EVALUATION OF Pb-FREE BGA SOLDER JOINT RELIABILITY ON Ni-BASED SURFACE FINISHES USING ALTERNATIVE SHEAR AND PULL METROLOGIES
Authors: Kuldip Johal, Hugh Roberts, Thomas Beck, Sven Lamprecht, Hans-Juergen Schreier, and Christian Sebald Company: Atotech USA Inc./Atotech Deutschland GmbH Date Published: 1/31/2007
Pan Pacific Symposium
Abstract: During the relatively short history of the electronics packaging and assembly industry, few innovations have impacted the design and manufacturing sectors as much as the ball grid array (BGA). Widespread acceptance of the BGA has occurred for most electronics applications. In the case of BGAs, the nature of the solder bond was quite different from that of the earlier leaded through-hole and SMT components. As a result, new testing methodologies were required to augment the data resulting from more traditional solder spreading and wetting evaluations. Consequently, the ball shear test and cold ball pull test represent two relatively fast and simple methods for measuring the effectiveness of bonding achieved through reflow of BGA solder balls. However, there have been some recent developments that may raise questions regarding the applicability of previously obtained test data. Most of the global electronics producers are now actively responding to the newly effective Pb-free requirements. The electronics manufacturing chain, including material suppliers, IC substrate fabricators, EMS/ODM companies, and OEMs, is continuing to learn about Pb-free assemblies. In some cases, parameters for testing methodologies are also changing in response to the combined effect of continued miniaturization and Pb-free requirements. This technical paper summarizes the findings and conclusions from a comprehensive examination of Pb-free solder ball bonding, comparing ball shear and cold ball pull testing results. The paper compares results obtained from conventional speed (slow) ball shear and cold ball pull testing with the results from newer high-speed test parameters. Relationships between ball shear and cold ball pull results are examined. Findings are also compared for two different substrate surface finishes: a process-of-record electroless nickel / immersion gold finish (ENIG), and an electroless nickel / electroless palladium / immersion gold finish (Ni/Pd/Au). Testing variables include surface finish deposit thickness, solder ball diameter and solder resist opening diameter, ball shear speed and cold ball pull speed. Test results are presented as graphical analyses of ball shear and cold ball pull forces and fracture modes, as well as microscopic examinations. Interpretation of the results includes determinations of the fracture modes for the various test methods and deposit conditions. Conclusions are offered regarding the two test methods and parameters, the performance of the two surface finishes and their interaction within Pb-free applications.