What Problems Should be Concerned about when Choosing a Solder Paste?
2022-02-10
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Since the official implementation of EU RoHS on July 1, 2006, lead-free solder pastes have been increasingly used in various fields to replace leaded solder pastes. The research of lead-free solder paste has brought technology development, but there are also some process problems in the applications. What are the process problems of lead-free solder paste applications? What problems should be concerned about when choosing a solder paste?
1. What are the process problems of lead-free solder paste application?
1.1 Printability
Taking SAC305 as an example, since the density of SAC305 is 7.4g/mm3, which is lower than that of SnPb alloy (8.4g/mm3), the printability of SAC305 solder paste is worse than that of SnPb solder paste, and it is easy to stick to the squeegee. Ensuring good printability is very important to improve SMT production efficiency and reduce production costs. In the case of the same alloy composition, the printability can only be improved by adjusting the flux composition. Printability includes the aspects of the ability to fill the apertures, wettability, thermal collapse resistance, and humidity resistance.
By adjusting the flux composition and proportion, lead-free solder pastes can have the same high-speed printing operation ability as leaded solder pastes.
1.2 Wettability
The wettability of lead-free solder alloys is worse than that of leaded solder alloys, and the processability and effect of soldering are inferior to those with lead. Therefore, the activity of the flux used in lead-free solder paste is usually stronger than in leaded solder paste, allowing it to better purify the soldered metal surface during the soldering process, prevent solder oxidation, improve wettability, and ensure excellent solder paste quality and soldering process capability.
1.3 Melting point
At present, the alloy solder powder used in lead-free solder paste in the electronics industry is mostly based on SnAgCu alloy. The melting point of this alloy is about 217-219°C, which is 34°C higher than that of the SnPb alloy. Hence, it will lead to adverse effects during the reflow process.
1.4 Self-correction of surface tension
Due to the weakened self-correction ability of the surface array packaged devices (BGA, CSP, FCOB, etc.) during the lead-free reflow process, the alignment accuracy of the solder paste printing and placement processes is improved.
1.5 Repair
The difficulty of repairing has increased, and the success rate of repairing has decreased.
2. What problems should be paid attention to when choosing solder paste?
2.1 Consistency of solder paste batches
For solder pastes, the most important evaluation parameters are the viscosity of the solder paste, the metal content of the solder paste, and the particle size distribution of the alloy powder. It is important to know the actual values of these parameters and the range of specifications provided by the suppliers. Because the actual evaluation of materials will generally only be carried out on one lot per supplier, it is compulsory to determine whether each supplier can guarantee the consistency of quality from batch to batch and whether there are sufficient testing methods to ensure the consistency.
2.2 Reliability
The selected solder pastes used on the PCBs must have long-term reliability. The minimum requirements are that solder paste must meet the SIR standard of IPC/STD-004 and the electromigration requirements of GR-78, especially lead-free solder pastes.
2.3 Technical support and services
Problems will inevitably occur during the solder paste application, so the selected supplier must have sufficient technical support to ensure that solder paste problems can be quickly resolved. In addition, suppliers with outstanding technical support capabilities can also assist users in solving process problems.
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