No-clean solder pastes

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. 

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

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. 

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. 

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-dean solder pastes are hydrophobic therefore water cannot be used unless a saponifier or other clean agent also is used. No-dean paste residues on the stendl can be removed using alcohol-based cleaners or solvents. Always clean a stendl promptly after use, as dry solder paste is difficult to remove from fine apertures. Stencils will require regular underside wiping of paste residue, and an alcohol-based cleaner. 

Hand wipe and immersion N/A cleaning for removal of no-clean water soluble and rosin solder paste, white residue associated with isopropanol cleaning, and flux residues... 

Dual tank system for stencil (29 inch maximum) and pallet cleaning RA, RMA, no-clean synthetic and water soluble (OA) flux residues, solder paste (ambient), and wet SMT adhesives (38-43 °C, ultrasonics)... 

AQ-201SC Semi-automatic ultrasonic (40 kHz) stencilcleaning system for solder paste and adhesives removes rosin (R, RMA, and RA), water-soluble (OA), and no-clean pastes Stencil wash, stencil wash plus, stencil wash AD SMT, stencil cleaner N/A N/A... 

Model 6000 and 6000 LC Fully automatic and programmable SMT stencil cleaner [for RA, RMA, no-clean and water soluble (OA), adhesives and solder paste, 72-in. max.] SMT detergent 440-R N/A N/A... 

Although commonly used solder paste for both THT and SMT production contains 63-37 eutectic tin-lead solder, other metal formulations are available, including 96-4 tin-silver (silver solder). The fluxes available are also similar, with typical choices being made between RMA, water-soluble, and no-clean fluxes. The correct decision rests as much on the choice of flux as it does on the proper metal mixture. A solder paste supplier can best advise on solder pastes for specific needs. Many studies are in process to determine a no-lead replacement for lead-based solder in commercial electronic assemblies. The design should investigate the current status of these studies as well as the status of no-lead legislation as part of the decision making process. 

Certain no-clean fluxes are not compatible with each other. Their chemical interaction could result in corrosion on the PCA. To avoid corrosion issues, solder paste flux, wave solder flux, and repair flux should be tested separately and in combination for SIR and ECM with the board surface finish. 

Fenner, M., Solder Paste for No Clean Reflow, Soldering and Surface Mount Technology, No. 13, February 1990. 

Soldering flux is applied prior to component placement and soldering. Flux choice should be tailored to the hot-bar process.The flux can be either liquid or paste. TTie flux chosen should be tested to resist charring and the development of polymerized decomposition products, or lacquers, which adhere to both circuit board and hot-bar. Residue build-up on the bar can adversely inhibit hot-bar performance by diminishing thermal transfer. The residue can also become thick and uneven enough to prevent the hot-bar from squarely contacting component leads. Baked-on aqueous-clean flux residues can make flux cleaning difficult. With no-clean flux, overheated residues may detract from the visual appearance of the printed circuit assembly. 

Reflow. The reflow process attaches SMT and some TH components to the PCB by melting solder paste to form solder joints using an oven with a controlled thermal profile (see Fig. 57.17) and, in some cases, a controlled atmosphere (often N2). Reliability problems that can arise due to improper reflow parameters can be gronped into three categories damaged components, poor solder joints, and, for no-clean assemblies, cleanliness issnes. 

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