Wetting force/time

Second, droplets impact and coalesce on the powder bed surface if mixing or wet-in time is slow. Third, droplets spread and penetrate into the moving powder bed to form loose nuclei, again coalescing, if wet-in is slow. In the case of high-shear processes, shear forces break down overwet clumps, also producing nuclei. 

The maximum wetting force produced The higher the force the better—provides information on the basis metal solderabiUty, the surface preparation, or lack thereof. Low forces recorded on a test, even if the wetting time is fast, is of major concern. 

The impact of stressing (steam exposure or dry bakes or temperature/humidity) on the robustness of the plating can easily be determined as a function of reduced wetting forces or increased wetting times. 

The ratio of polymer viscosity values, imder which a substantial drop in local interfacial CB concentration is observed, differs for various polymer pairs. The greater is the difference in wetting forces of polymers, the more times the viscosity of the second (having higher wetting force) polymer, can exceed the viscosity of the preliminarily filled polymer. So, this ratio is about four for the PE + PMMA blend, and about two for the PS+PE blend. The best conditions for particles to localize at the interface are provided when viscosity of the second polymer is slightly lower than that of a preliminarily filled polymer component. 

The general concept of what happens during an advancing scan is that each time a portion of the liquid front comes out of one of these cavity bands, the wetting force will abruptly Increase. The size of this increase will depend on the steepness of the cavity and also on the fraction of the total three-phase interface that is emerging from the cavity. Of course, if two separate cavities are encountered at the same time, they would appear as one. The slower the scan rate, the less likely this is to happen, which explains why more jumps can be detected at slower speeds. 

The wetting balance is an instrument that measures the wetting forces that a liquid exerts on a substrate. The substrate, often in the form of a thin plate or wire, is initially positioned over a bath of the liquid. The bath is then raised at a specified constant speed whereby the substrate enters the liquid edgewise. After a specified immersion depth of the substrate s leading edge in the liquid bath is achieved, the bath advance is halted. A dwell time at this immersion depth is preset during which time both the bath and the sample are held stationary. Following the dwell period, the specimen is withdrawn from the bath at a specified speed. Forces may be measured for the entire period that the substrate is in contact with the liquid. 

In general, a complete understanding of the wetting balance force-time curve for solders awaits a rigorous theoretical analysis capable of resolving aU the dynamic aspects of the solder flow noted above. 

FIG. 41 Wetting balance results for eutectic Sn-Pb and two lead-free alloys. The test consists of monitoring the time required to change the force exhibited on a coupon from compressive (solder is pushing on the coupon) to a tensile force (solder is pulling on the coupon). This change in force direction (or vector) is called the zero force time. (From Ref. 26.)... 

Wettability A wetting balance test assesses the force resulting when a copper wire is immersed in and wetted by a molten solder bath. A large force indicates good wetting, as does a short time to attain a wetting force of zero and a short time to attain a value of two-thirds of the maximum wetting force. Fmax > 300 pN, to < 0.6 sec, t2/3[Pg.669]

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