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Effect of Flux Quantity on Sn-Pb and Pb-Free BGA Solder Shear Strength

Mandar Painaik, Cookson Electronics Semiconductor Products,
Daryl L. Santos, Binghamton University
A. James McLenaghan, Prashant Chouta, Cookson Electronics Process Technology
Stacy Kalisz Johnson, Agilent Technologies

ABSTRACT

The global movement to eradicate Pb from electronic assemblies has made it necessary to manufacture Pb-free packages. While the industry has identified potential Pb-free alternatives, much work still needs to be done in the component and board level reliability. Ball Grid Array (BGA) packages are commonly used in the electronics industry today.

Flux deposition is a standard step in the BGA sphere-attach process. Deposited flux requires precise measurement, since it plays a critical role in the formation of the BGA solder joint. Flux measurement is difficult, as flux is clear or amber colored, making it invisible to standard optics techniques.

The patent-pending Ultraviolet Fluorescence Intensity Mapping (UV-FIM) technique is used in this research effort for flux measurement. BGA spheres are attached using different quantities of flux. A correlation is made between the solder joint shear strength and the flux quantity for Sn-Pb and Pb-free (96.5Sn-3.5Ag, 95.5Sn-4Ag-0.5Cu) BGA solder joints.

Keywords: BGA, Flux, Shear Strength, Pb-Free.

EXPERIMENTAL PROCEDURE

  • The test vehicle used is a device with 22 mil pads for 30 mil spheres. The alloys used for the sphere are 96.5Sn-3.5Ag, 95.5Sn-4Ag-0.5Cu. A generic water-soluble flux was used.

  • Flux deposition was performed via stencil printing. Two stencils with the same 17.6 mil apertures, 2 mil and 5 mil thick, were designed for this purpose.

  • The Agilent FXB was used for measuring the flux deposits on the pads.

  • Spheres were placed on the fluxed pads using the Camalot-Matrixx positive pin sphere placement machine.

  • Spheres were reflowed using an Omniflow 7 reflow oven. Different reflow profiles based on the material supplier recommendations were used for Sn-Pb and Pb-free solder alloys.

  • Ball shear testing of the spheres was performed using an Instron 5566 to assess the solder joint integrity. The acceptance criteria for a 30 mil solder sphere on a 25 mil pad is 1 kg.

ANALYSIS AND DISCUSSION

In order to compare the shear strength results, Analysis of Variance (ANOVA) was performed on the data using Minitab and Statistica data analysis software.

  • ANOVA analysis for the 63Sn-37Pb solder alloys shows that the shear strengths for the two levels of stencil thickness are statistically different.

  • ANOVA analysis for the 96.5Sn-3.5Ag solder alloy shows that, although the 5 mil thickness has a higher mean strength than the 2 mil thickness, the difference is not statistically significant.

  • ANOVA analysis for the 95.5Sn-4.0Ag-0.5Cu solder alloy shows that the shear strengths for the two stencil thicknesses are statistically different by an average of approximately 0.25 kgf. The 5 mil thickness has a higher mean shear strength than the 2 mil thickness.

  • In addition, the variation in the shear strength is less for 5 mil stencil thickness than for 2 mil thickness, suggesting that a stencil thickness of 5 mil results in a more stable process.

The net difference in the percent pad coverage for the two stencil thicknesses is greatest for Sn-Ag-Cu and is least for Sn-Ag, with the Sn-Pb in the middle.

Die drawing

The figure shows that the flux thickness is nearly the same for all alloys for the 2 mil thick stencil, but the 5 mil thick stencil results in maximum thickness for Sn-Ag-Cu and the least thickness for Sn-Pb. Thus, the pad coverage and flux thickness (flux amount) has a strong effect on the ball shear strength.

CONCLUSION

The findings of this research effort indicate that flux quantity does have an effect on the BGA ball shear strength. Another important factor that affects the shear strength is the flux pad coverage. Higher flux thickness and pad coverage resulted in greater shear strength. Thus, it can be concluded that the flux quantity is an important factor in the BGA sphere-attach process and needs to be monitored for better solder joint quality.

BIBLIOGRAPHY

Reference available upon request.

FOR MORE INFORMATION

Mandar Painaik, mpainaik@cooksonelectronics.com
Daryl L. Santos, Ph.D. santos@binghamton.edu
A. James McLenaghan, jmclenaghan@cooksonelectronics.com
Prashant Chouta, pchouta@cooksonelectronics.com
Stacy Kalisz Johnson, stacy_johnson@agilent.com


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