How to Evaluate the Comprehensive Performance of the Selected Solder Paste?
The problem that customers often encounter when choosing solder paste products is the evaluation of the comprehensive performance of the solder paste. According to Fitech’s accumulated experience in solder paste solutions, the following points should be paid attention to when applying solder pastes.
1. The rheology of solder paste：
The chemical reaction between the flux and the solder alloy affects the rheology of the solder paste and is critical to the printability of the solder paste. Once the solder alloy composition is determined, the key to ensuring solder paste printability and solderability is the flux. Using flux correctly can improve solderability and reduce soldering defects. Solder paste with good wettability can avoid tombstoning after soldering, while poor wettability leads to higher electrical resistance and capacitance shift. Therefore, to choose a solder paste should, it is necessary to conduct a process test to see if the printability can meet the actual application and quality requirements, such as analyzing the rolling, filling, and release properties of the solder paste during printing, observing whether the printing quality changes at 1-hour intervals and testing the viscosity changes for 1 to 8 hours. According to the experimental results, the solder paste suited to your product and process requirements can be selected.
2. The stencil life of solder paste:
Any solder paste must print well when first placed on the stencil, and it must have excellent printability after being placed on the stencil for a few hours. When the material is exposed to air, the solder paste viscosity will gradually increase due to the volatilization of the solvent. Based on this, measuring the change of solder paste viscosity with time will obtain the actual stencil life of solder paste. If the stencil life of the solder paste is short, frequent adjustments to the printing parameters are required to ensure printing quality.
3. The "pause-response" performance of solder paste:
In the manufacturing environment, the equipment usually needs to be repaired, and the placement machine needs to be stopped for feeding, so there will be at least 0.5h of downtime. At this time, it is required that the performance of the solder paste should not degrade. Solder paste's ability to accommodate this downtime and guarantee good performance when reprinted after downtime is defined as "pause-to-response" performance.
4. Anti-Shear reduction performance:
Solder paste should be designed to ensure that the solder paste has the characteristics of shear thinning during printing. This feature ensures that the solder paste can better fill the stencil apertures. The solder paste viscosity will restore its original value at the end of the printing stroke and reduce again in the next printing stroke. In continuous printing, the viscosity of some solder pastes does not recover between printing strokes, which means that the viscosity of the solder paste will continue to decrease, eventually leading to collapse and bridging during fine-pitch printing.
5. Adhesion of solder paste
Since the formulations of lead-free and leaded solder pastes are very different, it is necessary to determine whether the lead-free solder paste has good adhesion or the ability to maintain a stable component placement position. Over time, changes in solder paste properties can affect its ability to hold components in place. There are test methods for measuring solder paste adhesion in the IPC/J-STD-005 standard.
The above ideas are the main points that should be paid attention to when choosing solder paste. In the actual solder paste selection process, more factors need to be considered. If you have questions about the solder paste selection, please follow Fitech, and we will be happy to answer your questions.