There are a number of issues regarding the use of Sn-Ag-Cu alloys,including the solderability and long-term reliability of the solderjoints, which require further study. The lower solderability of Sn-Ag-Cusolder can alter the interface and microstructure of the solder jointformed because of the differing reaction rates between the molten solderand substrate surface. This also has an impact on the nature and extentof the intermetallic compounds formed at the interface, as theintermetallic is generally more brittle than the base metal. This cannegatively impact the solder joint reliability. In this paper we reporta study on the effect of solder volume on intermetallic layer formationand thickness. For lead-free soldering this could prove to be veryimportant, as a wide range of devices and components of varying jointsize, e.g. plastic quad flat pack (PQFP), ball grid array (BGA),chip-scale packaging (CSP), and flip chip, may need to be assembled on atypical board. This means that the nature and thickness of theintermetallic layer formed for each joint size will be different. In thestudy, solder joints of different sizes representing different deviceswere used for evaluating the effect of solder volume on intermetalliccompound formation. The layer thickness and microstructure were analyzedusing scanning electron microscopy (SEM). SEM analysis was also carriedout on joint micro-sections, which has undergone temperature cycling toevaluate the effect of intermetallic layer the joint reliability. Ourresults show that increasing the solder volume (and solder joint size)does not significantly affect the growth of the intermetallic layerthickness. Therefore the intermetallic layer thickness provides thelower limit for solder joint design for ultra-fine pitch flip-chipapplications
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