Wetting, solder spreading

F. G. Yost, F. M. Hosking, and D. R. Frear, The Mechanise of Solder Alloys Wetting and Spreading (Van Nostrand Reinhold, New York, 1993). 

F. G. Yost, "Fundamentals of Wetting and Spreading with Emphasis on Soldering", in The Metal Science of Joining. M.J. Cieslak, et al. Editor (The Minerals, Metal, and Materials Society, Warrendale, PA, 1992) p. 49. 

The need for higher processing temperatures limits the assembly process window of Pb-free solders. A higher nominal temperature is needed to accommodate the temperature variation at components across a circuit board to ensure melting of the solder and adequate wetting and spreading at each interconnection. On the other hand, the maximum temperature must be limited to prevent thermal damage to heat-sensitive devices and the circuit board. 

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. 

A minimum temperature of 230°C is required to ensure adequate wetting and spreading by the Pb-free solder on circuit board pads as well as on component leads and terminations. Temperatures exceeding 245°C increase the likelihood of thermal damage to larger, plastic-molded packages (e.g., BGA and QFP devices). When temperatures exceed 260°C, there is the potential for thermal degradation of passive chip components (chip capacitors, indnctors, or filters) as well as to circuit board structures (e.g., vias) and laminate materials. In the case of Sn-Pb eutectic solder that melts at 183°C, the minimum process temperature of 215°C ensures adequate solderability, which is 15°C lower than that of a nominal Pb-free process. The available Sn-Pb process window is 215°C to 260°C, or a AT equal to 45°C, rather than the AT of 30°C for the Sn-Ag-Cu Pb-free solders (T di = 217°C). 

FIGURE 441 Comparison of soldering to copper (a) and nickel surface (b) with a weak organic solder flux. Note that in the case of the copper surface, the solder has wet and spread, characterized by a low wetting angle (the extent of wetting is indicated by the dashed outline). In the case of the nickel surface, the flux was ineffective in penetrating the oxide layer and solder was not able to wet to it. Instead, the solder beaded up on the surface of the nickel. (Courtesy of Hewlett-Packard.)... 

Another issue with printing lead-free solder paste is stencil aperture design. Traditionally, stencil aperture size is reduced in relation to PCB pad size. This ensures the stencil aperture seals, or gaskets, to the PCB pad. Gasketing reduces solder paste that can get under the stencil and eventually cause wet solder bridges if not cleaned properly. Lead-free solder paste does not spread as well, so... 

Wettability is the capacity of the molten solder to react with the base metal at their mutual interface and establish a bond there. The wettability of liquid solders is largely an intrinsic property between the specific solder composition and the base metal composition. On the other hand, spontaneous spreading is impacted by several parameters base metal surface condition, wettability (solder/base metal interface reaction), and the flux. The synergistic roles of the molten solder, base metal, and flux on the combined properties of wetting and spreading, that is, the alloy s solderability, can be described by Young s equation ... 

One approach uses a single coating—the protective finish. As the terminology implies, the protective finish prevents the loss of base metal solderability due to rapid reoxidation prior to soldering. The role of the protective finish in a soldering process is illustrated in Fig. 20 [5]. The protective finish is initially wetted therefore, it must be solderable. Then, the coating is dissolved into the molten solder. The solder proceeds to wet and spread over the underlying (pristine) base... 

In those cases in which it is impractical to sustain base metal solderability, a metalhc coating is applied to the base metal. The molten solder wets and spreads over the coating and, ultimately, makes a bond to it. This coating is referred to as the solderable layer. Typically, a protective finish... 

Prasad, R. Surface Mount Technology, Van Nostrand Rheinhold New York, 1986 478-480 pp. MacKay Eluxes and flux action. In The Mechanics of Solder Alloy Wetting and Spreading, Yost, F. Hosking, M., Frear, D., Eds. Van Nostrand-Rheinhold New York, 1993 35-98 pp. 

The two most common techniques for the quantitative and qualitative assessment of the wetting and spreading performance of solders are the wetting balance and the sessile drop technique. Each method possesses certain advantages over the other and is discussed in Secs. 3.4 and 3.5. However, there are certain experimental conditions that are common to either method and merit brief consideration. 

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