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Why the solder paste should be tested for viscosity <br> The viscosity of the solder paste is the main characteristic index of the solder paste property, and it is also an important factor affecting the printing performance. The viscosity is too large, the solder paste is not easy to wear through the leak hole of the template, and the printed graphics are incomplete; When the viscosity is too low, the solder paste is easily peeled off during printing. Although the solder paste on the market has been identified, the quality of the solder paste will deteriorate with transportation and storage for a long time (the excellent storage temperature of the solder paste is 2 ° C ~ 10 ° C, the storage period is six months).
There are several reasons for determining solder paste viscosity:
1 tin powder content, the higher the content, the higher the viscosity and vice versa;
2 flux content, the higher the flux content, the lower the viscosity, and vice versa;
3 alloy powder particle size, the larger the particle, the lower the viscosity, and vice versa;
4 Store the temperature of the solder paste. The higher the temperature, the lower the viscosity. Product quality will vary depending on the manufacturer. Therefore, discriminating and determining the viscosity of solder paste is also an important workflow for SMT manufacturers to test the viscosity of solder paste.
method 1
Instrument composition: BROOKFIELD viscometer DV2T, HELIPATH STAND, TC-550MX
Test procedure: set 5r/min, select the appropriate T-type rotor (guaranteed the torque percentage range of 10~100%), adjust the range of the lifting bracket, so that the depth of the T-shaped rotor can be immersed in the sample is 0.3cm~ Up and down movement between the 2.8cm area (minimum distance between the rotor and the bottom of the test container is not less than 1cm); set the test time of 5 cycles (the lift bracket moves up and down once for one cycle); start the test and continuously record the test process The viscosity changes. The temperature of the sample controlled by the Boleri TC-550MX during the test was maintained at 25 °C ± 0.25 °C.
Evaluation of results: The maximum and minimum viscosity values ​​measured in the first two cycles were recorded as N1 and N2, respectively; the maximum and minimum viscosity values ​​measured in the last two cycles were recorded as N3 and N4, respectively. The viscosity values ​​Y1 and Y2 of the solder paste samples were calculated according to the formula Y1=(N1+N2)/2 and Y2=(N3+N4)/2.
If Y1≤(1+10%)Y2, the final viscosity of the sample is recorded as Y2; if Y1>(1+10%)Y2, the result is regarded as invalid. After the sample is allowed to stand still, the test is repeated and the above method is performed. The result is judged.
Method 2
Instrument composition: BROOKFIELD viscometer DV2T, screw adapter, TC-550MX
Test procedure: Install the screw adapter on the DV2T, so that the spiral rotor is immersed in the solder paste sample (be careful to prevent the sample from covering the spiral pump outlet), set the speed to 10r/min; start the test, when the reading is stable for more than 1min, no change, Record data. The temperature of the sample controlled by the Boleri TC-550MX during the test was maintained at 25 °C ± 0.25 °C.
Evaluation of results: The reading when the stability is recorded is the viscosity value of the solder paste sample.