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March 2014 • SMT Magazine 77 and production tests (hot slump, solderballing, wettability, reflow process window, graping abil- ity). Finally, the cleaning ability will be assessed. Experiments The pastes used for this evaluation were all made of SnAg3Cu0.5 (SAC305) alloy with type 3 (25/45 microns,-325/+500 mesh) particle size. The selected water-soluble pastes are named A, B and C and the no-clean pastes are named D, E and F. The flux designation according to J-STD-004A and the metal content are gathered in the Table 1. We ensured that all the pastes used for the study were recently manufactured (4–8 weeks). Before starting the evaluation, the aspect and the viscosity of the pastes were checked in order to ensure that they were compliant with the ini- tial specifications. acid Index, Non-volatile residue and viscosity The metal contents of the solder pastes (MC, expressed in percentage) and their non-volatile residues (NVR, expressed in percentage of the total flux medium) were measured although the ability of a solder paste residue to be cleaned is more linked to its quality than to its quantity. The acid index (Ia) was determined by acid-base titration. The dynamic viscosity was determined us- ing two types of viscometers: Brookfield and Malcom (Figure 1). The viscosity was first mea- sured with a spindle type viscometer Brookfield DVII at 5 rotations per minute (RPM) using a TF spindle. Measures were made without mix- ing, and after mixing, at 20°C and 25°C accord- ing to the test method described in IPC-TM-650 method 2.4.34. The viscosity with a spiral pump viscometer Malcom PC1-TL was determined at 10RPM and 25°C. The results are listed in Table 2. The viscosity data are given in Pascal-seconds (Pa.s). All the results were found in accordance with the initial specifications. The highest vis- cosity was found for paste C, which also had the highest metal content. The acid indexes of the water-soluble solder pastes were low compared to the ones of the no-clean pastes: a value of 5 was even found for paste B. The non-volatile residues were a bit higher for water-soluble sol- der pastes than for no-clean ones. Tackiness The tackiness was assessed using our inter- nal test which is designed to measure the tack force of a solder paste with an applied force of 1g/mm 2 or 4g/mm 2 : Low applied forces are more representative of the actual use of a sol- der paste at the pick-and-place step and allow a feaTure rELIaBILITY aSSESSmENT OF NO-cLEaN aND WaTEr-SOLUBLE SOLDEr PaSTES, ParT 1 contin ues Figure 1: Schematic representation of a) brookfield viscometer (source: brookfield) and b) Malcom viscometer (source: Malcomtech).