Lead-free solder pastes

SMT detergent 440-R (10%-12% for RA solder paste, 10% for RMA solder paste, 6%-12% for lead-free solder paste, and 10% for SMD adhesives and post-solder flux [rosin and OA] buildup)... 

Sanders, J. R., Chute, S., Soma, J., and Pouts, C., A Comparison of Cleaning Technologies for New Lead-Free Solder Paste Formulatons, Proceedings of SMTAI International, 2005, pp. 871-875. 

Amalu, E.H., Ekere, N.N. Mallik, S. (2011). Evaluation of Rheological Properties of Lead-Free Solder Pastes and their Relationship with Transfer Efficiency During Stencil Printing Process. Materials and Design, Vol. 32, (February 2011), pp. 3189-3197. 

Durairaj, R., Mallik, S., Seman, A. Ekere, N.N. (2009a). Investigation of Wall-Slip Effect on Lead-Free Solder Paste and Isotropic Conductive Adhesives. Sadhana, Vol. 34, No.5, (October 2009), pp. 799-810. 

Jackson, G.J., Durairaj, R. Ekere, N.N. (2002). Characterisation of Lead-Free Solder Pastes for Low-Cost Flip-Chip Bumping. Proceedings of the 27 Annual lEEE/CPMJ/SEMI/ International Electronics Manufacturing Technology (lEMT) Symposium, San Jose, CA, USA, July 2002, pp. 223-228. 

In terms of printability, tack, slump, and solder balling, there is no clear and consistent difference between the Sn-Pb and lead-free solder pastes (Ref 59-60), because these performances depend on the solder paste formulation, not directly on the solder alloy. Very clear and consistent differences have been observed, however, in wettability between the tin-lead and lead-free solder pastes. In general, the wettability of lead-free solder paste is not as good as the tin-lead solder paste. For example, lead-free solder paste exhibits very limited spreading on OSP during reflow, and exposed corners after reflow are quite common, unless overprint or round corner pads are used. The difference in wettability between OSP and ENIG surface finishes, which is already evident for the tin-lead solder, becomes even more pronounced for lead-free solders. This has been observed with a variety of solder paste and flux formulations from a number of vendors. 

M. Arra, D. Geiger, D. Shangguan, S. Yi, F. Grebenstein, H. Fockenberger, K.H. Kerk, and H. Wong, Performance Evaluation of Lead-Free Solder Paste, Proceedings of SMTA Conference, Oct. 2001, p 850-857... 

Tin-lead and lead-free (Sn95.5Ag3.8Cu0.7) solder pastes were printed on printed wiring boards with various BGA and CSP design features. After paste printing, the boards with tin-lead and lead-free solder pastes were subjected... 

Mixed assembly, i.e., tin-lead solder ball with tin-silver-copper (SAC305) solder paste, also showed a higher level of voids when compared to lead-free assemblies (Fig. 22). The tin-lead and SAC solder reach liquidus at 183 and 221 °C (361 and 429 T), respectively. Therefore, the tin-lead solder balls become molten well before the lead-free solder pastes, possibly blocking the paste outgassing. In spite of the voids, it was shown that all lead-free assemblies and all surface finishes produced class III, 0-9% area voids, specified by IFC 7095. 

Experience has proven that there is no difference in the actual printing of lead-free solder paste vs. tin/lead solders. Formal... 

Another issue with printing lead-free solder paste is stencil aperture design. Traditionally, stencil aperture size is reduced in relation to PCB pad size. This ensures the stencil aperture seals, or gaskets, to the PCB pad. Gasketing reduces solder paste that can get under the stencil and eventually cause wet solder bridges if not cleaned properly. Lead-free solder paste does not spread as well, so... 

What lead-free solder paste is being used ... 

The lead-free transition will have some impact on the stencil printing process. Printability of solder paste depends on flux formulation, but not for all alloy types. Experience has proven that there are no differences when printing lead-free solder pastes than when printing tin/lead solder pastes. Formal testing has verified that the printed volume of several lead-free solder pastes and tin/lead solder pastes were the same statistically when using the same stendl, printing equipment and boards (Figure 2). 

Printability of lead-free solder paste will not change, but its spread during reflow will, which may require tightening of the stendl-printing process. One possible issue is print accuracy, or the alignment of the printed solder paste onto the PCB pad. Because lead-free alloys do not spread or wet as well as tin/lead, any solder paste that is not accurately printed onto the PCB will stay close to where it was printed after the reflow soldering process. Figures 3 and 4 depict the same deposits before and after reflow for QFPs and passives. 

Figure 3a. and 3b. OFP printed with lead-free solder paste before and after reflow. There is a lack of spread during the reflow process. 

Qualification studies and field experience have uncovered significant issues with the material, including shortened shelf life of several types of lead-free solder paste and significantly different results regarding voiding. Both phenomena have a potentially common root, which is oxidation of solder powder during production. 

Possible Effects of Lead-Free Solder Paste Systems on ... 

With the move to lead-free solder pastes, problems in the cleaning and removal of flux residues from soldered assemblies are to be expected due to soldering-process variations. Because of the change of flux formulations, especially with respect to new components or changes in solvents used, resin, and thixotropic concentrations, the removal of unsoldered solder paste also may be affected. The lead-free pastes used in this study were tested additionally for the ease of removal from stencils and misprinted circuit boards. 

In this test, multiple, stainless steel stencils and PCBs were prepared with the latest lead-free solder pastes available. After an hour of drying time, each stencil, in conjunction with a misprinted board, was cleaned at room temperature in spray-in-air equipment for three to six minutes (Table 2). Subsequently, cleaning tests were repeated at room temperature in ultrasonic equipment. Test substrates cleaned were visually inspected under a microscope (lOx) and tested for solder-paste residues. It was shown that all tested cleaning agents removed all lead-free solder pastes (Table 3). Differences were observed in the cleaning times for the different cleaning applications. 

The removal of flux residues from soldered assemblies is more difficult than the removal of solder paste from stencils and misprints. In cooperation with a reputable local university, various lead-free solder pastes were printed onto standard test substrates and then soldered in a reflow oven at the specific temperature profiles of each respective solder... 

Cleaning Time Required for Complete Removal of Lead-Free Solder Pastes... 

Unsoldered Lead-Free Solder Paste Cleaning Results... 

Based on the results of both test series conducted on 40 different lead-free solder pastes, some important conclusions were reached. For the removal of the uncured lead-free solder pastes from stencils and misprints, the results obtained were comparable to those of eutectic pastes. 

Figure 1. Typical process window for lead-free solder paste.

Hart, P.J. Henderson, D. Puttlitz, K.J. Tonsi, N. Sarkhel, A. Woychik, C. Characteristics of some lead-free solder pastes for microelectronics package assembly. Semicon West 2000 (San Jose, CA) July 2000. 

Matsushita introduced its first lead-free solder minidisc player in October 1998, the MJ30. The minidisc player was assembled using surface mount technology and a Sn-Ag-Bi-Cu lead-free solder paste. For through-hole assemblies, the eutectic Sn-Cu alloy was selected. The company... 

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