Thermal fatigue damage in solder joints is believed to be closely related to microstructural evolution. In this study, a microstructural approach will be taken to evaluate the lifetime of Sn-3.0Ag-0.5Cu lead-free solder joints before the initiation of thermal fatigue cracks. In order to clarify the relation between the microstructural change and the initiation of thermal fatigue cracks, a series of thermal cycle tests were performed using fabricated PCBs on which various chip resistors were mounted. The following results were obtained via SEM observations. First, both the β-Sn and the Ag3Sn phases grow as the number of cycles N increases. This phase growth is characterized by phase growth parameters Ss and SA in the β-Sn phase and the Ag3Sn phase, respectively, which are defined as the average phase size to the 4th power. The phase growth proceeds such that the parameters increase proportionally to N. Furthermore, simple relations exist between the average number of cycles leading to the initiation of thermal fatigue cracks Ni and the average increase in the parameters per cycle ΔSs or ΔSA. That is, power law relations ΔSs = C2Ni−β and ΔSA = C3Ni−γ are determined. These relationships will enable us to evaluate the lifetime of the solder joints before the initiation of thermal cracks based on observations of microstructural evolution.
The rate and saturation value of water absorption were determined for nylon plastics molded in different shapes by injection molding. The degrees of crystallinity and its distribution in the specimens were estimated.The change of dielectric properties caused by water absorption and the degree of crystallinity were measured in a frequency, ranging from 50KC/sec to 20MC/sec. Behavior of the absorbed water and mechanism of polarization were discussed.The results are summarized as follows: 1) Water absorption gets larger in the order of nylon 610, 66, 6, 6/66 copolymer, and decreases by containing some additives such as graphite or metal powder.2) Water content of nylon is proportional to the 0.4-0.5th power of time, independent of the kind of nylon, the shape of specimen, the relative humidity, etc.3) The average degree: of crystallinity gets smaller in the order of nylon 66 (37.8%), nylon 610 (30.4%), nylon 6 (24.7%).4) The dielectric properties of nylon containing water are in non-linear relation with water content.5) At a definite temperature and a definite degree of crystallinity, the higher the water content is, the smaller the internal friction for polar groups to orientate, which the internal friction for polar groups becomes larger with the increase of crystallinity.
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