Electronics Assembly Knowledge, Vision & Wisdom
Using Rheology as a Measurement Potentially Predictive Tool
Using Rheology as a Measurement Potentially Predictive Tool
The types of rheology measurements studied here show trends that mirror the print volume and volume reproduceability for three pastes.
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Authored By:
Mitch Holtzer, Karen Tellefsen and Westin Bent
Alpha Assembly Solutions
South Plainfield, NJ, USA

Summary
Industry standards such as J-STD-005 and JIS Z 3284-1994 call for the use of viscosity measurement(s) as a quality assurance test method for solder paste. Almost all solder paste produced and sold use a viscosity range at a single shear rate as part of the pass fail criteria for shipment and customer acceptance respectively.

As had been reported many times, an estimated 80% of the defects associated with the surface mount technology process involve defects created during the printing process. Viscosity at a single shear rate could predict a fatal flaw in the printability of a solder paste sample. However, false positive single shear rate viscosity readings are not unknown.

Intuitively, solder paste is subjected to several shear rates during the printing process. A squeegee forcing paste into an aperture may be the highest, and most critical strain rate applied to solder paste during the process. Separation of the stencil from the printed circuit card may be the second most important. Paste interaction with the squeegee is also a critical step for a high yield process. Very low shear force behavior may be an indicator of the potential for slumping.

Using a cone and plate fixture on a variable speed rheometer, this study will examine the correlation between rheology and print volume and print volume repeatability of the pastes. The goal is to determine if a specific set of viscosity vs. shear strain rate curves can predict the defect rate of a solder paste, and possibly create a more accurate prediction of the solder pastes value in use to users. The expectation is that this work will lead to a rich sequence of further valuable studies.

Conclusions
The three types of rheology measurements studied here all show trends that mirror the print volume and volume reproduceability for the three pastes used in this study. A low area ratio, 0.54 amplified the correlation of the rheology measurements to the print deposit reproduceability.

The paste sample size required for rheological testing is an order of magnitude lower than what is required for spiral viscosity testing. Both suppliers and users could see a significant reduction in wasted material if rheology would become the standard for solder paste inspection.

Obviously, more testing, including a round robin test by the IPC-JSTD-005 would be required before this type of test method could be elevated to an industry standard

Initially Published in the SMTA Proceedings

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