Lead-free solder wetting

The process window for lead-free soldering requires higher peak reflow temperatures and longer times above liquidus. Soldering in a nitrogen environment improves wetting and reduces oxidation of the flux residues and the solder. Due to the higher reflow temperature. 

Solderability Lead-free solder does not wet as well to solderable surfaces as leaded solder does, so pad size, solder quantity, and flux type and quantity must be taken into account. 

Surface Finishes. The search for alternatives to hot air solder leveling (HASL) has been ongoing for several years, primarily because of the inherent inconsistency in the quality of the HASL finish. For example, the thickness (and therefore, solderability) of HASL is difficult to control. In areas with a very thin layer of HASL, consumption of Sn by the formation of tin-copper intermetallics will render the areas non-wet-table. The HASL finish is typically non-flat (with a dome shape), making it difficult to deposit a consistent amount of solder paste during solder paste printing and difficult to place fine pitch (<25 mil) devices. The HASL process itself is not as clean and easy to control as some plating processes. The current move towards lead-free solder has provided the additional impetus towards alternative surface finishes. 

Reflow Process. The key parameter for the reflow profile is the peak temperature. Adequate reflow temperature is needed for the solder to melt, flow and wet, interact with the copper on the pad and the component termination, and form sound intermetallic bond when cooled and solidified. Typically, 30 °C (55 °F) superheat (above the melting temperature) is desired. Eor lead-free soldering, because of concerns about the thermal stability of the components, efforts are needed to minimize the soldering temperature. For SAC alloy with the eutectic temperature at 217 °C (422 °F), the minimum reflow... 

Rework for lead-free solders has been found to be more difficult, because the lead-free solder alloys typically do not wet or wick as easily as the Sn-Pb solder due to their difference in wettability. This can be easily seen with QFP packages. In spite of these differences, successful rework methods (both manual and semi-automatic) have been developed (Ref 74-75) with lead-free solders (Sn-Ag-Cu, or Sn-Ag), for many different types of components. Most of the rework equipment for tin-lead can still be used for lead-free solder. For area array packages, it is helpful to use a rework system with split vision and temperature profiling features. The soldering parameters must be adjusted to accommodate the higher melting temperature and reduced wettability of the lead-free solder. The other precautions for tin-lead rework (such as board baking) still apply to lead-free rework. 

It may be noted that the tin-lead solder is shiny with excellent wetting. The only lead-free solder joint that comes close to this feature is Sn96.5 Ag3.5, a eutectic solder. All lead-free solders show appearance of a graininess and lack of good wettability. 

Fig, 10 Optical photographs of J-lead packages assembled with Sn-Pb solder and four lead-free solders. Note differences in solder wetting angle and appearance. The lead-free eutectic solder (Sn96.2Ag3.5) has the closest appearance to tin-lead solder. 

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... 

Printability of lead-free solder paste will not change, but its spread during reflow will, which may require tightening of the stendl-printing process. One possible issue is print accuracy, or the alignment of the printed solder paste onto the PCB pad. Because lead-free alloys do not spread or wet as well as tin/lead, any solder paste that is not accurately printed onto the PCB will stay close to where it was printed after the reflow soldering process. Figures 3 and 4 depict the same deposits before and after reflow for QFPs and passives. 

The wave-solder-pot temperature for lead-free soldering should range from 265° to 270°C. The specific temperature to use depends on board layout and if pallets are used, and should be adjusted to maximize wetting. The dwell time in the wave should... 

Due to the lower wetting speeds associated with alternative lead-free solders, flux activation will be a critical factor in paste performance. No-clean and water-washable solder pastes are being designed so they do not require nitrogen reflow and can produce reliable solder connections with good wetting in air. Water-washable solder pastes with their higher concentration of activators will solder most metal finishes adequately. No-clean solder pastes require careful finish and paste attribute selection. 

In principle, compounds for MID can be manufactured with a very wide variety of fillers from the groups of metallic (only for MID processes that do not require wet-chemical conductor metallization) or ceramic materials. At this time some fillers are coming into widespread use on account of their verifiably excellent thermal properties, offering considerable benefits for MID production (e.g., reduction in hotspots for lead-free soldering processes) and MID use (e.g., integrated thermal management in highly loaded MID such as LED modules). The most important of these fillers are... 

Hwang, J.S. Lead-free solder the Sn-Ag-Bi system. Surf. Mt. Technol. June 2000,14 (6), 20-22. Bradley, E. Hranisavljevic, J. Characterization of the wetting and melting of Sn-Ag-X solders. IEEE Trans, of Electron. Packag. Manuf. Oct. 2001, 24 (4), 255-260. 

Takemoto, T. Miyazaki, M. Effect of excess temperature above liquidus of lead-free solders on wetting time in a wetting balance test. Mater. Trans. JIM 2001, 42 (5), 745-750. 

Many emerging lead-free solders are comprised of 3 or more alloying elements. Loomans et al. [172] evaluated the spreading of Sn-Bi eutectics with the addition of 1% ternary elements (Fig. 51) and found that the contact angle increased with the addition of each ternary element investigated. As seen in Fig. 55, the presence of Zn always increased the contact angle (i.e., decreases wetting). 

Tsung-Yu Pan, J.M. Nicholson, H.D. Blair, R.H., et al. Dynamic wetting characteristics of some lead-free solders. Proceedings of the 7th International SAMPE Electronics Conference, Parsippany, NJ June 20-23, 1994 343-354. 

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