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Cleaning
December 31, 1969 |Estimated reading time: 7 minutes
By Norbert Löw and Andreas Muehlbauer
The advance of lead-free solder paste has stepped up its pace recently. Hence, a very rapid transition to lead-free products can be expected in the near future. Just as with the introduction of other technology, Japan is once again the driving force behind the new development. The lead-free solder paste "train" is picking up speed because of market forces and the Asian leadership role, apart from isolated cases, and now has become unstoppable. Question: Is cleaning (of lead-free assemblies) friendly again?
Within only a few years, a significant number of professional associations, bodies and industrial project groups have established themselves at national and international levels, while a multitude of reports, articles and papers on the subject of lead-free solder assembly has been published. In the meantime, extensive know-how has been gathered on potential alternative solder paste systems, and exhaustive studies also have been conducted on printing and soldering with lead-free solder systems.
In the field of electronics cleaning, such changing processing conditions naturally have been monitored with considerable interest. For example, the higher soldering temperatures in conjunction with recently developed flux systems are indications of the "return-to-cleaning" trend.
Alternative Solder Paste SystemsAlthough the Sn/Ag/Cu alloy currently is discussed widely as an alternative solder paste system, the tests described here include a wider range of paste systems from various manufacturers. Consequently, the studies concern a total of 25 different lead-free solder pastes supplied by 13 manufacturers. (The table indicates the compositions of the tested materials, listing the alloys and share of fluxes and solids, e.g., colophon).
There are two important process changes that affect cleaning applications with respect to lead-free solder-paste systems:
- Conventional reflow soldering temperatures (~225°C) increase significantly (245° to 260°C) with most lead-free solder paste systems. (Exception: Sn/Bi.) Their flux systems, when exposed to higher thermal stresses, generally result in residues very difficult to remove.
- Little information currently is available on their removability.
Potential Cleaning Agents. Three types of cleaning systems are basic in the industry: solvent-based cleaners (in water-free and semiaqueous processes), water-based cleaners and aqueous alkaline surfactant cleaners.
The type of contamination determines the chemical formulation of a cleaning agent. For example, whereas a pH-neutral cleaner removes unsoldered paste from stencils and misprinted printed circuit boards (PCB), alkaline cleaning agents are effective in removing fluxes after soldering.
Unsoldered Solder Paste RemovalThe industry primarily uses stencil printing processes for solder paste application. Most stencils are stainless steel and must be cleaned at regular intervals (e.g., at the end of a shift) by equipment specifically designed for this purpose. The actual printing process is integrated into manufacturing and is fully automatic. Nevertheless, misprints still occur, particularly when the press is run at full speed, resulting in paste printed in the wrong areas or blurred. Such misprints are cleaned and then returned to the production process.
The studies involve stainless steel stencils and unprinted PCBs that are contaminated with the lead-free solder pastes listed in the table. After a one-hour drying period, the "misprints" are cleaned from two to six minutes in a typical stencil cleaning line at room temperature and a spraying pressure of 1.5 bar (Figure 1).
Figure 1. A two-tank stencil cleaning line. The PCBs are cleaned for two to six minutes at room temperature and under a spray pressure of 1.5 bar.
The cleaned test substrates then are examined under a microscope (10X magnification) to check for paste residues. In the test, the cleaning agents easily removed all lead-free solder paste systems without any problems. Moreover, it was confirmed that solvent-cleaning systems still achieve the shortest cleaning times and are characterized by the widest process window. The specific results are as follows:
- Solvent-based cleaners: two to three minutes
- Water-based cleaners: three to four minutes
- Aqueous alkaline surfactant cleaners: four to six minutes.
Post-solder Flux RemovalRemoving flux from soldered assemblies proves to be far more critical than from unsoldered solder paste from stencils and misprints. In a series of tests conducted in cooperation with the Munich Technical University, the lead-free solder pastes (1 to 24) are printed manually on standard test substrates and then soldered in a reflow furnace with due consideration of the specific temperature profile for each paste.
Figure 2. A four-chamber ultrasonic cleaning system. Immersion in demineralized water is at 50°C.
Ionic contamination measures were carried out on each of the sets of soldered test substrates with an ionograph*. Because the test substrates are unassembled soldered PCBs, lower values are expected even before cleaning is completed. The three remaining sets of soldered test substrates are cleaned with a typical representative of the mentioned cleaners in a customary cleaning line. The following process parameters are applied:
- Solvent: 10 minutes in an ultrasonic immersion system at 50°C, rinsing with demineralized water (Figure 2).
- Water-based cleaner: 10 minutes in a spraying system at 50°C, rinsing with demineralized water.
- Surfactant cleaner: 10 minutes in the spraying system at 50°C, rinsing with demineralized water (Figure 3).
Figure 3. Process diagram of a single-chamber spray cleaning system, effective for use both with water-based and surfactant cleaners.
After cleaning, the test substrates are examined for residues under a microscope (40X), followed by an ionic contamination measurement of each substrate. Finally, the substrates are subjected to an additional qualitative color-reaction test to detect flux activator residues (flux test) that are not discovered by ionic-contamination measurement. This simple test indicates the carbonic acid-based activators of fluxes, thereby providing a precise survey of the local distribution of contaminants.
Figure 4. A comparison of the cleaning methods in ionic-contamination measurements before and after processing.
A visual inspection for white residues produces good results with all three cleaning methods. Only occasionally are white residues discovered (approximately five percent of the total number of cleaned substrates), but these are removed completely through process optimization. Figure 4 compares the results of ionic-contamination measurement before and after cleaning with the different cleaning methods.
Figure 5. Photomicrograph of a PCB cleaned with an aqueous alkaline medium after performing the color reaction test.
The ionic measured results for the three cleaning systems are good. Color reaction test results for activator residues are less positive. At the soldered points, the latter are apparent on 20 percent of the tested substrates in spite of the good values associated with ionic-contamination measurement. Such residues are colored blue by the color reaction test, as shown in Figure 5 (no residues are visible before the color reaction test). Ionic-contamination measurement also supplies an excellent result with 0.06 μg (equivalent to NaCl/cm2). However, activator residues still are discernible after the color reaction tests. Therefore, it can be assumed that the situation would have been even more extreme without cleaning.
This example confirms that routine measurement of ionic residual contamination on soldered PCBs often is insufficient to detect all remaining residues. The cleaning process is optimized in the next step and the color reaction test once again is performed. The result is that activator residues no longer can be detected, particularly in the processes using solvent- and water-based medias.
SummaryThe following statements can be made as a result of the findings of this study involving three fundamentally different cleaning systems solvents, MPC cleaners and surfactant cleaners:
- Removing unsoldered, lead-free solder pastes in stencil and misprint cleaning provided results comparable to those obtained with lead-based solder pastes. Experience has shown that changeover problems are not to be expected.
- In removing flux residues from soldered PCBs, a relatively high percentage of activator residues are detected in spite of low ion equivalents. However, this is avoided by optimizing the cleaning process. Accordingly, qualified specialist companies must carry out corresponding tests before changing over to lead-free solder pastes. This applies particularly to those applications in which climate safety and long-term guarantee claims are of special importance.
- Westek ICOM 2000.MPC is a registered trademark of Zestron Corp.
DR. ANDREAS MUEHLBAUER may be contacted at Zestron Corp., 21641 Beaumeade Circle, Suite 315, Ashburn VA 20147; (703) 589-1198; Fax: (703) 821-9248; E-mail: A.Muehlbauer@zestron.com; Web site: www.zestron.com.
ABCs of SMTAqueous cleaning: A water-based method that may include the addition of neutralizers, saponifiers, surfactants, dispersants and anti-foaming agents.
Azeotrope: A mixture of two or more polar and nonpolar solvents that behaves like a single solvent to remove contaminants.
Cold cleaning: An organic solvent cleaning process in which liquid contact completes residue solution and removal after soldering.
Dispersant: A chemical added to water to improve its ability to remove particulates.
Neutralizer: An alkaline chemical added to water to improve its ability to dissolve organic acid flux residues.
Saponifier: An aqueous solution of organic or inorganic bases and additives for removing rosin and water-soluble fluxes via emulsification or detergent action.
Solids: The percentage by weight of rosin in a flux formulation.
Surfactant: A chemical added to water to decrease surface tension and improve wetting.
Ultrasonic immersion: A cleaning technique using cavitation, or the creation of bubbles via high-intensity sound waves, in a cleaning liquid. The resulting agitation "scrubs" the immersed boards.