Lead in solder pastes

AS OF JULY 1, 2006, THE WEEE DIRECTIVE WILL ELIMINATE THE USE OF LEAD IN SOLDER PASTE. THE INDUSTRY IS INTENSIVELY DEVELOPING SOLDER-PASTE SYSTEMS THAT SERVE AS AN ALTERNATIVE TO THE TIN/LEAD (SNPB) SOLDER PASTE USED. BUT SWITCHING TO LEAD-FREE PASTE WILL INCREASE THE NEED FOR CHEMICALLY SUPPORTED CLEANING. By Umut Tosun... 

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. 

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... 

A well-trained and experienced formulation chemist knows how to overcome these hurdles. Oven contamination and flux-management systems were common topics of discussion before lead-free became a buzz-word. When oven contamination is a problem with lead-bearing solder paste, it will be more of an issue with lead-free SAC alloys. Increased process temperatures will decompose the condensed flux. This results in contaminants that are in-... 

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... 

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... 

While the exact printing performance of individual lead-free solder pastes vary, in general, a reduction in paste roll, transfer characteristics, or print quality has not been observed. That is, when comparing the paste roll on a solder paste stencil, there is no noticeable difference in paste bead characteristics except color as noted in Figure 3, which compares the paste roll between eutectic Sn-Pb and Sn-Ag-Cu pastes. In addition, the release quality of lead-free pastes from a stencil has not been noticeably different compared to eutectic Sn-Pb paste. 

Activators in solder pastes often consist of organic acids that degrade with time and temperature. These acids have been selected to react with oxides of lead-containing solders. They are selected because their activity is near optimum in the vicinity of the melting points of these solders. The effectiveness of activators typically decreases at higher temperatures or over time at elevated temperatures. 

As noted earlier, lead-free alloys require high reflow temperatures compared to eutectic Sn-Pb— with thermal profile plateaus in the range of 180-190°C, which are too high for fluxes formulated for eutectic Sn-Pb solder. The oxidation of rosin in air and the increased polymerization at elevated temperatures result in residues that are very difficult to clean with standard solvents. Flux vehicles selected for use in lead-free solder pastes are dramatically different from those used in eutectic Sn-Pb solder pastes. It was determined that cleaning products that preformed very well with eutectic Sn-Pb technology did not remove some residues left as a result of the new flux vehicle formulated for a lead-free solder [11]. 

Material cost was another issue of concern. With the introduction of lead-free, economies of scale worked against the MD player product. No other manufacturer was using a lead-free solder in significant volume. When first introduced in 1998, the cost of the lead-free solder paste was about SI 50/kg. Then in 2000, it dropped to 100/kg. By 2003, the cost of the lead-free solder paste was only marginally higher than eutectic Sn-Pb with the downward trend continuing. 

In the past, lead solder was used to seal canned foods and lead pipes were used to carry drinking water. Tetraethyl lead was once routinely added to gasoline as an antiknock agent certain vehicles may still use leaded gasoline (e.g., agricultural vehicles). Lead was commonly used in paint, with certain formulations containing up to 50% lead. 

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