Lead-free solder cleaning residues

Methyl laurate (surfactant) Rosin, no clean and lead-free solder flux residues... 

In wireless communication products, the effect of no-clean fluxes on the radio frequency (RF) signal integrity of a product is a matter of concern. As RF frequencies increase, every part of a circuit, such as the conductor traces, the solder mask, and the flux residue, all add to the overall circuit design and tolerances. The RF impact of lead-free solder and flux residues (Ref 49) needs further study. 

No-clean flux residue can vary in color from clear-transparent to amber. The color depends on the resin systems used in the flux formulation. Modified resin systems tend to give pale, transparent residues, while rosin-based no-cleans tend to give amber-colored residues. The residue also will be affected by the thermal profile it sees (hotter thermal profiles will darken the flux residue). Therefore, residues from lead-free no-clean solder pastes typically are darker than those of the Sn/Pb systems. The chemistry of the no-clean flux system also will determine if the flux residue remains on top of the soldered joint, or flows out to the perimeter of the soldered joint. 

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. 

Conductive adhesive materials are very cost-competitive with solder materials, only require a relatively low-temperature cure, and leave no residues, and thus require no subsequent cleaning step. This is a significant advantage compared to lead-free solders that require a high-temperature reflow and, depending on the flux utilized, may also require a subsequent cleaning step. The print speeds of conductive adhesives are comparable to solder pastes. 

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]. 

Since the process temperature for most lead-frees are higher than for eutectic- or near-eutectic Sn-Pb solder, any flux residues associated with No-Clean soldering are more thoroughly baked onto PWB snrface metals. This inhibits electrical test probe contact. Even with today s No-Clean solder pastes and with Sn-Pb solders, electrical probing can be a challenge. Often the residues that cover test points necessitate multiple seating cycles of the test probes to penetrate the flux residue. 

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