| Two key technologies used by the electronics industry are chip technology and packaging technology. Solder plays a crucial role in both of them. During the last decade, there has been a strong worldwide environmental movement towards lead-free electronic products.The interfacial reaction of Sn-3.5Ag and Sn-4.0Ag-0.5Cu solders on Electroless Nickel, Immersion Gold (ENIG) metallization after high temperature storage (HTS) testing was investigated from a metallurgical point of view. It was noticed that only Ni3Sn4 IMCs were found in the Sn-Ag system, while two kinds of interfacial products, (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5 existed in the Sn-Ag-Cu system. During soldering and aging, Ni from the electroless Ni(P) was consumed to form IMCs, P atoms were accumulated and formed the P rich Ni(P) layer. In Sn-3.5 Ag solder joint, more Ni atoms were consumed comparing with Sn-4.0Ag-0.5Cu solder joint and one dark layer between Ni3Sn4 and Ni(P) could be observed. The interfacial layer between the Sn-Ag-Cu solder and electroless Ni(P) coating showed better thermal stability than eutectic Sn-Ag solder since no spalling was observed.The interfacial reactions between the eutectic Sn-0.4Co-0.7Cu alloy and ENIG metallization was investigated after reflow soldering. Common Sn-4.0Ag-0.5Cu and eutectic Sn-0.7Cu solders were used as references. Two types of IMCs were found in the solder matrix of the Sn-0.4Co-0.7Cu alloy, namely coarser (Co,Cu)Sn2 and finer (Cu,Ni)6Sn5 particles, while only one ternary (Cu,Ni)6Sn5 interfacial compound was detected between the solder alloy and the ENIG coated substrate. It was noted that the type of the interfacial IMCs in all the solder joints was the same one-(Cu,Ni)6Sn5 and the thickness of the interfacial IMCs layer in the Sn-Co-Cu solder joint was also similar to that of Sn-Ag-Cu and Sn-Cu solder joints.Furthermore, the coupling effect in both Sn-3.5Ag-3.0Bi and Sn-8.0Zn-3.0Bi solder joints in sandwich structure was studied as a function of reflow time. The coupling effect between the ENIG metallization and the Cu substrate was confirmed since the type of IMCs on Ni(P) layer changed from Ni-Sn phase to Cu-Sn phase, apparently as a result of the diffusion of Cu atoms from the opposite Cu substrate. Furthermore, the growth rate constant of ternary (Cu,Ni)6Sn5 IMCs between Sn-Ag-Bi solder and Ni(P) substrate was 3.8×10-10cm2/s. One complex alloy Sn-Ni-Cu-Zn was formed at the Sn-Zn-Bi/Ni(P) interface; however the growth of this complex alloy on the ENIG coated substrate was suppressed and it's growth rate constant was 2.93×10-12cm2/s. The growth rate constants of interfacial Cu6Sn5 in Sn-3.5Ag-3.0Bi and Cu5Zn8 in Sn-8.0Zn-3.0Bi were 1.44×10-10cm2/s and 1.36×10-10cm2/s, separately. The diffusion coefficient of Cu atom in the molten Sn-3.5Ag-3.0Bi solder at 240℃was calculated to be about 1.1×10-5cm2/s.Fourth, PBGA package was assembled on the FR-4 PCB and the temperature cycling (TC) was carried out in a systematic manner for two different TC profiles in a single chamber Heraeus climate cabinet. The first TC profile ranged from-55℃to 100℃and the second one ranged between 0℃to 100℃. It was found that Ag3Sn IMCs coarsen in the solder matrix and interfacial (Cu,Ni)6Sn5 IMCs layer growth during the temperature cycling. The growth rate constants of (Cu,Ni)6Sn5 for TC test ranging from -55℃to 100℃was 3.43×10-15cm2/s and another one ranging from 0℃to 100℃was 2.30×10-16cm2/s. The common failure mode of the solder joints analyzed in this work were cracks in the solder matrix. No fatigue cracks were found to propagate through the interfacial intermetallic layer for all the cases. The Weibull lifetime for TC test from -55℃to 100℃was 5415 cycles the second one ranged from 0℃to 100℃was 14094 cycles, both of them were qualified.The low cycle fatigue behavior of Sn-8Zn-3Bi solder joint, which is one promising lead-free candidate for low melting temperature soldering, was investigated using single lap shear samples. For the test loading condition using±40μm amplitude, the average lifetime of Sn-8Zn-3Bi was longer than that of Sn-37Pb, while in the±60μm displacement loading case, the average lifetime of Sn-8Zn-3Bi was shorter than that of Sn-37Pb solder joint. The fatigue cracks originated from the corner and prorogated through the solder joints in most cases. The 2D Finite element (FE) modeling work could described the strain and stress distribution in agreement with the experimental observation well, but the modeled stiffness of PCB was too low and allowed PCB to bend too much. 3D FE simulation based on the dynamic hardening model was performed and the Coffin-Manson equation was given based on results from experiment and simulation both:Nf= 0.0294 (△γ)-2.833...
|
PDF Full Text Request