Abstract
To meet the urgent demands of future electronic packages,the solder joints need to be increasingly miniaturized.Although the size of solder joints has surpassed 50—100μm range. further decrease is still necessary.Mechanical property of solder joints is a key factor that influences the reliability of electronic packages and assembly products.As a consequence,it is very important to understand the fracture behavior of solder joints,which can better predict the reliability of solder joints in electronic interconnections.Compared to the large solder joints,the mechanics behavior for the small solder joints is very different,resulting in a series of reliability issues.In this paper,shear test of the as-reflowed and aged Sn3.0Ag0.5Cu solder joints with the diameters of 200—600μm on Cu pads was conducted,and fracture behavior was observed using SEM.The results show that the shear strengths of both as-reflowed and aged solder joints decrease with increasing the solder joints volumes.At the initial stage of aging process,the shear strength of solder joints decreases remarkably, and will not decrease much with increasing aging time.For the large solder joints,the fracture occurs close to the interface,and the solder joint shows strong brittleness.Whereas,for the small solder joints,the fracture occurs within the bulk solder,and the solder joint shows ductility.SEM images at the interface of solder joints and solder bulk show that the Ag_3Sn intermetallic compounds within the bulk solder and Cu_6Sn_5 at the interface region have a prominent effect on the shear property and the propagation of the fracture,which is the key factor for the volume effect of the fracture behavior of the solder joints.The Ag_3Sn phase inside the small solder joints has fine particle-like morphology and dispersively distributes in the bulk solder,which strengthens the solder joints,however,the Ag_3Sn phase inside the large solder joints has feather-like and dendritic morphologies and makes the joint become brittle.
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