Low-temperature Solder Paste Process Advantages
Environmental issues are becoming increasingly important to organizations and individuals alike. The electronic assembly industry, like everyone else, is becoming more conscious about environmentally friendly processes and materials. This movement brings low-temperature processes and materials back into the spotlight because they can help with energy consumption and a reduced carbon footprint for assembly processes.
Tin/bismuth alloys have been used for a while in lead-free assembly processes in electronic assembly, one of the main concerns being the compatibility with the SAC (SnAgCu) spheres from ball grid array (BGA) and chip-scale package (CSP) components. Due to lower process temperatures, new chemistry platforms for solder paste need to be developed to fulfill both the assembly process requirements and the final assembly reliability. Solder joints are the result of the soldering process consisting of the temperature gradient action over the solder paste volume at the interface between pins and pads [1, 2]. Overall, the solder joint performance is described by the intermetallic formation at the interface of pad and solders volume as well as the pin and solder volume [3, 4]. The reduction of carbon footprint and the potential dollar savings add to the appeal of the low-temperature process .
The advantages of a low-temperature assembly process have long been a target for the assembly industry. Many attempts have been made by material suppliers to come up with an appealing chemistry platform that will allow the use of a low-temperature solder paste on an existing SMT line without major equipment changes (if any at all). Of importance, from the assembly process point of view, were the printability of the new platform and voiding performance. The printing performance of the proposed chemistry platform was evaluated using transfer efficiency (TE) for two aperture shapes—circle and square—as process indicators .
For the printing experiment, we used a DEK horizon printer with a four-mil stainless still stencil. We used a Keyence microscope for the visual inspection after printing and a Koh Young 8030 for the paste volume measurement after printing.
Four different printing experiments have been carried out to gather data for the following printing process indicators:
- Finest feature to print
- Print volume repeatability
- Response to pause
- Stencil life
This process indicators will provide a valuable indication of the rheological performances of the solder paste. After the reflow, we evaluated:
- Drop shock
- Thermal cycling
The stencil design used both square and round apertures 100–250 µ starting with an aspect ratio (AR) of 0.25 to force a failure and find the breaking point for the printing process. The finest feature to print was evaluated using the transfer efficiency process indicator. The combined transfer efficiency for the printing process for square and circle apertures is shown in Figure 1.
Figure 1: Combined transfer efficiency results for square and circle shaped apertures.
As pass/fail criteria, we used 50% transfer efficiency for the minimal acceptable value with the condition to have all values above that value, including any outliers that may occur.
Print Volume Repeatability
Print volume repeatability is the next process indicator used to characterize the paste performance. The print volume repeatability examines the printed solder paste volume repeatability for a given aperture at different printing speeds.
In Figures 2–4, we have the results for AR= of 0.6, 0.75, and 1, respectively. We used four printing speeds for this test with the printing parameters adjusted for optimum output for each speed.
Figure 2: Print volume repeatability for AR=0.60.
Figure 3: Print volume repeatability for AR=0.75.
Figure 4: Print volume repeatability for AR=1.
Response to Pause
The response to pause test evaluates the printed solder paste volumes variability for the first four consecutive prints, which provides valuable information about the capacity of a paste to recover after pause (i.e., the print strokes needed for the paste to recover after pause).
The response to pause test was carried out using the following process conditions:
- Out of jar
- After kneading
- One-hour pause without wipe
- One-hour pause with wipe
- Two-hour pause with wipe
Figure 5 shows the data for the 12-mil circles with AR=0.6, Figure 6 presents the data for 15-mil circles with AR=0.75, and Figure 7 presents the data for 20-mil circles with AR=1.
Figure 5: Response to pause data for AR=0.6
Figure 6: Response to pause data for AR=0.75
Figure 7: Response to pause data for AR=1.
Stencil life was evaluated by measuring the printed solder paste volumes for a given aperture. The classification was made based on the solder paste volume variability between the different printing conditions. The time between prints was used as a process variable (one-hour intervals up to 12-hour intervals were used). Figure 8 shows the data for the 12-mil circles with AR=0.6, Figure 9 presents the data for 15-mil circles with AR=0.75, and Figure 10 presents the data for 20-mil circles with AR=1.
Figure 8: Stencil life data for AR=0.6.
Figure 9: Stencil life data for AR=0.75.
Figure 10: Stencil life data for AR=1.
For the reflow step, a seven-zone Vitronics oven was employed. The reflow profile employed for the low-temperature alloys and SAC305 spheres is shown in Figure 11.
Figure 11: Low-temperature reflow profile.
The voiding experiment was designed to consider the main contributors to the void formation and generate comprehensive data that will separate the paste influence in voids generation process from the other contributors. For the mixed alloy voiding performance, the BGA256 component was used with SAC305 spheres. For the joint formed with HRL1 alloy by itself, an LGA256 component was used. We also used the reflow profile from Figure 2 and ran the reflow process in both air and nitrogen atmosphere. Both electroless nickel immersion gold (ENIG) and organic solderability preservative (OSP) pad finishes were used for this study to eliminate their contribution, if any, to the results. Figures 12–14 show the voiding performance results and typical X-ray images for air nitrogen reflow.
Figure 12: Combined voiding performance for OM-550, HRL1 alloy, solder paste (data).
Figure 13: Combine voiding performance for OM-550, HRL1 alloy, solder paste (pictures, air reflow).
Figure 14: Combine voiding performance for OM-550, HRL1 alloy, solder paste (pictures, nitrogen reflow).
The mechanical performance of the joints formed with the HRL1 alloy from the solder paste and the SAC305 from the BGA spheres were evaluated using JEDEC service condition B for drop shock. The drop shock data is presented in Figure 15.
Figure 15: Low-temperature reflow profile.
Thermal Cycling Test
The IPC 9701-A method was used for the thermal cycling test. Table 1 shows the thermal cycling performance of the joint formed in a low-temperature reflow process using a solder paste with HRL1 alloy and BGA components withSAC305 spheres.
Table 1: Thermal cycling performance.
In this body of work, we have summarized the evaluation results of our latest low-temperature chemistry platform that is capable of using powder made of HRL1 and off-eutectic alloys. Based on the data presented, we make the following conclusions:
- The printing performance of the new platform (OM-550) is on par with the performance of the existing SAC chemistry. Concerning rheology, there are few differences from existing SAC chemistries used today in the industry, which allows for the new platform to be dropped into an existing printing process using the normal fine-tuning procedures to optimize the printing process and ensure a smooth transition from a PCB assembly process using SAC alloys to a low-temperature assembly process.
- Based on the printing results, the chemistry proved consistently printed on stencils with an AR equal to or larger than 0.6 (as observed in the testing, even lower ARs were possible during our testing).
- The printing process indicators show consistent performance for the new low-temperature assembly platform that facilitates the new assembly process implementation with minimal impact on the assembly line equipment and makes it possible to achieve the full benefits of a low-temperature process with minimal cost impact.
- The required reflow process can be easily achieved using existing equipment. The reflow-temperature envelope is shaped similarly to the envelope of a SAC reflow process.
- For a reflow envelope, as previously mentioned, we observed that a peak temperature of 190°C or higher is preferred for the formation of a mixed alloy joint with improved mechanical performance.
- The performance of the mixed joint formed with HRL1 alloy is substantially superior to the one formed with the eutectic SnBiAg alloy; this was very close in performance to a joint formed with SAC305 alloy.
- The new material is enabling the use of a low-temperature PCB assembly process that brings the full benefits of low environmental impact through lower energy consumption and the reduction of the carbon footprint for the assembly process with minimal impact on the joint mechanical properties.
OM-550, HRL1 solder paste proved to be a strong option for the applications where a low-temperature process can be employed, including assembly where mixed alloy joints are going to be formed.
1. R. Strauss, SMT Soldering Handbook, Butterworth-Heinemann Linacre House, Oxford, 1998.
2. Traian C. Cucu, Norocel-Dragos Codreanu, & Ioan Plotog, “Reflow process using lead-free materials: Basics and comparison with tin-lead process,” 2006.
3. Klein Wassink, Soldering in Electronics (2nd Edition), 1989.
4. P. Svasta, I. Plotog, T. Cucu, A. Vasile, & A. Marin, “4 P Soldering Model for Solder Joints Quality Assessment,” ISSE 2009, The 32nd International Spring Seminar on Electronics Technology, proceedings from Brno, Czech Republic, May 2009.
5. Lenovo Newsroom, “Lenovo™ Announces Breakthrough, Innovative PC Manufacturing Process,” 2017.
6. Ioan Plotog, Constantin Jianu, Carmen Turcu, Traian C. Cucu, & Norocel D. Codreanu, “Multi-criterial Approach for Implementing of Lead-free Technology,” 4th European Microelectronics and Packaging Symposium, proceedings from Terme Catez, Slovenia, May 2006, pp. 301–306.
This article was originally published in the proceedings of SMTA International 2017.
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