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STEP 3: Solder Materials
December 31, 1969 |Estimated reading time: 6 minutes
By Brian Toleno
While many solder material topics could be addressed, an overview of no-clean pastes deserves attention. This detailed Q&A answers common and relevant no-clean paste questions.
Q: Does “no-clean” mean no residue is left on the soldered board after reflow?
A: No. No-clean solder pastes will leave a small amount of resinous residue after reflow around, or on, the solder joint. The amount of residue left after soldering depends on the amount of solids contained in the solder-paste flux system. Many of the pastes in use contain 50-70% solids in the flux system. The lower the solid content, the lower the flux-residue volume.
Q: How is no-clean paste specified?
A: 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 of 88 to 90%.
Q: What do flux residues look like?
A: 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.
Q: Does no-clean flux residue causecorrosion? Is it conductive?
A: No-clean residue is not corrosive in nature, and the flux residue is designed to remain on the board after soldering. No-clean flux systems have high electrical resistance. They also are designed to be hydrophobic, meaning they do not pick up water from the atmosphere, and are benign. No-clean flux residues pass the J-STD-005 requirements for copper corrosion, halide content and surface-insulation resistance. No-clean solder pastes are used in many applications - from consumer to military.
Q: What thermal profile is recommended for no-clean solder pastes?
A: 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 of 205-220°C with a time above liquidus of 30-60 seconds is a good starting point. For lead-free systems such as 95.5Sn3.8Ag0.7Cu, a ramp-to-spike is more common with a ramp rate of 1-2°C up to a peak of 235-240°C, with a time above liquids (for SAC387, 217°C) of 40-75 seconds. For a soak profile, a typical soak temperature of 150-165°C for 60-120 seconds before ramping up to the peak is more common.
Q: What can I expect if the thermal profile is not according to the manufacturer’s recommendations?
A: Proper profiling is essential to achieve the best soldering results, which are critical with lead-free soldering. A high peak temperature will cause flux residues to darken and char, while the solder joint surface may become dull due to oxidation. Excessively high or long soaks will break down the activator package to the point where no activator is left when the solder powder melts. This may result in solder balls and poor wetting. Excessive hot slump also may occur and cause bridging and solder balling. If the peak temperature is too low and the time above liquidus temperature is too short, wetting may be incomplete.
Q: How can I remove no-clean residues from a board after soldering?
A: No-clean flux residues are designed to remain on the board after reflow. If removal of the residues is required, solvents or saponifier/water solutions may remove the residue.
Q: Is flux residue pin-testable?
A: Yes, in most cases, flux residues differ from one paste to another. Some are designed to shatter when contacted by pins, while other residues are soft and can be penetrated by probes. Various pin designs exist, including spear single-point designs and crown multi-points. Multi-point pins generally give fewer false-failures. The nature of the flux also will play a role in false failure readings. Some fluxes may accumulate around the perimeter of the soldered joint, while other fluxes can sit on top of the solder joint, making pin testing without false failures more difficult. To reduce false failures, institute a procedure to periodically clean test probes.
Q: Is nitrogen needed for the best soldering results with lead-free no-clean pastes?
A: Nitrogen is not required in most cases to achieve good wetting and reliable solder joints, even with lead-free alloys, such as SAC305 or SAC387. Nitrogen will help in the appearance and solder wetting, but it is not required.
Q: How do work environments, temperature and humidity affect no-clean pastes?
A: 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 some water pick-up and reduce paste viscosity, leading to slump and bridging issues. Higher temperatures will cause reduction in viscosity, while cooler temperatures will cause higher viscosity and printing problems, such as poor aperture fill.
Q: What metal finishes can be soldered with no-clean pastes?
A: 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.
No-clean paste shown in packaging ready for use with dispensing or screen printing equipment.
With lead-free alloys, wetting generally is less than that of Sn/Pb systems. This may leave exposed corners or edges on the pads. If full coverage is required, then a change in stencil design is needed so that the stencil aperture covers 100% of the pad.
Q: What dispensing solder pastes can I use?
A: Dispensing pastes in a no-clean process should be compatible and of similar chemistry. The viscosity and metal percentage of dispensing solder pastes will be lower. Dispensing pastes traditionally have viscosities in the range of 300,000 to 600,000 Centipoises, and metal loads of 85-88%. If a dispensing no-clean paste is required, it also will meet the J-STD-005 for corrosion, surface insulation and halide content.
Q: What stencil cleaner should I use?
A: No-clean solder pastes are hydro-phobic; therefore water cannot be used unless a saponifier or other clean agent also is used. No-clean paste residues on the stencil can be removed using alcohol-based cleaners or solvents. Always clean a stencil 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.
Q: How do you clean misprinted boards?
A: Misprinted boards are cleaned of solder paste effectively through complete immersion in an appropriate solvent. Alcohol-based solvents are acceptable choices. Simply wiping a board may not remove fine solder-paste particles, and could lead to paste trapping itself around the well formed by the solder mask, resulting in solder balls after the board is reflowed. Bare copper boards with organic surface preservatives must be soldered immediately after cleaning because the cleaning process will remove the coating, and oxidation will render the board difficult to solder.
Q: Should wave-solder flux and no-clean solder paste be compatible?
A: Yes. You should be certain that no-clean paste and no-clean liquid flux are chemically compatible and of similar specifications. Do not use no-clean paste and water-washable flux followed by water washing, as the water may affect the no-clean residue and turn it slightly white.
Brian J. Toleno, Ph.D., application engineering team leader, the electronics group of Henkel, may be contacted at (949) 789-2554; e-mail: brian.toleno@us.henkel.com.