Investigating the thixotropic behaviour of Type 4 solder paste during stencil printing

Abstract

Purpose A measurement method has been developed to reveal the viscosity change of solder pastes during stencil printing. This paper aimed to investigate thixotropic behaviour, the viscosity change of a lead-free solder paste (Type 4). Design/methodology/approach The viscosity change of the solder paste during stencil printing cycles was characterised in such a way that the time-gap between the printing cycles was modelled with a rest period between every rheological measurement. This period was set as 15, 30 and 60 s during the research. The Cross model was fitted to the measurement results, and the η0 parameter was used to characterise the viscosity change. The number of printing cycles necessary for reaching a stationary state in viscosity was determined for various rest periods. Findings It was found that the decrease in zero-shear viscosity is significant (25 per cent) in the first cycles, and it starts to become stationary at the sixth-seventh cycles. This means a printing process can provide the appropriate deposits only after the 7th cycle with the investigated Type 4 solder paste. Originality/value Time-dependent rheological behaviour of solder pastes was studied in the literature, but only the viscosity change over continuous time at constant shear rates was examined. The time-gap between stencil printing cycles was not considered, and thixotropic behaviour of solder pastes was also neglected. Therefore, the authors developed a measurement set which is able to model the effect of time-gap between printing cycles on the viscosity change of solder pastes.

Residual free solder process for fluxless solder pastes

“…After six initial printings on a test substrate, the printing was done on the PCB. As described by Krammer (Krammer et al, 2017), after this number of initial printing cycles and a rest period between the printings of <30 s, the viscosity of the solder paste no longer changes and the printing process can provide proper paste deposits.…”

Rheologically Assisted Design of Conductive Adhesives for Stencil Printing on PCB

“…Finally, the stencil separates from the substrate, thus transferring the injected paste as printed deposits on the substrate. Relationships between the rheology of pastes and the quality of the printing have been established [5][6][7], and a printability map for solder pastes was proposed [8]. The rheological modelling of the process was also earlier performed [9][10][11] and improved by taking on board the vast literature on the rheology of solder pastes (see, e.g., [12] and references therein).…”

“…The rheological characterization of solder paste usually encompasses step shear rate tests (including jumps and intermittent shear) [6,8], creep and recovery [6,14], ramps in steady shear [4,5,7,8,13,14] with thixotropic hysteresis loops [8,13,14], and small [4,5,13,14] and large amplitude oscillatory shear [4,5,13]. More recently, extensional testing [15] was reported, as extensional deformation is considered important during stencil lifting, though such rheometry is very demanding for pastes.…”

Electrochemical Migration of ENIG surface finish in Na2SO4 Environment

“…In order to optimize the production processes of printed circuit board (PCB) assemblies, it can be found simulation studies [1,2]. Even more reliability issues are published related to solder alloys or surface finishes used in PCB assemblies [3][4][5][6][7][8][9]. This reliability study focuses on a humidity induced failure mechanism called ECM.…”