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Matching Cleaning Process to Solder Paste
December 31, 1969 |Estimated reading time: 9 minutes
By Gérard Abidh, Avantec SA
How can we determine PCBA cleanliness in a cleaning process? How do cleaning materials change with environmental restrictions? What is the best type of cleaning for a chosen solder paste? These example experiments include contaminant, detergent, and process set-up parameters. An additional parameter, cleaning solutions that are environmental friendly, is also considered.
Cleaned electronic circuits represent only a small part of the electronic assembly market, as most companies use a no-clean process. In several sectors – aeronautics, medical, automotive – cleanliness and reliability are crucial. Post-assembly cleaning of no-clean solder paste residue is necessary. These sectors have demanding requirements for cleanliness and chemical reliability after PCB assembly.
Requirements vary with the particular product assembly. For example, the aerospace industry needs long-term reliability, while the medical industry prioritizes highly reliable circuits for implants and other devices. Some products are covered with a protective varnish or conformal coating that requires the surface to be thoroughly cleaned before the sealant is applied.
From a chemical point of view, flux and solder paste residues common to high-reliability/long-lifecycle products must be compatible with solvents or detergents used to clean them. Co-development of materials in this area leads to global solutions for assembly cleaning.
Contaminants
Contaminants on PCBs are mainly organic, brought about during PCB manufacturing and during the assembly process. Fluxes have a deoxidizing function; they deoxidize the pads and components. In the case of solder paste, they deoxidize the solder powder, ensuring good wettability of the alloy. Fluxes contain organic acids, resins, and other materials to perform this function.
The term no-clean implies, according to current norms, that residues are chemically safe and can be left on the PCB. The corrosion test, surface insulation resistance (SIR) test, electromigration, and other specialized tests determine the halogen/halide content used to verify no-clean safety for the finished assembly. However, for high-reliability applications, no residues or other contaminants can remain on the circuit board.
Be certain that cleaners used on these PCBAs are compatible with the residues left by the soldering processes. In fact, if halides are hidden or trapped under components, partial cleaning can cause disastrous consequences by releasing these ions. This can lead to dendritic growth that can short out circuits.
Technology Evolution
PCB assemblies are becoming denser, which means that higher-quality cleaners are needed to remove no-clean residues. The distance between components and the board continues to decrease as more leadless chip carriers, microBGAs, chipscale packages (CSPs), 0204s, etc. are in use.
Solder paste and no-clean flux residues are more difficult to clean than older flux formulae that contained resins with aggressive activators. In addition, reflow profiles for lead-free solder pastes are approximately 20°C higher than those for leaded pastes.
Figure 1. Upstream cleanliness parameters relate to the PCB and contaminants such as flux residues, glue or adhesive remnants, and other matter accumulated in the SMT assembly process.
Cleaning products must meet increasing challenges due to both the technology’s evolution and to legislative changes – norm CEE 648, REACH – regarding what chemicals can and cannot appear in cleaner formulations. New processes have been created during the last years to replace old technologies, like the now-eliminated CFCs, ETDs, ESs, HCFCs, etc. Chloride-free solvents and water-based formulations have replaced these outmoded cleaners.
Experiments
The goal of every cleaning process is to make a “dirty” board “clean.” Dirty means different things for different product assemblies. Therefore, a dirty board is defined as one that doesn’t meet the criteria for the next step of the production process. Contrary to a dirty board, a clean board meets these criteria. The retouch ratio is generally an efficient and practical way to define cleanliness criteria for a board. Cleaning consists of removing harmful substances that could cause the board to fail cleanliness tests and make it unacceptable for use. These tests include visual exam, ionic contamination, SIR/electromigration, compatibility with coatings, etc.
Cleaning
To control the transition from a dirty board post-soldering to one that meets a standard of cleanliness, control the parameters encountered at each step of the production process. We can define two distinct families of parameters: upstream and downstream. Upstream parameters mainly refer to the board itself and its contaminants (Figure 1). Let us define the formulation parameters and the application parameters. Formulation is the flux composition, typically about 15 components such as resins, activators, solvents, thixotropic agents, and other additives. Application refers to the chemical nature of the materials – no-clean, lead-free, water-soluble – reflow profile, nature of the substrate – surface treatment, size, geometry, presence of foreign elements, metallic components, etc. – and the waiting time between reflow and cleaning.
Downstream parameters refer to the cleaning process (Figure 2). Like the upstream parameters, we have the formulation parameters of the cleaning fluid and the parameters for their application. The cleaner composition can be an aqueous solution, a solvent, or a mix of the two. Solvents, hydrocarbons, or modified alcohols are used in a water-free cleaning system such as a co-solvent process – a mix of hydrocarbon and hydrofluoroether – or a vacuum process.
Aqueous cleaning materials are formulated using water as the primary ingredient. These compositions contain alkaline agents to promote saponification of residual organic materials, third solvents giving hydrophilic and lypophilic properties that allow a solubilizing action of organic components in a aqueous environment (these solvents are generally alcohol or derived from alcohol), and surfactants to decrease the surface tension and increase the speed at which the pollutants are removed. Some cleaners can be used in both pure and diluted states. The pure state of the product promotes fast solubilization of organic residues not cleanable in water, whereas diluted, the cleaner allows for saponification of residual organic contaminants on the PCB. These aqueous materials applications use a dipping process (with submerged jets) or a spraying process for the cleaning action.
Application parameters for the cleaning process include the number of baths, cleaning cycles, cleaner concentration, bath temperature, contact time, type of mechanical action used, and the saturation level of the cleaning bath.
A successful cleaning process optimizes all parameters. To resolve the most pressing questions in the modern cleaning industry, we study how to test for cleanliness, the best reflow profile (as related to cleanability) for halogen-free no-clean paste, how well a cleaner matches a given solder paste, and how to set cleaning parameters for optimum results.
As there are several factors and importance levels, dividing the parameters into areas helps to eliminate nonessential data while allowing us to focus on the important elements.
Figure 2. Downstream parameters relate to the cleaning step, such as what equipment and cleaners are used, and in what fashion.
How can we determine how well our PCBA can be cleaned? We can change the flux or organic parts of the formulation (downstream parameters) while keeping the process constant. Four solder pastes are used.* The process will remain, for example, an aqueous detergent** at 50% dilution, with 5 minutes in the spray. This is followed by rinse in deionized (DI) water for 1 minute at 20°C and 1 minute at 50°C. Dry time reaches 20 minutes at 70°C with ventilated hot air. Cleaning results vary due to the composition of the flux (organic part), which then can be optimized based on the tests, obtaining a paste that is easier to clean. Ionic contamination testing and visual inspection are used to evaluate the results. With a lead-free co-developed solder paste, Lead-free Paste A, the detergent cleans with good visual results and 0.511 µg/cm2 ionic contamination; another co-developed lead-free paste, B, shows good visual cleanliness and 0.296 µg/cm2 ionic contamination; the third, Paste C, is not cleaned by the detergent and white traces are observed, with 0.743 µg/cm2. The same detergent is tested with a lead-free paste developed without regard of the cleaning formula, Paste D. Results are good visual cleanliness results and 0.509 µg/cm2 ionic contamination.
What is the best peak reflow temperature for a halogen-free no-clean solder paste? To isolate the effect of this parameter, we will vary the peak reflow temperature (a upstream parameter), while holding constant all other variables, such as time between reflow and cleaning, component density, etc. Ionic contamination, visual inspection, and SIR tests post-cleaning are used to determine profile compatibility with cleanliness standards. At 235°C, visual results indicated good cleaning, ionic contamination hit 0.532 µg/cm2 (equivalent to NaCl), and the PCB failed a SIR test. At 240°C reflow, visual results remained good, ionic tests hit 0.215 µg/cm2 (equivalent to NaCl), and the board passed SIR. Raising the reflow temperature to peak at 245°C brought ionic contamination results to 0.657 µg/cm2 (equivalent to NaCl), while visual results remained good and the PCB passed SIR. Clearly, the reflow profile directly influences cleaning quality. By optimizing the cleaning result, one can also evaluate the best reflow profile.
How can one determine the best cleaner to use for a chosen solder paste? The only variable here is the cleaner itself, a downstream parameter. A common test can determine if product A cleans better than product B for a chosen solder paste, with a given reflow profile and with similar cleaning conditions for both products. The set up includes cleaning product concentration at 30%, temperature at 50°C, cleaning time at 10 minutes under submerged jets, with rinsing under DI water for 10 minutes, twice. Drying is performed with ventilated hot air. If product A shows white residue at visual inspection and ionic contamination of 0.211 µg/cm2, and cleaning product B shows 0.223 µg/cm2 results for ionic tests but no white residues, the observation is made that even though the ionic contamination results are similar, one product fails the visual test. This can occur when residue is non-ionic. In this case, other variables must be adjusted to get the optimum cleaning results.
How can we determine the optimum cleaning parameters for a chosen solder paste? The solder paste and its reflow profile must be defined for this test. Only the downstream cleaning parameters (temperature, concentration, and cycle time) of the aqueous solution vary in a given process. The pressure of the jets is fixed, as are the rinsing and drying steps. The only cleaner used is the aqueous detergent. Results are shown in Table 1.
Table 1. Varying cleaning fluid concentration, cleaning time, and temperature of the solution clearly affects results in standard cleanliness tests.
Several conclusions can be reached. We can determine an optimal setting for a chosen solder paste when the reflow profile is known. Each of the parameters has a different level of importance. It is best to vary a parameter where the consequences on the cleaning are weighty rather than one where the consequences are small. Some threshold values for parameters can be reached. These are called limit values. Below these values, varying the parameters does not change the results appreciably.
Conclusion
We can see that these questions’ answers are complex because the parameters themselves are complex. We also note that these questions bring an almost infinite number of possibilities. Modeling a complete design of experiment considering all the factors and degrees of importance seems impossible. However, we showed that parameters upstream and downstream are all-important; the more we control these parameters, the more the results can be optimized.
We can combine experimental plans and answer several questions at the same time. Considering cleaning products and assembly products together, chemically, allows us to enlarge the field of action and work on the different possible areas. This optimizes the results’ quality. Of course, there is no miracle solution that will work for every production run. Each production line is different and other parameters can be identified than those shown here. A combination of knowledge gained from experimentation and chemistry with knowledge from experience yields successful cleaning processes.
Solder pastes included: AVANTEC ECOREL FREE 305- flux1 (Paste A); ECOREL FREE 305-flux2 (B); ECOREL FREE 305-flux 3 (C) and another paste of the same type (D).
Gérard Abidh, R&D manager, may be contacted at AVANTEC SA, 26 avenue du Petit Parc, 94683 Vincennes, France; (864) 992-4540; gabidh@avantec.dehon.com; www.avantec.dehon.com.