Journal of SMT Article


Authors: Marie Cole, Matthew Kelly, Mario Interrante, Grego
Company: IBM Corporation and Celestica, Inc
Date Published: 10/1/2006   Volume: 19-4

Abstract: The solder joint microstructure and reliability of 937 I/O Ceramic Ball Grid Array (CBGA) packages assembled on several Printed Circuit Board (PCB) test vehicle designs were studied in these evaluations. These packages were 32.5mm square, full array, and primarily 1.0 mm pitch.

In the first study, the packages used Sn3.8Ag0.7Cu solder balls. Primary card assembly was performed using no clean (NC) lead free alloy solder paste, as well as tin lead (Sn/Pb) solder paste. The purpose of this evaluation was to study both pure lead free (Sn3.0Ag0.5Cu - SAC 305 and Sn3.8Ag0.7Cu - 405 systems) and mixed assembly (SAC BGA balls with Sn/Pb card assembly) solder joint reliability performance relative to that of the conventional Sn/ Pb alloy. This evaluation included different surface finishes on assembly test vehicles such as OSP and ImmAg, and was done in both air and nitrogen.

Additional studies were completed on another PCB test vehicle using a water soluble Sn3.8Ag0.7Cu (SAC 405) solder paste. Geometry factors such as ball, package pad and card pad diameters were evaluated. Package ball alloys SAC 305 and SAC 405 were compared. Mixed assembly performance under slightly different conditions was compared to the results from the above assembly matrix.

Solder joints formed were examined “as assembled” and also after accelerated thermal cycling (ATC) using optical and Scanning Electron microscopy, EDX and X-ray methods.

This paper contains references to “lead free” solder alloys. Lead free has become an industry standard term meaning that the solder alloy should contain less than the RoHS Directive limit (1,000 ppm) for lead. It does not mean that the solder alloy will necessarily contain no lead at all.

Key words: SAC CBGA, SnAgCu CBGA, mixed assembly

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