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SMT-Feb2015

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48 SMT Magazine • February 2015 based material that is made by a given process, there is a "threshold strain" and a "threshold temperature" that cause tin whiskering. This threshold temperature is not necessar- ily the same as the recrystallization tempera- ture. It is critical to recognize that the recrys- talization temperature changes with the system (composition) and can also be affected by the initial strain level and initial grain size. Thus it is important to be able to capture the anticipat- ed changes from the pure tin when an impure tin system is used. How a selected testing tem- perature is related to the threshold temperature is the key to what test results will be obtained. Threshold strain and the thresh- old temperature work hand-in- hand. Tin whiskering engages both nucleation and grain growth. The energy to drive grain growth is relatively low and its growth rate is slow and easily slowed further or impeded to a halt by the pres- ence of second phase particles or solute atoms in the lattice. The measured activation en- ergy for grain growth of im- pure metals is often very high. On top of that, tin has a low recrystallization temperature as well (the recrystallization temperature of pure tin is nominally at 30 o C), thus the impact of the testing temper- ature can be very tricky. For instance, how the applied test- ing temperature in relation to the solubility temperature of im- purities or its de-anchoring temperature from the grain boundaries dictates the grain growth pattern—growth within the lattice or the growth out of surface. That is when the test- ing temperature is just below the solubility/de- anchoring temperature; when the testing tem- perature is above this temperature makes differ- ence. The external temperature (testing temper- ature) drives the kinetics of defect dynamics in the tin layer by affecting stress relaxation, grain boundaries and atomic mobility-related mecha- nisms. When applying a high temperature (rela- tive to tin's recrystalization temperature), it is expected to impede the continued grain growth along the protruding direction, resulting in short whiskers. It is also worth noting that tin's recrystallization temperature changes with the level of its purity. In other words, when adding elements into tin, tin's behavior in relation to the external temperature (testing temperatures) will change since its recrystallization tempera- ture is elevated. The propensity of a tin deposit to grow whiskers strongly depends on its intrinsic struc- ture and the state of energy including the level of strain, grain size and crystallographic orien- tation, composition, surface condi- tion, and the external tempera- ture (or testing temperature). The threshold temperature is lowered when there exists a higher level of strain in the tin material. When the strain is sufficient, the lower thresh- old temperature results in lon- ger and needle-like whiskers in contrast to many short and nodular whiskers that are fa- vored by a higher temperature due to higher nucleation rate. When impurities are present, whiskers also tend to grow in short and nodular form. Grain boundaries play a key role in the mechanism of whisker formation. Whis- kering can be much facili- tated when the grain bound- ary structure contains both the recryslallized grain boundaries in low-energy state and the high angle, high-energy grain boundaries. To obtain both types of grain boundaries, the adequate tem- perature is a prerequisite. On the other hand, when the applied temperature is too high, the driving forces and the mechanism for whisker- ing will be annihilated. A higher temperature alters the relative diffusion rate between grain boundary diffusion and the lattice diffusion. So where is the "green house" temperature? Find- ing this green house temperature is the main task in designing the test. Feature when the strain is sufficient, the lower threshold temperature results in longer and needle-like whiskers in contrast to many short and nodular whiskers that are favored by a higher temperature due to higher nucleation rate. when impurities are present, whiskers also tend to grow in short and nodular form. " " TIN WHISKerS: WHy TeSTING TeMPeraTure CaN CHaNGe THe OuTCOMe continues

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