrhenius Equation. For Device #II, although its pins were coated with Ag like #I. The IMCs evolution in them were quite different. At the device side, the IMCs evolved from only Ag3Sn to Cu6Sn5 (inside) + Ag3Sn (outside) and then Cu3Sn (inside) + Cu6Sn5 (middle)+Ag3Sn(outside). At the Cu wire substrate side, the IMCs developed from Ni3Sn4 to (Cu, Ni)6Sn5 and then (Cu, Ni)6Sn5 (outside) + (Ni,Cu)3Sn4(inside). The only difference between device #II and #I was the layer inside Ag coating was Cu and Ni respectively. As Cu atoms can diffuse faster then Sn atoms, they can cross the Ag/Ag3Sn layer to increase the Cu proportion in the solder and then the Cu proportion in the IMCs at the surface of the Cu wire. In this system, the increase of the thickness of the IMCs were faster with activation energy as 62.7KJ/mol. The tensile strength was decreased rapidly because of the IMC category change. For Device #III, although there was no Cu at the device side, the solder bump thickness was 8 times of the first two. The thickness increase along with time was the fastest with an activation energy 62.0KJ/mol. Meaning the Cu diffusion from solder was faster than from the device.
Ying Zhong, Phd. Candidate
Harbin Institute of Technology
Harbin, Heilongjiang

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