No-clean soldering

Gutierrez, S., and Tulkoff, C. (1994), Benchmarking and QFD Accelerating the Successful Implementation of No Clean Soldering, in Proceedings of the lEEE/CPMT Int. Electronics Manufacturing Technology Symposium, pp. 389-392. 

A process that has found widespread use in practice is semi-aqueous cleaning with ZESTRON FA. This cleaner was especially developed for removal of no-clean soldering fluxes in ultrasonic batch or spray under immersion cleaning systems. It is used in the cleaning bath, rinsed with demineralized water, and then dried with hot air (see scheme in Figure 14.8.13.). 

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. 

Do not use water to clean a no-clean soldered board unless the solder paste manufacturer recommends it, as it may result in a sticky, white residue on top of flux residues. This may impact electrical surface insulation resistance and corrosion. 

As mentioned previously, it is important to heat the flux sufficiently to activate it (make it chemically reactive) and to burn off the activators by the end of the soldering process. This is true even of no-clean solder chemistries. If an unactivated, insufficiently heated no-clean solder flux is left on a board, the weak organic acids in the flux residue could cause long-term corrosion. Usually temperatures between 80° and 150°C (depending on the flux composition) are sufficient to activate a flux fully. Avoid pooling flux. To minimize the possibiiity of corrosion. 

Iman, R., et al., Evaluation of a No-Clean Soldering Process Designed to Eliminate the Use of Ozone Depleting Chemicals, IWRP CRADA No. CR91-1026. 

For no-clean boards, the same solder paste used for the initial assembly should be used for the repair. For aqueous-cleaned boards, often a no-clean solder chemistry is used to preclude the need for another wash cycle. If solder paste is not added, then a paste flux (identical to that comprising the solder paste) should be used. This will obviate solder chemistry interactions and reduce likelihood of long-term corrosion. 

No-clean solder flux formulations can be used for rework and repair even if the PCA was manufactnred with an aqueous clean chemistry, although the reverse is not recommended. Sometimes when a no-clean board is subjected to an aqueous cleaning, the no-clean flux residue takes on a white, gnmmy characteristic that is conducive to dendritic corrosion, which can result in soft or hard electrical shorting. Saponified aqueous cleaning can be nsed, but must be tested for effectiveness in removing the polymerized flux residue from the no-clean process and for compatibihty with the selected no-clean flux to avoid generation of corrosive byproducts. This is particularly important under connectors, area-array devices, and other components with low headroom between the underside of the body and the PWB surface. 

D. Shangguan and G. Gao, Lead-Free No-Clean Soldering for Automotive Electronics, Solder. Surf. Mt. Technol, 26 (1997), p 5-8... 

J. Csonka-Peeren, Quantifying Parasitic Induced by No-Clean Solder Paste Residues at RF Frequencies, Proceedings of APEX 2002... 

M. Duffy et al., RF Characterization of No-Clean Solder Rux Residues, IMAPS International Symposium on Microelectronics, 2001... 

No-clean solder pastes are specified by alloy type, flux type, powder diameter, and metal percentage by weight. Traditional alloys are SN63 and SN62 alloys. Printing pastes usually will have a metal percentage of88to90%. 

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. 

O What thermal profile is recommended for no-clean solder pastes ... 

Every no-clean solder paste is different, containing different activator packages, resins and solvents. Chemistry differences require users to follow the recommended thermal profile. A thermal profiler is needed, with thermocouples attached to key areas of the board to be soldered, to determine the exact profile a board will experience. For traditional SN63 no-clean solder pastes, a ramp up of 1-2°C, a soak of 120-160°C for 2-3 minutes, and a peak temperature... 

No-clean solder pastes are not hydro-phobic or prone to water pick-up. The recommended environment for an SMT process is 40-60% relative humidity at temperatures of 68-78°F. Low humidity, such as 10% relative humidity may cause the paste to dry out faster, giving lower abandon and open times at the stencil. Higher humidity may cause... 

No-clean solder pastes can solder most popular metal finishes adequately due to improvements in the activator packages. Gold over nickel, bare copper with organic surface preservatives, silver immersion, tin plates, and hot-air leveled boards are popular, while component terminations such as tin, tin/lead, silver, silver palladium, and nickel are used. Solder pastes can be designed to solder specific surfaces and maintain the non-corrosive and electrical resistance required to qualify them as no-clean pastes. 

Should wave-solder flux and no-clean solder paste be compatible ... 

Due to the lower wetting speeds associated with alternative lead-free solders, flux activation will be a critical factor in paste performance. No-clean and water-washable solder pastes are being designed so they do not require nitrogen reflow and can produce reliable solder connections with good wetting in air. Water-washable solder pastes with their higher concentration of activators will solder most metal finishes adequately. No-clean solder pastes require careful finish and paste attribute selection. 

Schneider, A.F. Understanding the requirements of J-STD-004 and its relationship to the soldering requirements of J-STD-OOIB. (http //www.alphametals.eom/products/fluxes/pdf/jstd004.pdf). Rubin, W. Steinberg, J. Niemezura, P. Moyer, D. The importance of hahdes in no-clean solder pastes. Heraeus SMT Materials Technical Paper, retrieved March 16, 2003. http //www.4hcd.com/ techprrs/halides. 

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