Snap-off distance

The nonplanar surface created by camber adversely affects subsequent metallization and assembly processes. In particular, screen printing is made more difficult because of the variable snap-off distance. Torsion bar printing... 

In the off-contact printing process, tiiis is arguably the most important parameter in the screen printer setup. If it is too large, the screen will rapidly lose tension and the print will lose definition. If it is too small, the tension on the screen during the print will not be sufficient to transfer the paste to the substrate. Referring to Figure 5.19, the magnitude of snap-off is dependent on the size of the screen, with a ratio of maximum screen dimension to snap-off of 200 1. A list of typical snap-off distances for a sample of standard-size screens is shown in Table 5.3. 

If the screen is not exactly parallel to the substrate, the snap-off distance will change across the print, causing variability in the print definition and thickness. 

Adjust screen tension and snap-off distance to obtain screen release over the entire screen area. 

Stencil printing is very similar to Ihis process except that it is normally conducted with zero snap-off distance, i.e., with the stencil s bottom surface in contact with the tape layer. Also in the case of stencil printing, higher squeegee speeds are used in the via-filling process to provide increased ink shear forces, thereby facilitating the flow of ink into the vias. 

A key factor in the commercialization of surfactant-based mobility control will be the ability to create and control dispersions at distances far from the injection well (TJ ). Capillary snap-off is often considered to be the most important mechanism for dispersion formation, because it is the only mechanism that can form dispersions directly when none are present (39,40). The only alternative to snap-off is either leave-behind, or else injection of a dispersion, followed by adequate rates of thread breakup and division to maintain the injected lamellae. 

As defined by Radke and Ransohoff (Equation 7), the "snap-off" capillary number, C, contains the effective grain radius, R the permeability, K anS the relative permeability of the nonwetting phase, k ( ). In field applications, the values of all of these parameters are set by the reservoir. Also contained in C are the total superficial velocity, U, and the distance between injection and production wells, L. Within narrow limits, L can be changed by... 

Figure 5. Two force-distance plots obtained when the PEO 900 kDa has been physically adsorbed to the silicon nitride surface in the presence of 0.1 N KNO3. A pristine site compression cycle is shown by the curves A. Occasionally, a compression cycle would yield a force-plot similar to the curves B. Here, a series of a esive "snap-offs" upon retraction is most likely due to the ripping of PEO chains off from one of two surfaces after they have bridged the two surfaces. The cantilever spring constant was 0.064 nN/nm and the frequency of oscillation was IHz.

Fig. 4. Measurement of force-distance curves (schematic representation). The sample is approaching the tip (1) at some distance the gradient of the force overcomes the cantilever spring constant and the tip jumps into contact (2) further movement up causes a deflection of the cantilever (3), during the retraction the tip sticks usually much longer (4) and snaps off, when the spring constant overcomes the force gradient (5). Adapted with permission from Ref 21.

Figure 13.10 Measurement of the snap-off separation with an amplitude modulation AFM. [a] Piezo displacement Az vs. time, (b) Voltage pulse, (c) Tip oscillation. Zj represents the average tip-sample distance. The snap-ofF separation is given by Dsnap-off = Z -t- Azsnap-off- The zero position in the y-axis represents the position of the surface. Relative humidity, RH = 55%. Voltage pulse of 9 V and 1 ms. (Data adapted from Ref. 53.]...

However, molecular adhesion is very different. This falls off in a very short distance of separation. As a consequence, these molecular adhesion forces cannot be measured with a meter ruler, but need a nanometer scale. The adhesion force may be high when the molecules are touching, but even a separation of one nanometer causes the force to drop almost to nothing. Thus the surfaces snap apart in a brittle fashion, totally different from the other types of adhesion force. The area under the curve is very small, hi other words, the energy of molecular adhesion may be negligible. Taking this energy for one square metre of joint, we define the work of adhesion ITin Joules per square metre. 

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