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April 2015 • SMT Magazine 37 measured after complete removal of their resi- dues. However, in order to observe the effect of non-cleaned or poorly cleaned corrosive resi- dues, it was decided to measure the surface in- sulation resistance of all the pastes according to IPC-TM 650 method 2.6.3.3 without prior cleaning. The solder pastes were printed onto IPC B24 coupons (track/space width 0.4/0.5 mm) and the coupons were reflowed using P1 thermal profile. Additional coupons for pastes A and D were made and cleaned before SIR as references. Boards were placed in the chamber and the test took place at 85°C and 85% relative humidity for 168 hours. A graph is presented in Figure 8, where values of water-soluble solder pastes without cleaning all drop under the limit within the first 24 hours; A and C remain under the limit till the end of the test while B recovers. Pastes D, E, F as well as pastes A and D after resi- due removal all meet the requirements. Den- drite growth was observed for pastes A, B and C (pictures of C and D are presented in Figure 9). Cleanability Water-soluble solder pastes are designed to be cleaned using hot water only as they already contain saponifiers and/or surfactants allowing their removal. On the other hand, no-clean sol- der pastes cannot be cleaned using hot water only. Nevertheless it was decided to submit all the pastes to the same cleaning tests. The wetting test boards with OSP finish were used: one CSP 84 3 rows 0.5 mm pitch 7 mm² and several 0603 chips were placed (Fig- ure 6). After reflow with P5 thermal profile, the boards were cleaned in a water-based spraying machine. Five conditions were used: cold de- ionized (DI) water, 50°C DI water, 65°C DI wa- ter, 50°C DI water with 5% detergent, 50°C DI water with 25% detergent; each cleaning cycle was set to 10 minutes and was followed by a rinsing step with DI water and a drying step of five minutes at 80°C with hot air. The cleanli- ness was evaluated by visual inspection under binocular. Table 8: copper mirror and corrosion test results. rEliaBiliTy aSSESSMENT OF NO-ClEaN aND WaTEr-SOluBlE SOlDEr paSTES, parT ii continues FeAture