Issue link: https://iconnect007.uberflip.com/i/270052
76 SMT Magazine • March 2014 ability on different finishes). Finally the residue cleanability will be assessed. The IPC SIR test (method IPC-TM-650 2.6.3.7) will be also per- formed to conclude the study. Both standard- ized tests and production tests will be used to evaluate the performance of these two kinds of solder pastes. Introduction Solder paste is a compound that typically consists of a fusible alloy and some type of de- oxidizing flux. Different pastes can have a va- riety of compositions, though a typical formu- la consists of powdered solder mixed in with a gel-like flux material. According to standard J-STD-004 "Requirements for Soldering Fluxes," solder pastes are classified into three types based on the flux types: (1) rosin-based, (2) water-sol- uble and (3) no-clean. In the past, solder pastes were cleanable. There are generally two differ- ent types of cleanable pastes: water cleanable type (water-soluble paste) and chemical clean- able type (cleaned by solvent or detergent). With the increase in packaging density, combined with the complete ban on CFCs by the year 2000 (the Montreal Protocol), the elec- tronics industry was strongly driven to imple- ment water cleanable process and no-clean processes. Furthermore, as the workability and reliability requirements of the process are be- coming stricter and more precise, water-soluble solder paste and no-clean solder pastes have been developed accordingly. As the name im- plies, flux residues from the water- soluble sol- der paste are designed to be cleaned using wa- ter. For example, for RF circuits and high-speed circuits, for performance reasons it is usually essential that assemblies have no flux residue after reflow. Many electronics manufacturers with a wide variety of products also prefer clean assemblies, such as medical equipment and military assemblies. With the current challenges of the minia- turization of SMT components, narrower spac- ing between components and substrates, lower stand-off and higher temperatures used for lead- free applications, solder paste residue becomes more difficult to clean. Water-soluble pastes are also commonly used in hard-to-solder applica- tions when solvent cleaning is not feasible or in applications where no-clean residues may pose cosmetic issues. Water cleaning is an easy process to control, and the equipment is relatively easy to maintain, though the cost-effectiveness issue comparing to solvent base cleaning, becomes one of the hot debate topics in recent times. From the application point of view, the ob- vious advantage of converting to water-soluble solder paste is the elimination of CFCs from PCB cleaning processes. However, this solder paste technology is more sensitive to normal processing variables. Water-soluble materials often exhibit production sensitivity to mois- ture: Moisture which enters the paste can de- grade printability and promote solder balling. It has also been noted that water-soluble pastes typically have reduced tack times and may be less compatible with long reflow profiles. The lower level of activation associated with no- clean pastes may reduce the process window for full solder wetting of components. The properties of six recent lead-free solder pastes will be described: three of them are wa- ter-soluble and three of them are no-clean. The characteristics of each paste will be reviewed. Then, printing performances will be evaluated through standardized laboratory tests such as viscosity, tackiness and slump and through tests in a printing machine. The reflow properties will be investigated through both standardized feaTure rELIaBILITY aSSESSmENT OF NO-cLEaN aND WaTEr-SOLUBLE SOLDEr PaSTES, ParT 1 contin ues Table 1: Solder pastes characteristics.