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The Quantitative Assessment of Mixed BGA Joint
The Quantitative Assessment of Mixed BGA Joint
An investigation to evaluate the soldering process and the reliability of solder joints of SnAgCu PBGA components formed using the Sn-Pb solder paste.
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Authored By:
HongqinWang, Guanghui He and Daojun Luo
China Electronic Product Reliability and Environmental Testing Institute
Guangzhou, China

Summary
A careful experimental investigation was undertaken to evaluate the soldering process and the reliability of solder joints of SnAgCu PBGA components formed using the Sn-Pb solder paste. Initially, a series of the reflow soldering profiles were developed to assure the complete mixing of the SAC ball and Sn-Pb solder. Analysis of as-assembled solder joints revealed that the degree of mixing between the BGA SAC ball alloy and the Sn-Pb solder paste varied with the different reflow soldering profile. The peak reflow temperature was observed to have a significant impact on the solder joint microstructural homogeneity.

Serious crack defects of the complete mixed solder joints happened after the waving soldering process. Then a series of the reflow soldering profiles and tests were developed to solve the crack phenomenon and the mechanism of the crack phenomenon was illustrated with the quantitative assessment of mixed solder joints.

The reliability of mixed solder joints after optimazation was studied with an accelerated thermal cycling(ATC) test in the temperature range of -40C to 125 C for a maximum of 3000 cycles in accordance with IPC 9701A standard.

Conclusions
In this paper, the properties of the mixed solder joints of SnAgCu PBGA components formed using the Sn-Pb solder paste was investigated, which was assembled by the reflow soldering and then experienced the following waving process.

The homogeneous microstructure of the mixed solder joints could be obtained by the optimization of the reflow soldering parameters. Crack defects happened when the specimen went through the waving soldering, where the peak temperature of the mixed solder joint reached higher than the melting point of the mixed solder alloy in the waving soldering period.

To avoid such cracks, the reflow soldering should assure the even distribution of Pb element in the mixed solder joint through the optimization of reflow temperature profile; and the temperature of the mixed solder joint should be controlled below the melting point of the mixed solder alloy in the waving soldering period by the optimization of the waving soldering parameters.

The reliability of mixed solder joints after optimization could stand an accelerated thermal cycling(ATC) test in the temperature range of -40C to 125 C for a maximum of 3000 cycles in accordance with IPC 9701A standard without function failure, through the microscopic crack appeared at the interface of component pad and the solder.

Initially Published in the SMTA Proceedings

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