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78 SMT Magazine • March 2014 better tack force discrimination from one paste to another. The common standards measure the cohesion of a solder paste at higher forces, 15 g/ mm 2 for IPC-TM-650, method 2.4.44. The measurements were performed at room temperature (21°C +/-3°C) and (50% +/-10%) relative humidity. The first measure was done after printing and the evolution of the tacki- ness was followed. The follow-up of tackiness with an applied force of 1 g/mm 2 according to time is shown in Figure 2. The low limit is 1 g/ mm 2 (equal to the force applied). The pastes ex- hibit initial values between 1.2 (paste A) and 1.6 (paste F). All the pastes keep a tackiness above the limit for 24 hours except the paste C which adhesion starts to decrease between 8 and 18 hours. Internally, we have correlated the tacki- ness with the abandon time (idle time) on the stencil during printing: If a solder paste does not lose its tackiness for 24 hours during this feaTure rELIaBILITY aSSESSmENT OF NO-cLEaN aND WaTEr-SOLUBLE SOLDEr PaSTES, ParT 1 contin ues Table 2: Metal content, non volatile residue, acid index and viscosity measurements. figure 2: Tackiness vs. time.