A Study of Solder Paste Rheology for The Alternative Assembly and Reflow Technology (AART) Process
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The AART process allows for the simultaneous attachment of through hole and surface mount components on a mixed technology PCB through mass reflow soldering. Stencil printing is employed for the deposition of solder paste at the surface mount pads and the through hole locations. From a process standpoint, stencil printing involves the parallel movement of a squeegee blade, which exerts hydraulic pressure and shear force on the solder paste. The hydraulic pressure forces the solder paste through the stencil apertures whereas the shear forces result in reduction of the paste viscosity [1]. Subsequently, upon stencil release, the paste deposits should exhibit slump resistance, which is the ability of the paste deposit to retain its original printed shape without collapsing. The rheological behavior of the solder paste is of paramount importance for its stencil printing performance as well as its pre-reflow properties. The most important rheological properties of solder paste with respect to stencil printing are its viscosity and thixotropy. The viscosity of the solder paste reduces as the shear rate increases during the stencil printing process. The aforementioned shear thinning phenomenon is beneficial for the AART process as it significantly facilitates the flow of paste at the PTH locations. On the other hand, the thixotropic behavior of the solder paste determines the time-dependant restoration of the solder paste's viscosity to its initial value upon removal of shear stresses. In the AART process, a relatively fast recovery of paste viscosity will prevent sagging of the solder paste at the PTH sites. 159 Mat. Res. Soc. Symp. Proc. Vol. 515 © 1998 Materials Research Society
Considerable research efforts have been devoted to study solder paste rheology and its correlation to fine pitch surface mount printing [2-5]. However, there is a lack of information on the role of solder paste rheology in mixed technology PCB applications that contain both through hole and surface mount components. Flow and oscillation tests were performed with the use of a controlled stress rheometer in order to investigate the theological properties of the solder pastes used in the AART process and to gain an understanding of their stencil printing and pre-reflow performance. EXPERIMENTAL PROCEDURE Four solder pastes (Paste 1-Paste 4) with a metal content of 90% and an average particle size of 29 linm were investigated in this study. Pastes 1 and 2 consisted of a no-clean flux formulation whereas Pastes 3 and 4 were made of a water-soluble flux.
A TA-CSL 2 500
rheometer with a 2-cm parallel plate geometry was used throughout the experimentation. All the theological tests were performed at a controlled shear stress mode at a temperature of 25°C. The flow and oscillation tests were conducted in the stress range of 0-2500 Pa and 1-15 Pa, respectively. Furthermore, a frequency of 1 Hz was chosen for the oscillation tests. The separation distance between the parallel plates of the geometry was fixed at 290 rim. Documented studies by Bao et al
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