| As the Pb-free trend internationally is increasingly apparent, the domesticproduction is entering a transition period of mixed lead-free/tin-lead assemblies, whichis bound to face a series of issues on the reliability and compatibility when thetraditional tin-lead process lines are used. It is especially backward compatibilityprocess which means having lead-free component assembled on the printed circuitboard (PCB) under the tin-lead process technology that catches much attention, aboutwhich very few studies have been conducted and the data is limited with someconflicting conclusions. Therefore it has important practical significance to optimize theprocess for mixed assembly and predict the reliability life for thermal cycling test.Firstly finite element method (FEM) for mixed lead-free ball grid array (BGA)/tin-lead solder paste assembly was conducted, with the influence on the stress andstrain from peak reflow temperature and cooling rate analysised by determining thecritical solder joint. Also the stress and strain during thermal cycling without prestresswas studied, and reliability life of BGA for thermal cycling with/without prestress waspredicted and compared with tin-lead BGA. Then aging test and thermal cycling testwere conducted for two BGA component with different packaging reflowed by twokinds of processes, joints and interfacial microstructure evolution described, jointdefects observed and analysis, and of two types of processes compared. Besides,intermetallic compound (IMC) growth kinetics during aging was verified. Finally, thedyeing experiments for components were worked out, with failure modes summed upand MAP diagram made. The influence on the component reliability caused by differenttesting time, process and package was discussed.The results showed that: cooling rate had greater influence on the stress of reflowedjoint while peak reflow temperature did more on the strain, but with both elevated stressand strain did rise. Stress and strain of tin-lead BGA solder joints were more than that ofmixed lead-free BGA/tin-lead solder joints, but both did not reach the yield strength.Temperature rise and drop led to the same change of strain and opposite change of stress,which means solder joint was liable to fail. Heat preservation stage was good for stressrelaxation, while strain further rose and dropped respectively during high temperatureinsulation stage and low temperature insulation stage. The junction of upper substrateand the solder at the edge of joint experienced the greatest change on stress and strain,which was considered as the weakest point. Long thermal cycling life was predicted formixed lead-free BGA/tin-lead solder joints presenting equality, and life prediction wasnot affected by prestress, which only influence initial strain value phase without workingon strain change. In the study of reliability uniform microstructure was acquired under both reflow process, and the presence of Cu6Sn5, Ag3Sn and Cu-Ni-Sn ternary wasconfirmed. The growth of IMC during aging stage was proportional to the square root oftime, which was in line with bulk diffusion mechanism, and lead accumulation, crack andIMC fragmentation severely reduce reliability. However, no obvious lead aggregationand IMC thickness growth were observed during thermal cycling stage. The crackinitiation and propagation after long-period cycle was pointed out to be subject to stressaccumulation and weak interface. Through dyeing test and MAP diagram four kinds ofsolder joint failure was figured out, and the stress of four corners was greater than that ofsides and even center. Component of ceramic package possessed lower reliability due tomismatch of thermal expansion, with long-term aging or thermal cycling test having thesame result, and eventually the process Ⅱ was evaluated to be slightly better thanprocess Ⅰ. |
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