Surface finish Tacking

In terms of printability, tack, slump, and solder balling, there is no clear and consistent difference between the Sn-Pb and lead-free solder pastes (Ref 59-60), because these performances depend on the solder paste formulation, not directly on the solder alloy. Very clear and consistent differences have been observed, however, in wettability between the tin-lead and lead-free solder pastes. In general, the wettability of lead-free solder paste is not as good as the tin-lead solder paste. For example, lead-free solder paste exhibits very limited spreading on OSP during reflow, and exposed corners after reflow are quite common, unless overprint or round corner pads are used. The difference in wettability between OSP and ENIG surface finishes, which is already evident for the tin-lead solder, becomes even more pronounced for lead-free solders. This has been observed with a variety of solder paste and flux formulations from a number of vendors. 

A probe tack test may be regarded as a mechanical thumb in which a disc or hemisphere of standard material (e.g. brass) is brought into contact with the adhesive surface under a fixed load for a specified dwell time (e.g. 1 s) and then removed at a specified rate. The maximum force of removal is usually taken as the tack value, although sometimes the work or energy of separation is reported. In such a test, it is important to control accurately the variables probe material and finish, probe diameter and shape, load on probe, thickness of adhesive, dwell time, rate of debonding of probe from adhesive, and... 

Similar to gelcoats except that they cure tack free and are spray applied like paint, to seal and hide a substrate. They provide a hard, tough, durable and flat finish. However they cannot be used as gelcoats and are typically interior finishes only which - at least for the standard enamel - are not suitable for water immersion. Full surface preparation and application recommendations should be obtained from the manufacturer. 

In the formulation of early elastomer-based adhesives, it was soon learned that a simple system consisting of natural rubber alone gave only marginal performance as a finished adhesive. Most commercially available synthetic elastomers have little tack, either to themselves or to other surfaces. Modifiers were found to contribute improvement in the adhesive performance characteristics. 

There is no adhesive outside the joint and so no requirement for a tack free finish Where the adhesive may be exposed outside the joint (Figure 1.19), it is expected and it may be essential that the surface of the adhesive should be tack-free . 

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