Stress Relaxation in Sn-Based Films: Effects of Pb Alloying, Grain Size, and Microstructure

Abstract

Stress is believed to provide the driving force for growth of Sn whiskers, so stress relaxation in the Sn layer plays a key role in their formation. To understand and enhance stress relaxation in Sn-based films, the effects of Pb alloying and microstructure on their mechanical properties have been studied by observing the relaxation of thermal expansion-induced strain. The relaxation rate is found to increase with film thickness and grain size in pure Sn films, and it depends on the microstructure in Pb-alloyed Sn films. Measurements of multilayered structures (Sn on Pb-Sn and Pb-Sn on Sn) show that changing the surface layer alone is not sufficient to enhance the relaxation, indicating that the Pb enhances relaxation in the bulk of the film and not by surface modification. Implications of our results for whisker mitigation strategies are discussed.

Manipulating Crystallographic Texture of Sn Coatings by Optimization of Electrodeposition Process Conditions to Suppress Growth of Whiskers

“…[1][2][3][4][5] Whisker growth results in many serious short-term and long-term reliability issues in the electronic assemblies used in critical applications, for example aviation and space, defense, medical devices, etc., in which whisker-induced electrical short circuits and mass redistribution in conductors have resulted in complete failure of commercial satellites, defense systems, and medical equipments. 6,7 Alloying with Pb has been reported to reduce the extent of whisker formation on Sn coatings [7][8][9][10] ; this effect has been attributed to substantial reduction of the maximum compressive stress generated in Sn coatings on addition of Pb. 9,10 However, the worldwide acceptance of the European Union's Restriction of Hazardous Substances (RoHS) Act-2006 prescribing complete elimination of Pb from all electronic components 11 has resulted in re-emergence of reliability issues associated with tin whiskering.…”

“…6,7 Alloying with Pb has been reported to reduce the extent of whisker formation on Sn coatings [7][8][9][10] ; this effect has been attributed to substantial reduction of the maximum compressive stress generated in Sn coatings on addition of Pb. 9,10 However, the worldwide acceptance of the European Union's Restriction of Hazardous Substances (RoHS) Act-2006 prescribing complete elimination of Pb from all electronic components 11 has resulted in re-emergence of reliability issues associated with tin whiskering.…”

Major Driving Forces and Growth Mechanisms for Tin Whiskers

“…A plot of stress versus temperature (which is proportional to strain) is shown in Figure 2.5 for layers of Sn with thicknesses of 1 and 7.5 μm and Pb-Sn with thickness of 7.5 μm [26]. Note that the stress in the 7.5 μm Sn layer saturates at a lower stress value than the 1-μm-thick layer, which is consistent with the enhancement in relaxation inferred from the stress evolution due to IMC formation.…”

Atomic Layer Deposition (ALD) to Mitigate Tin Whisker Growth and Corrosion Issues on Printed Circuit Board Assemblies

“…[11][12][13][14] There are, however, many factors that may influence the growth of whiskers, including the physical characteristics of the Sn layers (e.g. thickness, 7,15 grain size and crystallographic structure), and despite an increased understanding of the processes that give rise to whisker growth the phenomenon is still not fully understood. 4,16 The influences on whisker growth of other factors such as current load [17][18][19] and electrostatic forces 20 have also been considered.…”