In surface assembly technology (SMT) production, the viscosity characteristics of the solder paste directly determine the window and soldering quality of the printing process. Excessive viscosity will lead to missing marks and incomplete graphics on the stencil; If the viscosity is too low, it is easy to cause defects such as bridge connection and collapse. Since solder paste is a typical thixotropic fluid, its apparent viscosity changes nonlinearly with changes in shear rate, temperature and time, so accurate viscosity testing is of key significance for incoming material inspection, process control and formulation development. Among a variety of test methods, rotary viscometers have become the most widely used standard configuration in the industry due to their ability to simulate the printing shear process and quantify thixotropic characteristics.
Test principles and core technologies
The application of rotational viscometers in solder paste testing is based on the classical model of rotational measurement mechanics. Its core principle is that the rotor is driven by a motor to rotate at a specific speed in the solder paste, the viscous torque on the rotor surface is measured, and converted into a viscosity value according to a hydrodynamic formula.
For common coaxial cylinder structures, the viscosity calculation formula can be expressed as:
η = K · (M / ω)
where η represents dynamic viscosity in pascal seconds (Pa·s); M is the measured torque; ω is the angular velocity of the rotor; K is the instrument constant, which depends on the geometry of the rotor and measuring vessel. By changing the rotation speed, the viscosity change curve at different shear rates can be obtained, and then the thixotropic performance of the solder paste can be evaluated.
Standard methods and key parameters
To ensure the accuracy and reproducibility of test results, the testing process must strictly follow the relevant standards and specifications. The following are the key steps and parameter requirements based on industry practices and standard requirements:
| Key dimensions | Specific requirements and instructions |
|---|---|
| Sample preparation | Let stand at 25°C±1°C for at least 24 hours before testing; Use a chrome-plated metal rod to gently stir in the "8" path for 1-2 minutes until even, avoiding air bubbles during the process. |
| Test environment control | The test vessel should be in a constant temperature environment of 23°C±0.5°C (or 25°C±0.5°C); The sample should be left in this environment for no less than 2 hours before testing. |
| Rotor and speed selection | Choose a T-type or spiral rotor to ensure that the torque percentage is between 10%-100%; The rotation speed is usually set to 5r/min or 10r/min to simulate printing shearing. |
| Thixotropic index calculation | Thixotropic coefficient Ti = η (low speed) / η (high speed), such as calculating the viscosity ratio of 3 rpm to 30 rpm; The general qualification range is between 1.5-3.0. |
| Data validity determination | When using multi-period testing, if the average value of the last two periods (Y2) exceeds 10% of the average value of the first two periods (Y1), the result is invalid and needs to be retested. |
Analysis of factors affecting test results
In practice, the test results of rotary viscometers are susceptible to interference by a variety of factors. Firstly, the inhomogeneity of the solder paste itself, such as the settling of solder powder particles or the separation of flux, can lead to local viscosity differences. Secondly, tight temperature control is essential, and the viscosity of the paste decreases significantly with increasing temperature, so deviations from the standard temperature will directly lead to measurement deviations. In addition, the depth of immersion of the rotor, the diameter of the test vessel, and whether air bubbles are introduced during the mixing process can affect the accurate measurement of torque. It is worth noting that according to the national metrology verification procedure JJG 1002-2005 "Rotational viscometer verification procedure", the equipment is regularly calibrated, and the uncertainty assessment of the measurement results is the technical basis to ensure the reliability of the data.
Application Practice
In actual production, rotary viscometers are not only used for the determination of single-point viscosity values, but also widely used in simulated printing processes. By setting the multi-gear rotation speed, the viscosity changes of the solder paste at different shear stages and the recovery ability after shear can be monitored. For example, viscosity at low rpm (e.g., 3rpm) reflects the ability of solder paste to hold at rest or in the awaiting state; The viscosity at high speed (e.g. 30rpm) simulates the filling behavior of the scraper when passing through the stencil at high speed. The viscosity value of solder paste generally used for fine pitch printing usually fluctuates in the range of 160-190 Pa·s at 10 rpm, but this needs to be determined in combination with the specific formulation and process requirements.
6. Conclusion
Rotational viscometers provide key physical parameters for SMT processes by quantifying the flow characteristics of solder paste under shear stress. From sample preparation and environmental control to data interpretation, every step of normalization is a prerequisite for effective results. Understanding the rheological behavior of solder pastes and using a rotational viscometer can help process personnel optimize printing parameters to ensure reliability and consistency in electronic assembly.
References
1. Zhang Beibei. Uncertainty Evaluation of Measurement Results of Rotating Solder Paste Viscometer[J]. Reliability and Environmental Test of Electronic Products, 2013(S1): 227-229.
2. Solder paste viscosity determination method. China Powder Network, Technical Articles.
3. Solder paste viscosity test. Technical exchange, according to IPC-650.2.4.44 and GB/T 2954.