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Tin/lead vs. Lead-free: A Question of Print Accuracy
December 31, 1969 |Estimated reading time: 8 minutes
The entire supply industry is feeling the impact of WEEE and RoHS regulations. While much progress has been made in preparation for this transition, one question lingers: Does the lead-free solder paste printing process need to be more accurate than the tin/lead process? This article aims to answer that question.
By Srinivasa Aravamudhan, Joe Belmonte, and Gerald Pham-Van-Diep, Ph.D.
One issue with lead-free solder paste is that it does not wet as well as tin/lead solder paste during reflow. Lead-free solder paste will not cover the PCB pad completely if a reduced aperture size is used in the stencil, or if the print is offset from the pad. There are several lead-free solder pastes from various suppliers, so there is a difference of how well a particular lead-free solder paste will perform with regard to wetting. The use of nitrogen atmosphere during reflow also affects how well a particular lead-free solder paste will wet to a PCB pad.
In this study, solder paste deposits were printed offset from circuit board pads intentionally and evaluated on how well they wet back and covered pads during reflow. Tin/lead and lead-free solder pastes from major solder paste suppliers were used.
Test Design Vehicle
The test vehicle used for this study is a 10" × 8" × 0.062", four-layer, FR-4 board with ENIG surface finish. The test vehicle has different types of components and packages that are widely used. In this experiment, the following components were selected:
- QFP208
- R0402 (horizontal and vertical)
- BGA256
- BGA36 (μBGA)
QFP208, R0402, and BGA256 components were selected because they are commonly used in many electronics products. BGA36 was selected to determine the print accuracy needed for fine-pitch components. The test vehicle has a circuit replication and different orientations for all the types of chip components. Certain components were offset intentionally in the stencil design to offset solder paste deposits.
Experimental Design
A designed experiment approach was used to evaluate the performance of tin/lead and lead-free alloys. Pastes (lead-free and tin/lead) from five manufacturers were selected. Tin/lead paste was used as a control. Table 1 shows the number of opportunities for defects per offset condition per component per paste.
Various levels in the paste offset (distance and direction) were incorporated in the stencil design, thereby reducing the number of samples required for the experimentation. Three boards per paste were printed and reflowed. The number of opportunities for defects, combined with board replication, provide significant statistical confidence in the data. The impact of reflow atmosphere on lead-free alloys was evaluated using an offset in QFP components.
Stencil Design
A 5-mil, laser-cut stencil with electropolish was used for the printing process. For the components, stencil aperture openings were offset progressively. Offset distances and offset progression for each component were:
- BGA36 - distance varied from 0-15 mil, in increments of 3 mil;
- BGA256 - distance varied from 0-21 mil, in increments of 3 mil;
- R0402 (H and V) - distance varied from 0-14 mil, in increments of 2 mil;
- QFP208 (X offset) - distance varied from 0-10 mil, in increments of 1 mil;
- QFP208 (Y offset) - distance varied from 0-14 mil, in increments of 2 mil.
Assembly Process
The assembly process for this study involves solder paste printing, reflow, and visual inspection.
Stencil Printing Process. A stencil printer* was used for the printing operation. Printing parameters were identified for each paste using a Design of Experiment (DOE) to achieve an optimal and consistent printing process performance. After each print, the board was inspected visually for print defects.
Reflow Process. A 10-zone reflow oven** was used to reflow assembled boards. The reflow oven was programmed to produce the required thermal profile specified by the paste manufacturer. For tin/lead paste, peak temperature was maintained at 212°C with a time above liquidus (TAL) of 65 sec. For lead-free pastes, peak temperature was maintained at 240°C with a TAL of 55 sec. For boards reflowed in nitrogen atmosphere, oxygen levels inside the oven were maintained at 200 ppm.
Post-reflow Inspection. An optical microscope was used for post-reflow inspection. Full inspection was carried out to determine soldering defects. Results from the visual inspection were used to evaluate and compare the performance of lead-free to tin/lead alloys for various components.
Figure 1. Acceptable solder joint for tin/lead solder alloys.
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Figure 2. Acceptable solder joint for lead-free solder alloys.
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Post-reflow Evaluation Criteria
Post-reflowed assemblies were inspected for soldering defects using the optical microscope inspection system. Lack of full-pad coverage (i.e., exposed pad area) after reflow was considered a defect. Figures 1 and 2 show acceptable solder joints for tin/lead and lead-free solder alloys. Figures 3 and 4 show defective solder joints for tin/lead and lead-free solder alloys.
Figure 3. Defective solder joint for tin/lead solder alloys.
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Figure 4. Defective solder joint for lead-free solder alloys.
BGA36 - Post-reflow evaluations show that defect percentages increase rapidly for offsets larger than 12 mils for tin/lead and lead-free pastes. The comparison of lead-free alloys show that Paste D appears sensitive to print offsets (i.e., produces defects from 6-mil-offset distances).
BGA256 - The defect percentage results show that defect percentages increase for offsets larger than 15 mils for tin/lead and lead-free pastes. The comparison of lead-free alloys shows that Paste D produces defects from 6-mil-offset distances. It also is noted that tin/lead alloys produce higher defect percentages compared to lead-free alloys. Figures 5 and 6 show reflow solder bumps for the best-performing (Paste A) and worst-performing (Paste D) pastes.
Figure 5. Reflowed image for best-performing paste (Paste A).
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Figure 6. Reflowed image for worst-performing paste (Paste D).
R0402H - Results for R0402 components in the horizontal direction show that lead-free pastes produced defects from a 3-mil-offset distance. The alloy comparison shows that lead-free pastes produced a higher number of defects compared to tin/lead counterparts. Figure 7 shows the defect percentage for all alloys with various offset distances for R0402 components in the horizontal direction. The comparison of lead-free alloys show that Paste B produces more defects compared to other alloys, and defects start from the 0-mil-offset distance. The figure also shows that tin/lead alloys produce the least number of defects (2% average defect rate) compared to lead-free alloys (20% average defect rate). These results indicate that lead-free alloys are more sensitive to print offsets compared to tin/lead alloys.
Figure 7. Defect Percentage for R0402H with various offset distances.
R0402V - Results for R0402H components in the horizontal position show that lead-free pastes produce more defects compared to tin/lead pastes. Lead-free pastes start to produce defects from 0-mil-offset distance. The comparison of lead-free alloys shows that Paste B produces more defects compared to other alloys, and the defect rate starts to increase from 0-mil-offset distance. Tin/lead alloys produce a defect percentage of 5% (average defect rate) compared to a 20% (average) defect rate of lead-free alloys. These results stress that lead-free alloys are very sensitive to print offsets and more accurate printing is needed to reduce the number of misprint defects.
QFP208 - The offset in the stencil opening was designed in four ways:
- Horizontal pads offset in the X direction;
- Horizontal pads offset in Y direction;
- Vertical pads offset in X direction;
- Vertical pads offset in Y direction.
To evaluate paste performance for QFP apertures, pastes from two manufacturers (Pastes A and D) were used. Paste D was also reflowed in nitrogen atmosphere to evaluate the effect of an inert atmosphere during the reflow process.
QFP208 horizontal pads offset in the X direction: In this study, the lead-free pastes produce more defects compared to tin/lead pastes. Results show tin/lead pastes did not produce any defects for all offset distances. The comparison of lead-free pastes when reflowed in air show that Paste A produces less defects than Paste D. Evaluation of inert atmosphere for the reflow process shows that nitrogen has a significant impact on defect reduction. Paste D when reflowed in nitrogen atmosphere produced no defects compared to Paste D reflowed in air atmosphere (20% defect).
QFP208 horizontal pads offset in Y direction: Results show lead-free pastes produce more defects compared to tin/lead pastes. The comparison of lead-free pastes show that Paste A produced less defects compared to Paste D for all offset distances. For reflow atmosphere evaluation, Paste D reflowed in nitrogen atmosphere produce less defects compared to the same paste reflowed in air. This indicates that Paste D needs a nitrogen atmosphere for complete reflow.
QFP208 vertical pads offset in X direction: Results show that the defect rate increases for tin/lead pastes from a 5-mil-offset distance. The nitrogen reflow atmosphere also increased wetting performance of Paste D and reduced the defect rate.
QFP208 vertical pads offset in Y direction: Results show that lead-free pastes produce more defects than tin/lead pastes. The comparison of lead-free pastes shows that Paste A produces fewer defects compared to Paste D for all offset distances. Results also show that tin/lead pastes did not produce any defects for all offset distances. Nitrogen reflow atmosphere reduced the defect rate for Paste D by increasing wetting characteristics under an inert atmosphere.
Conclusion
Lead-free solder pastes appear more sensitive to print offsets than lead-bearing solder pastes. This suggests that lead-free pastes are less forgiving to misprints than tin/lead.
It was observed that when tin/lead pastes were printed onto a non-wettable surface such as a solder mask, the reflowed paste did not migrate back to the pads. However, when a small percentage of tin/lead paste was in contact with a wettable surface (i.e., pads), reflowed paste pulled back and wetted the entire wettable surface. This can be attributed to higher defect rates for BGA components. For lead-free pastes, this was observed in the reverse direction.
The sensitivity to print offsets with lead-free alloys depends on the type of device being printed. While BGAs show little sensitivity, QFPs produced the most defects and would demand additional print accuracy. Evaluation of the reflow atmosphere shows that certain lead-free solder pastes require a nitrogen atmosphere to improve reflow and wetting characteristics.
*MPM UP3000 stencil printer, Speedline Technologies.** Electrovert OmniFlow 10-zone reflow oven, Speedline Technologies.
For a complete list of tables and figures, please contact the authors.
Srinivasa Aravamudhan, development engineer, Advanced Process Group, Speedline Technologies, may be contacted at (508) 520-0083; e-mail: saravamudhan@speedlinetech.com. Joe Belmonte, product manager, advanced process group, Speedline Technologies, may be contacted at (508) 541-4772; e-mail: jbelmonte@speedlinetech.com. Gerald Pham-Van-Diep, Ph.D., director of advanced development, Speedline Technologies, may be contacted at (508) 520-0083; e-mail: gpham-van-diep@speedlinetech.com.