Deposit Consistency
Mechanical reliability depends on the repeatable amount of paste applied to every pad on a circuit board. In the printing process, volume transfer variance measures the difference between the intended volume of the stencil aperture and the actual volume of the paste brick. This variation is usually expressed as a percentage of the theoretical maximum.
High variance leads to inconsistent fillets and increased defect rates during reflow.
Aperture Mechanism
Friction between the paste and the walls of the stencil aperture prevents a perfect release of the material. Factors such as aperture wall smoothness and stencil thickness contribute to volume transfer variance. Fine-pitch components are especially sensitive to these changes because the volume of paste is already very small.
If the transfer is too low, the result is a lean joint that may fail under mechanical stress. If it is too high, the excess paste can cause bridging between adjacent pins. Mechanical properties of the paste and the speed of the separation stroke determine the final volume.
Process Control
Automated paste inspection systems provide real-time data on the volume of every deposit. Tracking the volume transfer variance allows for identification of when the stencil needs cleaning or the paste needs replacement. Maintaining a stable printing process is the most effective way to improve the first-pass yield of an assembly line.
