This paper describes the results of a whisker formation study on SAC305 assemblies, evaluating the effects of cleanliness and lead-frame materials in room temperature/high humidity (25°C/85%RH). Cleaned and contaminated small outline transistors were soldered to custom designed test boards using Sn3Ag0.5Cu (SAC305) solder. Before assembly components were divided into two groups. The first group was cleaned using the method developed in this study. The level of contamination was 10 times below typical acceptable industry level and did not exceed 0.062 μg/cm~2 (0.4 μg/in~2) Cl. The second group was contaminated with NaCl. The piece part level of chlorine contamination 0.465 μg/cm~2 (3 μg/in~2) was selected to be within the industry levels encountered (no standards exist). After assembly, all the boards were cleaned, and half of them were re-contaminated to a level of at least 1.56 μg/cm~2 Cl (10.1 μg/in~2). 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. The influence of lead-frame material, cleanliness, and environment on whisker characteristics is discussed. Assembly contamination is an important consideration for whisker growth in harsh service environments.
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