The effect of wall-slip formation on the rheological behaviour of lead-free solder pastes
Introduction
Wall-slip occurs due to the displacement of the particles away from solid boundaries, leaving a liquid layer, which has apparent lower viscosity then the bulk viscosity [1], [2], [3], [4]. The formation of wall-slip is usually seen as a source of error in the rheological measurements and various steps have been taken to eliminate or reduce this phenomenon [5], [6], [7], [8]. However, in some cases the wall-slip could play a positive role in aiding the flowability of a material at various stages in processing, which could be related to the performance of the material in the real production environment. The factors that affect the wall-slip is summarised in the review by Barnes [1].
In the electronic manufacturing industry, the stencil printing process is the most popular method to deposit paste material (solder paste and isotropic conductive adhesives) onto the printed circuit board (PCB), as shown in Fig. 1. During the stencil printing process, the squeegee generates hydrodynamic pressures creating a paste roll and injects the paste into the apertures. In stencil printing process, two critical sub processes are the aperture filling and aperture release process [9], [10]. Successful aperture filling and release will greatly depend on the rheology of the paste material [11], [12]. When the paste is sheared by the squeegee, the viscosity of the paste must be low enough to flow into the aperture. During the aperture release process, the viscosity of the paste must be large enough to overcome the adhesion forces of the sidewall of the apertures. The aperture release process could be improved with the formation of a lubrication layer between the aperture and stencil wall. A good print is defined by the volume of the paste deposited on the interfaces, i.e. the deposited volume is equal to the aperture volume. However in a manufacturing environment, the volume of deposited is always less than the aperture volume. This is caused by the adhesion of the paste to the stencil aperture walls. Therefore, engineering a paste, which will easily flow through the aperture, as shown in Fig. 2, is critical in order to achieve consistent printing, which will ultimately reduce defects and increase production yield.
The rheology of solder paste is therefore considered to be one of the most important parameters, which affect the paste printing process. As component package and printed circuit board (PCB) densities continues to increase and the size of solder joints continues to decrease, proper characterisation and understanding of solder paste rheology is essential for achieving consistent solder paste deposition and hence quality and reliability solder joints. As dense suspension, the rheology of solder pastes is dominated both by flux–vehicle system and the particle size distribution of the solder powders. The commonly used solder pastes for stencil printing contain 0.5 solid volume fraction of solder powders. A slight increase in the solid volume fraction would lead to significant increase in viscosity, which results in poor printability.
Different types of geometries are available: such as cone and plate, parallel plates and T-shaped spindle viscometers is used to measure the solder paste viscosity. However, accurate measurement of the viscosity of a solder paste is still challenging due to the presence of wall-slip. The wall-slip occurs at the interface between the paste samples and measurement parts. It is a well known fact that in viscosity measurement, using parallel plate rheometer, the particles near the solid boundaries migrate into the bulk of the sample [13]. This leads to the formation of liquid rich layer at the boundary between the sample and the plate. The liquid rich layer contains lower solid volume fraction of particles and is often treated as pure liquid [14].
In order to obtain the true viscosity of a solder paste, correction has to be applied to the apparent viscosity so as to eliminate the effect of wall-slip. There have been quite a number of studies understating the effect of wall-slip on viscosity measurement of solder pastes [5], [6], [13], [14], [15]. However, there has been very few studies have attempted to utilise wall-slip formation as tool to characterise the flow behaviour of solder pastes. The aim of this study was to investigate the effect of the two lead-free paste formulations on wall-slip formation. The wall-slip model suggested by Yoshimura and Prud’homme [16] is utilised to understand the wall-slip phenomena in solder pastes. In this study we propose the use of slip velocity as parameter for characterising slip formation in the solder pastes.
Section snippets
Wall-slip model by Yoshimura and Prud’homme
The model developed by Yoshimura and Prud’homme [16] for parallel plate geometry is used to determine the true viscosity and wall-slip velocity of a dense suspension. The model is based on comparison between two parallel plate gaps height under same shear stress conditions. They assumed that the shear rate measured at the edge of the plate is the shear rate for the bulk fluid and slip occurred at both of upper and lower plate, as shown in Fig. 3. For a particular gap height, they determined the
Paste materials
Solder paste is a homogenous and stable suspension of solder alloy particles suspended in a flux–vehicle system, as can be seen in Fig. 4a. The flux–vehicle is a combination of solvents, thickeners, binders and fluxing agents [17], as shown in Fig. 4b. Solder pastes consists of three main constituents, namely:
- (a)
solder alloy particles which forms the base for the metallic bond,
- (b)
the flux system which helps to promote the formation of the metallic bond by providing a good wetting condition and for
True viscosity and slip velocity
The viscosity curve for dense suspensions usually consist of three regions; high viscosity Newtonian-like region at low shear stresses, followed by a shear thinning region (indicating structural breakdown) and finally a second Newtonian-like region at large shear stresses [18]. In the shear stress sweep experiment, apparent viscosity for P1 is lower than P2 by one order of magnitude, as shown in Fig. 8a and b. The measured viscosity drops with increasing shear stress for both solder paste
Conclusions
The study presented the investigation on the wall-slip formation in two lead-free solder pastes (samples P1 and P2). The wall-slip model developed by Yoshimura and Prud’homme [16] was used to calculate the true viscosity and slip velocity for lead-free solder pastes.
The viscosity increases with increasing gap heights for both solder paste samples and the true viscosity is higher than the measured apparent for both samples. The different in the true viscosity when compared with two different gap
Acknowledgement
The authors would like to acknowledge financial and materials support from UTAR Research Fund (UTARRF) and Henkel Technologies UK throughout the duration of this study.
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