SMT Magazine

SMT-Apr2017

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22 SMT Magazine • April 2017 by Mitch Holtzer, Karen Tellefsen and Westin Bent ALPHA ASSEMBLY SOLUTIONS Industry standards such as J-STD-005 and JIS Z 3284-1994 call for the use of viscosi- ty measurement(s) as a quality assurance test method for solder paste. Almost all solder paste produced and sold use a viscosity range at a sin- gle shear rate as part of the pass-fail criteria for shipment and customer acceptance respectively. As had been reported many times, an esti- mated 80% of the defects associated with the surface mount technology process involve de- fects created during the printing process. Vis- cosity at a single shear rate could predict a fatal flaw in the printability of a solder paste sample. However, false positive single shear rate viscos- ity readings are not unknown. Intuitively, solder paste is subjected to sev- eral 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 interac- tion with the squeegee is also a critical step for a high yield process. Very low shear force be- havior may be an indicator of the potential for slumping. Using a cone and plate fixture on a variable speed rheometer, this study will exam- ine 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 pre- dict the defect rate of a solder paste, and possi- bly create a more accurate prediction of the sol- der pastes value in use to users. The expectation is that this work will lead to a rich sequence of further valuable studies. Introduction Being able to predict the transfer efficiency and print volume reproducibility of solder paste are valuable data points for any circuit assem- bly process. Knowing how many good boards that can be expected from a jar of solder paste is critical for a contract manufacturer working on paper thin margins. Tier 1 automotive sup- pliers who in most cases, cannot re-work defec- tive assemblies, pay dearly for defects. Defective assemblies must be discarded, and production lines are stopped until the root cause of the de- fect is determined and corrected. Predicting Solder Paste Transfer Efficiency and Print Volume FEATURE

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