Austin American Technology Corporation

Removal of Lead-Free Flux Residues Using In-Line Aqueous Cleaning Process

Aqueous Cleaning of EAC Soldering Residues:
The development of a closed-loop cleaning system with advanced process and statistical control for PCBs and other eEffective Cleaning of electronic assemblies requires that the surface residues are either solubilized or dispersed by the wash medium or are forced off the surface by mechanical action. Residues encountered during the soldering process may consist of:

Water soluble materials, polar of non-polar;

Non-polar materials that are insoluble in water, but removable with water containing suitable additives;

Materials that cannot be removed practically by cleaning media, but some or all of the residues may be removable by mechanical action.

Water Soluble Fluxes: The term water-soluble does not necessarily mean that the flux residues will be water-soluble after the reflow process. Solder induced reactions from organometallic compounds or mineral salts. Tin salts are poorly soluble in water, but if left on the board can decompose slowly in the presence of humidity or moisture.

Water-soluble organic fluxes consist of a wide variety of ingredients. There is considerable variation within the materials used by different flux manufacturers, as well as in different fluxes supplied by the same vendor. The residues after soldering may not, however, show the same water solubility characteristics exhibited by the flux prior to soldering. The soldering process expose the flux to the very high temperatures (250-270°C) experienced when soldering high tin alloys. At these temperatures, chemical reactions such as oxidation or pyrolytic decomposition can occur. These reactions generate residues such as tin salts that may be insoluble in water. The degree to which these reactions can occur is a function of time and exposure temperature, as well as the specific chemistry of the flux.

Low solids (No Clean) Fluxes: This technology leaves behind non-ionic flux residues that are not detrimental to electrical performance, and/or to pin testability, and/or are barely visible. This is not necessarily a valid assumption when soldering high tin alloys. In order to ensure sufficient activity, the ratio of activator to rosin must be sufficient when soldering high tin alloys. Thus, it will be necessary to demonstrate that the degree and non-corrosive nature of the flux will not be detrimental.

Rosin Flux: Rosin is a naturally occurring material, consisting of organic resin esters and acids such as abietic, neo-abietic, primaric and palustric. These can be present in various proportions that may differ considerably between sources and even between individual lots of the same source.

Rosin alone has a mild fluxing activity. To enhance rosin's ability to wet high tin alloys, chemical activators are added to the rosin flux formulation. Activators, such as amine halides or organic acids, may consist of nonionic organic materials that become active at the soldering temperature. As a result, it may be desirable to remove rosin flux residues after soldering to expedite electrical testing and coating of the circuit assemblies, to eliminate the unsightly sticky residues, and to remove ionic activators that may cause electrical degradation.

Aqueous In-line Cleaning Process:
Electronic assembly post solder cleaning consists of wash, rinse and dry technology. Successful processing must incorporate four key variables: mechanical energy, solvency, time and temperature. The process conditions will also have an effect on the cleaning process.

Mechanical: In-line cleaning is a high volume cleaning process. The in-line process is comprised of prewash, wash, wet isolation, rinse, final rinse and dry processing steps. The length and number of wash, rinse and dry modules determine the conveyor speed required. Optimum operational parameters such as the wash chemistry, concentration, conveyor speed, temperature, pressure, volume and drying are process variables that can impact the overall results.

Prewash: Low-pressure impingement source that pre-wets the board. Provides a means of removing temporary spot mask.
Wash: Impingement consists of high pressure or high flow or a combination of both. Cleaning in the wash section is designed to dislodge or solubilize the flux residues. The closer the component/board stand-off, the higher the energy required to penetrate under the device. The flow rate of this section may depend on board complexity and flux/wash compatibility. One or more wash sections may be required, depending on the soil, line speed and cleanliness requirements.
Chemical Isolation: Low-pressure water spray following the wash section removes the wash chemistry prior to the rinse section. This allows the integrity of the rinse to remain pure as well as allow close loop processing through carbon ion exchange beds.
Rinse: High impingement or flow rinses free chemical wash water from under components and the surface of the assembly. The closer the component/board standoff, the higher the energy required. The size and degree of rinse will depend on conveyor speed and board complexity.
Final Rinse: A final rinse using high purity DI water. This section typically uses low pressure and volume.
Dry: Residual water is stripped from the board. It is desirable that drying be accomplished with hot high pressure air, not by evaporation.

Solvency: The media involved in aqueous based cleaning is quite simply either water alone or water plus an additive. When removing water soluble flux residues, water is only desirable. With high tin alloys, this may present a more difficult challenge due to higher reflow temperature, flux induced metallurgic reactions and insoluble tin salts.

The chemical activity of aqueous wash media increases when additives are used. Additives will be required when removing rosin, low solids no clean and some water soluble fluxed. The nature of the soil will determine the type additive required to remove the residue. Common additives are as follows:

Wetting Agents (Surfactants): Surfactants are highly concentrated materials that are effective at low concentrations in the wash water. These additives form micelles in the wash solution which consists of hydrophilic (water loving) and hydrophobic (oil loving) aggregates. They are effective in removing polar soils typically found when using water-soluble fluxes and some non-ionic soils such as finger oils and lotion
Saponification: Saponification is a chemical reaction in situ. The saponification ingredient is an alkaline material that reacts with rosin's carboxylic acid group, forming a soap that can then be removed with water. These cleaning agents are formulated with other critical ingredients such as solvents, surfactants, buffers, foam suppressors and inhibitors.
Non-reactive Additives: These additives do not react with the process residues being removed. The cleaning agents are comprised of solvent sprayable ingredients that solvate the residue. These cleaning agents typically do a better job removing low solids no-clean resin structures.
Organic Solvent Emulsions: Non-miscible organic solvents are used in combination with surfactants to form emulsion. These emulsions form finely dispersed droplets that wet and solubilize the contaminant.

Time: Time is the most elastic of the four variables. Increases in temperature, impingement, and solvency all allow a decrease in time. Likewise decreases in temperature, impingement, and solvency create a need for more time. Time can be impacted by outside parameters such as the flux formulation, reflow parameters, and alloy.

Temperature: Flux residues are either dissolved or saponified into the aqueous cleaning composition. Temperature of the cleaning bath is directly proportional to cleaning time and effectiveness. For every 10º C rise in temperature, cleaning activity doubles. Flux residues have a rosin or resin structure. Rosin and/or synthetic resins have a melting point around 80 ºC. Aqueous cleaning compositions heated at a range of 60-80 ºC rapidly soften the flux resin, allowing solubilization into the cleaning media. The viscosity and surface tension of the aqueous cleaning composition typically go down when heated to the desired temperature range.

Aqueous Inline Cleaning Study of High Tin Alloys:
The objective of this study was designed to determine if current inline cleaning processes will remove water soluble, low solids no-clean and rosin lead-free flux residues as a result of the soldering process. Twenty-six lead free solder pastes from 11 different manufacturers were evaluated. Two different inline cleaners were used, one with low pressure/high pressure impingement, and the second with high flow/low pressure impingement.

Process Variables:

Alloys: Sn/Ag/Cu, Sn/AG and SN/Ag/Cu/Sb
Flux: RMA (no-clean), no-clean hard residue, no-clean pin printable soft residue, and water soluble
Reflow: Ramp to spike reflow with a peak temperature of 240-245ºC.

Cleaning Machines: Both machines are typical of designs used for cleaning electronic assembly residues from eutectic Sn/Pb soldering processes. It is anticipated that high tin alloys will be more difficult to clean. This study will compare the results of a cleaning system that has an extended wash section to a machine that has a shorter wash.

Aqueous Inline Cleaner #1 has a longer wash section, lower flow but high pressure. The main wash has higher flow with a pressure of 40-50 psi. The high-pressure wash has lower flow but higher pressure of 80-90 psi. This machine was designed for removing low solids no clean flux formulations. Overall length of the machine is roughly 28 feet. The wash sections cover roughly 5 feet.

Aqueous Inline Cleaner #2 has a shorter wash section, typically designed for water-soluble and rosin based fluxes. This inline combines high flow with low pressure of 25-30 psi. The wash sections on this machine cover roughly 2 feet.

Cleaning Chemistry: Two chemistries were selected. The first is a VOC compliant saponifier. The second chemistry a low reactive solvent sprayable alcohol composition. Both chemistries are being used as EAC post solder assembly cleaners in industry today.

Saponifier (CSA): Concentrated aqueous cleaner designed for use in spray cleaning machines. The cleaning chemistry reacts with flux resin structure, allowing dissolution. Effective on water soluble, rosin and some low solids no-clean flux formulations.

Solvent Sprayable Alcohol (CSB): Concentrated aqueous alcohol cleaner designed for use in spray cleaning machines. This composition contains a mild activator that helps soften the residue followed by dissolution. This chemistry is effective on water-soluble, rosin and most low solids no clean flux formulations.

Time: A key rule worth consideration: more time is typically better, to a point. In-line #1 exposure in the wash was roughly 2.5 minutes versus 0.75 and 2 minutes for in-line #2. It is assumed that the longer exposure time in the wash will be a positive.

Temperature: Flux resins will reach a liquidus point around 180ºF. For instance, and increase from 140ºF to 150ºF can double the cleaning efficiency in the same amount of time. The closer the temperature approaches the liquidus point the quicker it will dissolve into the cleaning solution.

Key Variable Differentiation: When doing a designed experiment, only one variable should be changed. In this experiment, two separate machines were used. Each machine had different capabilities. As a result, several variables were different: time, temperature, impingement and pressure. The results will show the differences from a comparative viewpoint.

Water Soluble Flux: Four water soluble high tin lead free pastes were evaluated. Three of the four pastes evaluated have no residues left after the cleaning process. On some of the water soluble paste boards, there were more solder balls.

RMA Flux: Six RMA high tin solder pastes were evaluated. Overall results were poor. This is a clear indication that RMA high tin lead-free alloys flux residues are harder to clean than eutectic Sn/Pb.

No-Clean Soft Residue: Seven high tin lead-free alloys using no-clean soft residue flux were evaluated. Some good results were achieved when the part was exposed to the wash section for roughly 2 minutes. At shorter time exposure in the wash, results were marginal. This is another data point indicating a more difficult cleaning challenge.

No-Clean Hard Residue: Nine high tin lead-free alloys using no-clean hard residue were evaluated. As with the results using no-clean soft residue, there were some good results when exposure to the wash was roughly 2 minutes. At shorter time exposure in the wash, results were marginal.

Conclusion:
Aqueous inline cleaning is a robust process that has been successfully implemented for removal of water-soluble, rosin and no-clean flux residues. There is a wide variety of cleaning chemistries designed for use in aqueous inline processes; wetting agents, saponification, non-reactive additives and organic solvent emulsions. The selection of the flux formulation in either the paste composition or wave solder process will determine the cleaning chemistry required.

Lead-free soldering processes face challenges no only from soldering and handling, but also from a cleaning perspective. The flux formulation will require higher flux capacity, higher oxygen barrier capability and higher thermal stability. Cleaning lead-free flux residues will be more challenging due to higher reflow temperatures, increased residue and more tin-salt formation.

This experiment pointed out that existing aqueous cleaning processes would require adjustment from both a cleaning chemical and mechanical standpoint. It is possible to formulate more aggressive cleaning agents but there will be a key tradeoff being compatibility. From a mechanical standpoint, longer wash sections increasing exposure time to the wash chemistry will be key.

Industry consortiums between cleaning chemistry, solder paste, and cleaning equipment suppliers will allow the technology development to resolve industry-cleaning challenges when converting to a lead-free process.