Wetting, solder surface tension

The solder fillet is an overt manifestation of surface tension and wetting. Fillets are readily apparent in Fig. 44.2, which shows the fillet as a web of solder extending from the PWB bonding pad to the lead of the component. 

The wetting balance technique is a variant of the maximum pull (or detachment) method used to measure liquid-vapour surface tensions (Keene 1993). It is nowadays widely employed in the electronics industry to quantify wetting of solders, but has also been used for wetting studies in metal/ceramic systems (Naidich and Chuvashov 1983b, Nakae et al. 1989, Rivollet et al. 1990). As compared to the sessile drop method which needs planar substrates, solids of various geometry can be studied by this technique. 

Besides viscosity, the surface-wetting ability of underfills is critical to capillary flow. For capillary flow to occur, the underfill material must wet the surfaces so that the advancing contact angle is less than 90°. " Also, for capillary flow, the intramolecular forces of attraction among adhesive molecules must be weaker than the intermolecular attraction of the adhesive for the die, the substrate, and the solder surfaces.t This occurs when the surface tension of the underfill is lower than the surface energy of... 

Reflow The solder paste temperature exceeds the liquidus point and reflows, wetting both the component leads and the board pads. Surface tension effects occur, minimizing wetted volume. 

The intrinsic solderability performance of Pb-free solders is being improved by two means. First, new flux formulations are available that more effectively reduce the surface tension of the solder. Second, alternative surface finishes can be specified for the component I/Os and/or circuit boards that improve wetting and spreading activity exhibited by the Pb-free alloys. 

Strictly from an assembly process point of view, the mixing of Pb-free and traditional Sn-Pb solder can be beneficial. The Sn-Pb solder can improve the wetting and spreading performance of the Pb-free solder by two phenomena. First, Pb contamination lowers the molten solder surface tension of the solder. Second, the Pb contamination reduces the melting temperature of the Pb-free alloy. However, concerns are raised by the mixing of Sn-Pb and Pb-free solders and its effect on the long-term reliability of interconnections under thermal-mechanical fatigue environments. 

Reduce the interfacial surface tension between the solder and the substrate in order to enhance wetting... 

This is the most basic of requirements. The solder has to be in contact with the materials to be joined. The contact area of the solder is not wholly important as long as the solder is in point contact with the surface to be soldered when it reaches hquidus. Surface tension effects and metallurgical wetting will complete the spreading of solder contact. 

Onset of Solder Liquidus. In this step, the solder undergoes a phase change from solid to hquid.The Uquid metal solder wets onto fluxed metallic surfaces and flows along the component lead-to-PWB pad interfaces drawn by surface tension, capillary forces, and convective flows within the Uquid metal. This is the essence of soldering and effective solder-joint formation. 

The through-hole components (axially leaded parts, pin-grid arrays, solder-tail connectors, etc.) are inserted into their respective PTHs before or after surface-mount component placement. Once the surface-mount components and the solder-tail parts are placed, the pastebearing board is then passed through the SMT reflow oven. During the reflow process, the molten solder coalesces around the through-hole pins wetting between the pin and barrel. Surface tension and capillary action draws the solder down the barrel to complete the solder joint. 

FIGURE 47.16 Board sections illustrating pin-in-paste process (a) A PTH component pin inserted through solder paste squeegeed in and around PTH. (b) At reflow, the solder has melted and wet to pin and component leads. If solder volume is correct, surface tension effects will draw the solder between pin and barrel and a good solder joint will form, (c) The solder paste displaced by the pin insertion has melted into a ball around the pin tip. So much solder was displaced that voids are apparent between the pin and the PTH barrel. 

If you want a liquid to wet a surface (that is, 0 — 0, therefore cos 0 1), a balance is required between the numerator and denominator of equation 22.17 that is, Tsv 7(s should be approximately equal to -y . For example, solders are alloys whose liquids wet other metals because surface tensions have the appropriate values. By the same token, detergents and soaps help water wet solids (like synthetic fabrics) because they reduce the surface tension of water to the appropriate point. 

Surface-tension Test —The tombstoning performance of solder pastes has been speculated to be related to the surface tension of respective solder alloys. In this study, a wetting balance was used to measure the surface tension of solder alloys following established conditions. A piece of alumina coupon with a dimension of 2.5 X 0.4 X 0.062 cm was used for the testing. The solder pot was maintained at 245°C for the SnPb alloy and 260°C for SnAgCu alloys. The measured force with respect to time was translated to a graph of force vs. dip depth based on dip speed. When the alumina piece immersed into the solder for a certain depth where the meniscus became stable, the measured force was proportional to the displaced volume of solder. This force may be described as follows ... 

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