The Study on Low-Ag Content Sn-Ag-Cu BGA Solder Joints with various Pb-free Surface Finishes
Date Issued
2010
Date
2010
Author(s)
Chen, Chin-Liang
Abstract
Due to concerns about the drawbacks leading from the high-Ag content in the Sn-Ag-Cu Pb-free solders, the Ag content of the Sn-Ag-Cu solder is directly lowered in the present study. In addition, minor Ni and Ge elements are doped into the solder alloy to further optimize the properties of the low-Ag content solder. Interfacial reactions between each kind of solder alloy (Sn-3Ag-0.5Cu(SAC305), Sn-1Ag-0.5Cu(SAC105), and Sn-1Ag-0.5Cu-0.06Ni-0.01Ge(SAC105NG)) and various Pb-free surface finishes (Organic Solderability Preservative(OSP), Electroless Nickel Immersion Gold(ENIG), and Electroless Nickel Electroless Palladium Immersion Gold(ENEPIG)) after reflow and high temperature aging treatment were observed, and the mechanical strength of the solder joints were evaluated via Ball Shear(BS), Cold Ball Pull(CBP), and the newly-developed High Speed Ball Shear (HSBS) tests.
The results show that the ball shear and cold ball pull strengths of the Sn-Ag-Cu solder joints decrease obviously as the Ag content of the solder alloy reduces; however, after the minor Ni and Ge elements were doped into the solder alloy, the mechanical strength of the Sn-Ag-Cu-Ni-Ge solder can effectively be improved, and the degradation of the ball shear and cold ball pull strengths after high temperature aging treatment is less apparent in this five-element alloy than in the high-Ag content Sn-Ag-Cu solder. Unlike the ductile fracturing through the solder balls which is revealed in the ball shear and cold ball pull tests, the high speed ball shear test causes the solder joints to fracture along the interfacial intermetallic compounds. Thus, the morphology, chemical composition, and thickness of the interfacial intermetallic should all be considered to be factors influencing the high speed ball shear strength of each solder joint. For the case that the solders react with OSP surface finish, the high speed ball shear strength of the solder joint increases with the decreasing of the Ag content. On the contrary, the shock resistance of the solder joint deteriorates as the Ag content reduces when the surface finish changes to the ENIG as well as the minor Ni and Ge elements doping into the solder alloy. On the other hand, for the ENEPIG surface finish with an extra electroless palladium layer, its shock resistance can effectively be improved as comparing to the traditional ENIG metallization.
Subjects
low-Ag content Sn-Ag-Cu solder
Pb-free surface finish
BGA package
interfacial reaction
mechanical property test
ENEPIG
high speed ball shear strength
Type
thesis
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