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Rheology and Wetting Characterizations of Flux and Solder Paste
Rheology and Wetting Characterizations of Flux and Solder Paste
In this paper, we will discuss a solder paste wetting method. A high resolution solder wetting balance was used for the solder paste wettability analysis.
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Authored By:
Jinlin Wang, Ph.D.
Intel Corporation
Chandler, AZ, USA

Summary
Package failures related to solder joint reliability need to be addressed at the first level, second level, and package to socket interconnects. As the ball size, pitch, and ball to ball spacing are reduced, it poses a challenge to material and process development. Flux materials are typically used in the solder process to remove the oxides on the pad and solder surfaces. A clean surface results in a stronger solder joint. Due to the flux chemistry and formulation differences, the rheological behavior of the fluxes can vary. There are two types of commonly used flux materials in terms of their rheology properties: Newtonian and non-Newtonian fluids, depending on the formulation chemistry. Fluxes with non-Newtonian fluid behavior have complex rheological properties. Flux viscosity has always been an important material property for manufacturability.

It is critical to have a good understanding of the rheology properties of the flux and paste and its correlation with process and package parameters. This can help in optimizing the process conditions and reducing manufacturing related defects. The rheological properties and wettability of fluxes and pastes were studied. The rheological properties of flux and pastes were measured across a wide range of shear rates with various rheometers to correlate the rheological properties with process conditions. Wetting angles of the flux were measured for flux spread studies. The process of solder joint formation relies heavily on wetting parameters such as solder wetting force and wetting time in the presence of flux and/or solder paste materials. The correlation between wetting parameters, package assembly yield, and reliability data can provide a guideline for flux and solder paste material selection and process optimization.

The solder wetting balance test is one of the most common methods for solder wetting evaluation. This method can be used for flux, solder, and surface finish evaluations. The method with a molten solder pot, however, cannot be used for solder paste evaluation since the paste is a mixture of solder and flux.

In this paper, we will discuss a solder paste wetting method. A high resolution solder wetting balance was used for the solder paste wettability analysis. Solder wetting analysis for solder balls and solder paste with different pre-test conditions provide useful information for material selection and reflow process optimization studies. The effects of solder surface oxidation and paste aging on the solder-paste wetting interactions were investigated.

Conclusions
The rheology properties of flux and paste are quite complicated due to their non-Newtonian fluid behavior. The effect of time and deformation histories on the viscosity of flux and paste were studied. The low shear rate viscosity and yield have related to some process issues. The low shear viscosity has a workable window. Too high will cause stringing and too low will cause over spread and bridging. Stringing will cause to ball fall off from deposited paste. High shear rate viscosity for fluxes were measured with a capillary rheometer. Extensional viscosity and break time for paste were measured with an extensional rheometer. The required material properties can be very different from process to process. New process drives new material development and thus new characterization techniques need to be developed to face new challenges.

Wetting balance tests have been successfully performed for the characterization of paste and solder ball wetting behaviors for the resolution of HnP defects and NWO defects. In addition, the understanding of the effect of solder ball reflow in relation to paste wetting were also acquired in this work.

In general, wetting test results showed that paste aging and improper paste used in SMT resulted in significant reduction in the wetting force. However, in the case of NWO defects, the oxidation of the solder ball surface showed a measurable increased in wetting time with negligible effect in the wetting force. For the case of multiple-reflowed solder ball, a decreasing trend in the wetting force was observed.

The effect of solder paste aging on the solderability was studied. Solder pastes showed different levels of wetting in the Cu wire on paste test, which correlated with the NWO defects of the pastes. Aging has more impact on solder paste wetting properties than solder ball.

Initially Published in the SMTA Proceedings

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