-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Eutectic & Lead-free Defluxing in One Process
December 31, 1969 |Estimated reading time: 7 minutes
lead-free soldering has brought about several changes. This article determines if manufacturers can use existing chemistry-based cleaning processes to clean eutectic and lead-free alloys in the same equipment.
By Joachim Becht, Ph.D.; Umut Tosun; Harald Wack, Ph.D.; and Helmut Schweigart, Ph.D., Zestron; Dirk Ellis, Speedline Technologies; and Sia Afshari, RMD Instruments
he examination of the cleaning process for lead-free has raised a critical question: Can an existing chemistry-based cleaning process be used to clean both eutectic and lead-free alloys in the same equipment? A study was conducted to address this question, providing users with valid technical data to better assess associated risks when using one cleaning agent for both eutectic and lead-free products.
The presence of metal ions in a cleaning agent creates the possibility that a chemical reaction will occur. Most current defluxing processes rely on an alkaline pH level to facilitate the removal of contamination. A low or high pH level, respectively, can affect the redox potential between different metals. A redox reaction is one that includes a reduction of one metal, and the oxidation of its reaction partner.
Lead-based Ionic Residues
Ionic-contamination measurements quantify the sodium-chloride equivalent, and commonly are used to assess aspects of climatic reliability. Geometries and surface areas of populated assemblies are not taken into consideration. Accuracy and reproducibility fluctuate by ±2% for assembly sizes between 10-12". Results were analyzed in accordance with J-STD 001-C.
During the study, varying solubility phenomena with lead salts were observed. Some lead salts, i.e. Pb(NO3), are readily soluble in water. A random collection of customer-based bath samples showed lead levels below 10 mg/l (40 mg/gal.). The initial assumption of limited solubility was confirmed through laboratory tests with virgin cleaning agents at various concentrations and temperatures. In all cases, a white precipitate was formed with increased lead salt levels. This precipitate was analyzed and found to be a “temporary complex” between lead and constituents of cleaning agents. This illustrates that the solubility of lead and tin can depend on the cleaning agent.
Figure 1a. Lead and tin content per total cleaned board.
Samples were collected to fully understand risks and shortcomings of a mixed cleaning process. This provided valuable information on the levels of contamination, such as heavy-metal loading (i.e. lead and tin). All samples were taken from companies with altering cleaning agents; specific attention was given to ensure a variety of processed samples. Additional internal studies of numerous cleaning agents also showed that lead-free solder pastes have a propensity to solubilize tin. Tin salts were confirmed to be less soluble.1
A similar observation was made when examining samples for tin levels. Maximum levels of lead were found to be lower than 10 mg/l (40 mg/gal.). However, it was determined that under worst-case scenarios, the level of lead content in the cleaning agent would not surpass 50 mg/l. This was demonstrated by suspending 10 g of unsoldered paste (mix of two eutectic and two lead-free formulations) in 40 gallons of cleaning agent for four weeks, under simulated process conditions. These results were added as sample boards 12 and 13 (Figures 1 a/b). Despite the increased solubility of unsoldered lead and flux, values remained relatively low.
Figure 1b. Lead and tin content per total cleaned board.
Samples collected were from low to high throughput manufacturing plants. Despite large discrepancies in volume, the extent of lead solubilization has no linear relationship with respect to throughput. This is attributed to the constant precipitation and replenishment of fresh medium in high-volume applications. Apart from other heavy metals, the presence of both redox pairs (tin and lead salts) in a mixed process were confirmed. Quantities observed were used as benchmark values for future experiments. The number of boards used mirrored the surface area obtained from bath samples.2
X-ray Analysis
Suitable methods were needed to detect potential cross-contamination of lead, either through redox or ionic contamination. For this, X-ray fluorescence (XRF) technology was chosen. First, test samples were used to establish benchmarks. Both eutectic and lead-free samples were chosen, and differences were observed. The measurement of the lead-free board did not indicate any presence of lead within the system’s limit of detection (LOD). An experimental error determination also was performed. One hundred measurements were taken to achieve a statistically meaningful distribution. Experimental values of 0.0 and 0.3 mg/cm2 were then considered below 0.1% RoHS limits.
The measurement of the eutectic PCB, on the other hand, resulted in lead readings between 3.0-5.0 mg/cm2. The lead content was also measurable at values greater than 3.0 mg/cm2 from the back side of the board. The weight percentage equivalent of 3.0-5.0 mg/cm2, based on a general calibration, is about 3,000-5,000 parts per million (ppm). These analyses indicate that this method can effectively determine possible lead contamination. Additional samples were analyzed using XRF; data collected from these measurements show each sample’s composition and presence of lead in the second PCB (Figure 2). Results of this experiment indicate that such an instrument can effectively measure lead in PCB assemblies as an existing system with a limit of quantification exhibited as 50× higher than LOD for the two PCBs.
Experimental Lab Analysis
Using this analytical, XRF-based technique, the first experiments with identical test boards were conducted in a laboratory. Therefore, different product solutions were prepared at different temperatures, with a varying amount of lead.
Experiment 1: Cleaning agent A, 15% concentration, 50 mg/l lead salt, five minutes exposure time, room temperature (Table 1). Observations:
- Clear solution,
- Experimental error within 0.1 mg/cm2,
- No detectable lead contamination through redox reaction.
This initial experiment shows that lead is neither incorporated through a redox reaction, nor through ionic contamination. All measured values lie within the experimental error.
Experiment 2: Cleaning agent A, 15% concentration, 100 mg/l lead salt, five minutes exposure time, room temperature (Table 2). Observations:
- White precipitation of lead salts,
- Experimental error within 0.1 mg/cm2,
- No detectable lead contamination through redox reaction.
Interestingly, an increase in lead salts did not alter the magnitude of detectable lead. This provides an additional indication of low ionic contamination and reduced lead, mainly due to precipitation. It also was recognized that a second rinse step did not alter readings.
Experiment 3: Cleaning agent A, 15% concentration, 50 mg/l of lead salt, five minutes exposure time, 120°F (Table 3). Observations:
- Clear solution,
- Second rinse removes the remaining lead salts,
- No detectable lead contamination through redox reaction.
Based on initial experiments, an increase in temperature also shows no increase in lead incorporation, through either pathway.
Experiment 4: Cleaning agent A, 15% concentration, 50 mg/l lead salt, five minutes exposure time, 150°F (Table 4). Observations:
- Slight cloudiness,
- No detectable lead contamination through redox reaction.
This worst-case scenario (150°F) was included to further demonstrate that no detectable cross-contamination or reduction of lead was noticeable.
With these results, final trials were performed at an equipment manufacturer to simulate mechanical energies. Five hundred eutectic boards were pre-run to establish base values for lead and tin in the solution. Tin (32.1 mg/l) and lead (6.3 mg/l) were present, and maximum levels of these two redox pairs were confirmed. To allow for an easy correlation, the same cleaning agent was chosen when compared to previous samples.
Potential redox reaction had to be conducted first. Here, several different process parameters were chosen to illustrate all variables. Experiments at different concentrations had shown that concentration had little effect on the amount of lead or tin solubilized. A working concentration of 15% was chosen; however, it was determined that temperature and exposure time could influence the extent of lead reduction (Table 5).
Figure 2. Side-by-side comparison of lead-based and lead-free assemblies.
Data indicate no significant increase in lead. All values obtained are within observed readings, compared to reference values before cleaning. This suggests that not only is the amount of dissolved lead and tin relatively small, but no noticeable (or traceable) lead reduction (chemical reaction) occurs. This applies to worst-case scenarios with high operating temperatures and extended exposure times.
Conclusion
It was determined that a mixed process is feasible, provided users adhere to process variables. No lead contamination exceeding 0.1% was found. Among cleaning agents tested, lead levels were less than 10 mg/l (40 mg/gal.). No measurable reduction (chemical reaction) of lead was detectable. It also was observed that optimal rinsing can reach RoHS-compliance levels by eliminating ionic (lead-containing) contamination.
REFERENCES
1., 2. Contact the authors for specific experimental data. Future studies on pH-neutral cleaning agents for stencil-cleaning are under investigation.
Joachim Becht, Ph.D., head R&D, Zestron America: j.becht@zestron.com. Umut Tosun, application technology manager, Zestron America: u.tosun@zestronusa.com. Harald Wack, Ph.D., VP and executive CEO, Zestron America: h.wack@zestronusa.com. Helmut Schweigart, Ph.D., head application technology, Zestron Europe: h.schweigart@zestron.com. Dirk Ellis, product manager, Speedline Technologies: dellis@speedlinetech.com. Sia Afshari, product manager, RMD Instruments: safshari@rmdinc.com.