Abstract
This article describes the results of a whisker formation study on SAC305 assemblies, evaluating the effects of lead-frame materials and cleanliness in different environments: low-stress simulated power cycling (50–85°C thermal cycling), thermal shock (–55°C to 85°C), and high temperature/high humidity (85°C/85% RH). Cleaned and contaminated small outline transistors, large leaded quad flat packs (QFP), plastic leaded chip carrier packages, and solder balls with and without rare earth elements (REE) were soldered to custom designed test boards with Sn3Ag0.5Cu (SAC305) solder. After assembly, all the boards were cleaned, and half of them were recontaminated (1.56 µg/cm2 Cl−). Whisker length, diameter, and density were measured. Detailed metallurgical analysis on components before assembly and on solder joints before and after testing was performed. It was found that whiskers grow from solder joint fillets, where the thickness is less than 25 µm, unless REE was present. The influence of lead-frame and solder ball material, microstructure, cleanliness, and environment on whisker characteristics is discussed. This article provides detailed metallurgical observations and select whisker length data obtained during this multiyear testing program.
Highlights
After the European Union RoHS directive deadline was enacted in July 2006, the electronics industry began a transition to manufacturing with Sn-Ag-Cu (SAC) near-eutectic alloys to avoid the use of Pb-based solders
Because thin Sn plating is more prone to whisker growth, an 8-lm minimum tin plating thickness is recommended for whisker mitigation.[1]
Thermal cycling promotes whisker growth when tin is on low coefficient of expansion (CTE) base materials
Summary
After the European Union RoHS directive deadline was enacted in July 2006, the electronics industry began a transition to manufacturing with Sn-Ag-Cu (SAC) near-eutectic alloys to avoid the use of Pb-based solders. This change has had little impact on most commercial consumer products because of the limited life requirements and relatively benign use environments. Recent observations have demonstrated the ability of Pb-free solder joints to grow whiskers.[13,14] One challenge accompanying real electronic systems is the interplay among the component design and materials, manufacturing process, and service environments that contribute directly or conspire with one another to exacerbate whisker growth
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