Issue link: https://iconnect007.uberflip.com/i/1472190
62 SMT007 MAGAZINE I JULY 2022 show large variation and an overall decreasing trend in volume. Figure 4b shows the compar- ison between ECA and OM550 HRL1 paste. As can be seen, the solder paste volume is constant and its transfer efficiency is around 80% over a period of test. ECA, on the other hand, does not pass RTP test due to viscosity build-up. Once the printing parameters are opti- mized, it is the reflow profile that plays a crucial role in making the interconnection. Low soak and ramp profile were evaluated (Figure 5). In addition, conveyor speed and its effect on solder ball formation were also evaluated. is solder paste could be reflowed at low temperature. e conveyor speed of 28 inch/ min gave acceptable wetting on the metalli- zation pad (not shown). With too slow or too fast conveyor speed, solder ball formation was observed. A slow steady ramp (1°C/sec) per- mits moisture and solvents to evaporate grad- ually prior to rosin/resin soening. Hot slump is minimized. Pastes with long stencil life and tack time generally require a slow ramp so environmentally stable solvents can evapo- rate it and allows the solvent to gradually evap- orate. Study of different peak temperatures gave a good understanding of the paste's per- formance in wetting metallization pads. As can be seen in Figure 6, a 190°C peak reflow tem- ing characteristics of the pastes is consistent and in high throughput manufacturing, paste is likely to produce defect-free printing. e solder paste has excellent stability and metal content remains almost unchanged even aer 12 hours of continuous printing 3 . e binder and solvent system are formulated in such a way that viscosity buildup is restricted, and paste can be printed for hours without cleaning the stencil 3 . In addition, the high print definition and speed works in favor of paste. Even at higher printing speed, the observed transfer efficiency with solder paste was over 90%. Generally, ECAs do not have stencil/ screen life of more than four hours. Aer that, the viscosity of ECA increases rapidly and oen insufficient transfer and screen pore clogging is observed. Material waste is also a big prob- lem with ECA. ECA additionally needs higher pressure to print and through-cut stencils do not work with ECA. A response to pause (RTP) study was con- ducted which is measured by the difference in volume of solder paste deposition as a function of number of prints and pause time. is is to mimic a typical production scenario. A large variation in the print volume aer the pause is not acceptable as this causes end-of-line defects such as shorts and opens. A good sol- der paste shows less variation in the volume of the prints aer pause. However, another may Figure 4: a) Solder spread across the busbar with 600-micron aperture; b) Response to pause capabilities of ECA and solder paste. a b