Q1. What is recommended viscosity standard? A1: There is no standard. ASTM D4040-05 "Standard Test Method for Rheological Properties of Paste Printing and Vehicles by the Falling-Rod Viscometer" is a test method that might work. For more, look here http://www.smtnet.com/forums/Index.cfm?CFApp=1&Message_ID=33796
Q2. Which one is better, higher viscosity or lower viscosity and what is relation of them to defect of production? A2: Viscosity is important. Paste that is: * Too thick (high viscosity) may lead to a bad print in the form of an insufficient volume. * Too thin (low viscosity) may lead to the paste slumping.
Q4. I usually put a solder paste in refrigerator and take it if i want to use i let it abt 2 hour then i mix with solder paste mixer? Am I doing right? A4: The typical warming or stabilization time for solder paste is four to six hours. Check with your supplier for specific recommendations.
Finally: Solder paste is a non-Newtonian fluid. Its viscosity is not only a function of temperature, but also of shear rate. Providing data for one is useless without the other. ["How Heat Generation in Stencil Printing Affects Solder Joint Quality" MHA Riedlin and NN Ekere; Surface Mount Technology (SMT) August 1999]
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