Investigation of solder paste rheological properties

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Dr. Krammer Olivér
Department of Electronics Technology

The widespread surface mount technology (SMT) applies the reflow soldering method for soldering. During surface mount reflow, the solder is deposited in paste form. It is essential to choose the optimal solder paste for a certain production. The alloy, the paste type and the condition of the applied solder paste plays a key role concerning the quality and reliability of the final product. Knowing the rheological properties of a certain jar of solder paste, it is possible to apply again a previously used solder paste for production. Reusing such a solder paste might be important and economical for small and medium size enterprises, as well as for experiments and prototype production. With this solution, it is possible to reduce the costs of production without decreasing the quality of the product.

I investigated rheological and printing properties of some different lifetime solder pastes during my experiments. Based on literature survey I have measured rheological properties of solder paste which have the highest effect on stencil printing process. During the investigations I have measured the viscosity of solder paste and experienced how solder paste reacted during oscillatory stress tests. I have also measured the printing properties of different lifetime solder pastes. The results were evaluated by technological aspect.

According to my experimental results, I have found that the rheological properties, the printing properties and the lifetime of solder pastes are highly correlating to each other. With the aging of solder paste, the viscosity of solder paste increases, but the structural recovery (measured on the rheological curve) highly decreases, which makes the printing properties of solder paste worse.

Small extent of slamping was observed at the newly opened jar and the jar opened 3 days ago. This is a consequence of the high flux content which causes the lower viscosity also. This fact is confirmed by the measured low stress strain values (G’ = G”). These solder pastes perfectly suffice for printing ultra fine pitch deposits.

On the contrary, viscosity values are increasing at solder pastes opened 3 weeks and 6 months ago. This is the result of the evaporation of the flux. The degradation rate of solder pastes are shown by the low recovery values and the high deformation of the deposit. This is proven by the measured stress strain values, which are higher at these old solder pastes.


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