Brian Toleno columns | November 18, 2005

A Basic Guide to No-Clean Solder Paste

While many solder material topics could be discussed, a basic overview of no-clean pastes is one that deserves attention.
Because the overwhelming majority (80+%) of solder paste used in today's advanced electronics manufacturing environments is no-clean, a detailed Q&A on the most common and relevant no-clean paste is of particular benefit.

1. Does "no-clean" mean no residue is left on the soldered board after reflow?
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 will depend on the amount of solids contained in the solder paste flux system. Many of the pastes in use today contain anywhere from 50-70% solids in the flux system. The lower the solids content the lower the flux residue volume. The solids are the resins, gelling agents and activator used in the formulating the flux system.

2. How can no-clean residues be washed off a board after soldering?
No-clean flux residues are designed to be left on the board after reflow. If removal of the residues is required, solvents or saponifier/water solutions may satisfactorily remove the flux residue. Every flux chemistry is different, so follow the procedural recommendations of the manufacturer.

3. How is no-clean paste specified?
No-clean solder pastes are specified by alloy type, flux type, powder diameter and metal percentage by weight. Traditional alloys used in today's electronics are the SN63 and SN62 alloys. Printing pastes will usually have a metal percentage of 88 to 90%. At present, Type III powder, 45 microns to 20 microns, or -325+500 mesh powder is most recommended for fine pitch down to 16 mil.

4. What metal finishes can be effectively soldered with no-clean pastes?
No-clean solder pastes can adequately solder most popular metal finishes 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 these specific surfaces and still maintain the non-corrosive and electrical resistance required to qualify them as no-clean pastes.

With the Pb-free alloys the wetting is generally less than what is typically observed with the Sn/Pb systems. This may leave exposed corners or edges on the pads. If full coverage is required than a change in stencil design to have the stencil aperture cover 100% of the pad may be used to have complete solder coverage of the pad after soldering.

5. What do the flux residues look like?
No-clean flux residue can vary in color from clear transparent to amber. The color will depend 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 will also be affected by the thermal profile it sees (hotter thermal profiles will darken the flux residue). Therefore, the residues from Pb-free no-clean solder pastes are typically darker than those of the Sn/Pb systems. The chemistry of the no-clean flux system will also determine if the flux residue remains on top of the soldered joint or flows out to the perimeter of the soldered joint.

6. Does the no-clean flux residue cause corrosion? Is it conductive?
The no-clean residue is not corrosive in nature, and the flux residue is designed to be left on the board after soldering. No-clean flux systems have very high electrical resistance. They are also designed to be hydrophobic (they do not pick up water from the atmosphere) and are therefore 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 kinds of applications from consumer to military.

7. 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 will require one to follow the manufacturer's 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 see. For traditional SN63 no-clean solder pastes, a ramp up of 1-2 degrees Centigrade, a soak of 120-160ºc. for 2-3 minutes and a peak temperature of 205-220ºc. with a time above liquidus of 30-60 seconds is a good starting point. For Pb-free systems such as 95.5Sn, 3.8Ag, 0.7Cu, a ramp to spike is more common with a ramp rate of 1-2 degrees C up to a peak of 235-240oC, with a time above liquids (for SAC 387, 217oC) of 40-75 seconds. For a soak profile a typical soak temperature of 150-1650C for 60-120 seconds before ramping up to the peak is more common.

8. What can I expect if the thermal profile is not as per the manufacturer's recommendations?
Proper profiling is essential to achieve the best soldering results, this is more critical with the application of Pb-free soldering. A high peak temperature will cause the flux residues to darken and char, while the solder joint surface may become dull due to oxidation. Excessively high or long soaks will breakdown the activator package completely 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 may also occur and cause bridging and solder balling. If the peak temperature is too low and the time above the liquidus temperature is too short, wetting may not be complete.

9. Is the flux residue pin-testable?
In most cases, yes. Flux residues will differ from one paste to another. Some are designed to shatter when contacted by the pins, while other residues are soft and can be penetrated by the probes. Various pin designs exist, including spear single point and crown multi-points. Multi-point pins generally give less false failures.

The nature of the flux will also 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 one must institute a procedure to periodically clean the test probes.

10. Is nitrogen atmosphere needed to achieve the best soldering results with Pb-free no-clean pastes?
In most cases nitrogen is not required to achieve good wetting and reliable solder joints even with a Pb-free alloy, such as SAC305 or SAC 387. Nitrogen will certainly help in the appearance and in the solder wetting, but it is not required for good solder joints.

11. How are no-clean pastes affected by the work environment, temperature and humidity?
No-clean solder pastes are not hydrophobic and are not 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 a little faster and give lower abandon and open times at the stencil. Higher humidity may cause some water pick-up and reduce the viscosity of the paste leading to slump and bridging issues. Higher temperatures will cause reduction in viscosity also, while cooler temperatures will cause the viscosity to be higher and cause printing problems such as poor aperture fill.

12. What dispensing solder pastes can I use?
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 to 88 percent. If a dispensing no-clean paste is required, it will also meet the J-STD-005 for corrosion, surface insulation, and halide content. Multicore PN60 and RM89 are recommended no-clean dispensing pastes.

13. What stencil cleaner should I use?
No-clean solder pastes are hydrophobic and therefore water cannot be used, unless a saponifier or another clean agent is used with the water. No-clean paste residues on the stencil can be removed using alcohol-based cleaners or solvents such as Multicore SC-01. 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 like SC-01 can also be used here.

14. How do you clean misprinted boards?
Misprinted boards are more effectively cleaned of solder paste by complete immersion in an appropriate solvent. Alcohol-based solvents and SC-01 are acceptable choices here. Simply wiping a board may not remove the fine solder paste particles, and this could lead to paste trapping itself around the well formed by the solder mask around a pad. The result can be solder balls after this board is reflowed. Bare copper boards with organic surface preservative must be soldered immediately after cleaning since the coating will be removed by the cleaning process and oxidation will slowly render the board more difficult to solder.

15. Should wave solder flux and no-clean solder paste be compatible?
Yes. You should make sure that the no-clean paste and no-clean liquid flux are chemically compatible and of similar specification. You should 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.


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