Bubbles or voids

Fluidization may be described as incipient buoyancy because the particles are still so close as to have essentially no mobility, whereas the usual desire in fluidization is to create bed homogeneity. Such homogeneity can be achieved only by violent mixing. This is brought about by increasing the fluid velocity to the point of blowing "bubbles" or voids into the bed, which mix the bed as they rise. The increased fluid velocity at which bubbles form first is referred to as the incipient (or minimum) bubbling velocity. 

The paste or gel should be free from air bubbles or voids. 

In the absence of all moieties (i. e. gas or particulate matter) it is possible to deduce an equation relating the critical pressure (P ) which must be exceeded to create a bubble (or void) of radius in an ultrapure liquid (Eq. 2.18). Provided vapour pressure is neglected then ... 

This method has the advantage of not requiring a knowledge of the foil thickness t, but it becomes very difficult to count surface intersections for dislocation densities higher than about 10 cm". Clearly, measurements of the number-density of small dislocation loops or small inclusions (such as bubbles or voids) requires a knowledge of thickness t. 

The outside zone is used to store the reserve liquid precursor in a mass reservoir, which surrounds the preform in a fully immersed manner. Outside of the preform the liquid forms a turbulent boiling fluid with many bubbles due to cavitation and the outgoing gases. The cavitation phenomenon is a two-phase process in which bubbles or voids in the heated liquid precursor are formed. 

Particulate and Aggregative Fluidization. When the fluid phase is liquid, the difference in the densities of fluid and solid is not very large, and the particle size is small, the bed is fluidized homogeneously with an apparent uniform bed structure as the fluid velocity exceeds the minimum fluidization velocity. The fluid passes through the interstitial spaces between the fluidizing particles without forming solids-free bubbles or voids. This state of fluidization characterizes particulate fluidization. In particulate fluidization, the bed voidage can be related to the superficial fluid velocity by the Richardson-Zaki equation. The particulate fluidization occurs when the Froude number at the minimum fluidization is less than 0.13 [9],... 

When the solids-free bubbles or voids are present in the bed as in bubbling or turbulent fluidization, the bed of solid particles is fluidized nonhomogeneously. This state of fluidization characterizes aggregative fluidization. The distinct properties of aggregative fluidization are intense mixing of particles, bypassing of fluid through bubbles, and solids entrainment above the bed surface. 

For a plastic to be suitable for RM, it must flow well enough to cover the mold surface evenly. As a powder, it must have a small-enough particle size and a low-enough melt viscosity to flow without bubbles or voids. Because it will be in contact with air on the inside, it must have good thermal stability. PE usually has a melt index of at least 2 to 3, in order to flow adequately. Solid plastics generally must be reduced to a uniform powder (via grinders, etc.) that will go through a 35 mesh screen, which is about 0.020 in. (500 /im) in diameter. 

Hydrolytic stability Stencil-applied coating, shall show no evidence of bubbles or voids (3.4.3) IPC-TM-650, Method 2.3.10 — —... 

The second group of inhomogeneity sources are extraneous to the material itself and are the result of the preparation procedure. These extraneous sources include bubbles or voids, incomplete reaction of starting materials, contamination from the container, and motes (insoluble or slowly soluble particulate material) introduced through the starting material. 

Another important advantage of MR fluids is their relative insensitivity to temperature changes and contamination. This arises from the fact that the magnetic polarization of the particles is not influenced by the presence or movement of ions or electric charges near or on the surface of the particles. Surfactants and additives that affect the electrochemistry of the fluid do not play a role in the magnetic polarizability of the particles. Further, bubbles or voids in the fluid can never cause a catastrophic dielectric breakdown in an MR fluid. 

Some fluxes leave residues that are meant to remain on the board. Some fluxes are difficult to clean or may interfere with in-circnit test (ICT) probing or may hinder conformal coating or nnder-fill adhesion. Post-process flnx deposits can cause corrosion, especially in the electric field between adjacent conductors on the surface of the printed circuit board (PCB). Fluxes may impart gas bubbles, or voids, which may be frozen into the solidified solder (see Fig. 46.2). The resnltant voids may detract from the mechanical strength if they are large enough, are in abundance, intersect bonding snrfaces, or are in close proximity to one another. 

The actual chemical density was determined by using this procedure with increasingly finer particle sizes of a sample. The apparent density was plotted versus radius and extrapolated to zero to obtain a value for density free of the error associated with internal bubbles or voids. 

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