Predicting Solder Paste Transfer Efficiency and Print Volume

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Shear viscosity was measured while shear rate was continuously logarithmically ramped from 0.1 to 100 s-1 for a total measurement time of 2 minutes. 10 measurements were made per decade of shear viscosity.

Alpha-Figure 4-Apr2017.jpgFigure 4: Close-up of the cone and plate mechanism.

Results and Discussion

Table 1 details the metal loading and initial viscosities of the three pastes at room temperature (25°C). As mentioned above, the metal loading needs to be decreased as powder size distribution decreases in order to obtain the same 10 RPM Malcolm measured viscosity.

It is interesting to note that the divergence in viscosities tends to be greater at lower shear rates. Conversely, the viscosities converged at the higher shear rates.

Alpha-Table 1-Apr2017.jpg

Table 1: Metal Loading and Viscosity of 3 Pastes Used

The print volume reproducability (CpK) of the pastes made with Type 3, 4 and 5 powders when the area ratio was 0.79 was 2.89 ±0.14. This converts to an expected defect rate of less than 0.6 parts per trillion.

The performance of the three pastes diverged at a 0.54 aspect ratio. In other words, a 210 μ (.008”) square aperture in a 100μ (.004”) thick stencil. The paste made with Type 3 powder showed a CpK of 1.23. This translates in an estimated defect rate of greater than 100 per million opportunities. The paste using Type 4 powder showed a CpK of 1.82, translating to an expected defect rate of approximately 50 per billion opportunities.

Finally, the paste that used the type 5 powder showed a CpK of 3.48, translating to an infinitesimally small expected defect rate of 1 in 2.56 x 10-17 opportunities.

To read the full version of this article which appeared in the April 2017 issue of SMT Magazine, click here.



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