Wave soldering solder alloys

Lead-Free Wave Solder Alloy Selection Reliability is Key... 

In an ideal world, there would be a lead-free wave solder alloy that gives as good or better overall performance as Sn63/Pb37— a drop-in replacement. Unfortunately, this does not exist. What is obtainable in the market today are various lead-free wave solder alloys that possess different properties. For PCB assemblers, this means that they need to make a choice of alloy and for many, this means that they need to conduct extensive in-house testing to make an informed choice. 

Wave soldering alloy Melting point (°C) QFP pull strength (kgO Jumper wire pull strength (kgf) Elongation (%)... 

TABLE 10 Summary of Wave Solder Alloy Evaluations... 

FIG. 37 Thermal cycle tests for a 0.8-mm pitch QFP using several lead-free wave solder alloys. Thermal cycling conditions were -40°C to 85°C for 30-min dwell times. Electrode finishes tested were a) Sn-Pb finish, b) Sn-Bi finish, and c) Pd finish. The Sn-Ag-Cu lead-free alloy is generally observed to be the most resilient to electrode finish materials, outperforming even the eutectic Sn-Pb solder in most cases. (From Ref. 24)... 

However, up to now, most flexible circuit boards have been based on either polyester or polyimide. While polyester (PET) is cheaper and offers lower thermal resistance (in most cases reflow soldering with standard alloys is not possible), polyimide (PI) is favored where assemblies have to be wave or reflow soldered (with standard alloys). On the other side, the relative costs for polyimide are 10 times higher than for polyester. Therefore, a wide gap between these two dominant materials has existed for a long time, prohibiting broad use of flexible circuits for extremely cost-sensitive, high-reliability applications like automotive electronics. Current developments in the field of flexible-base materials as well as the development of alternative solder alloys seem to offer a potential solution for this dilemma. 

The impact of Pb-free technology on wave soldering has largely occurred in the equipment performance. It has been determined that the same solder bath temperatures that are used for Sn-Pb processes (250 to 270°C) are suitable for the Sn-Ag-XCu Pb-free alloys. Therefore, excessive dross formation and flux residue removal have not become a significant problem during equipment operation. The lack of shiny fillets with the Sn-Ag-XCu alloys has been addressed by modified alloys having Ni and Ge additions that alter the solidification process, which leads to shinier fillet surfaces. 

The Pb-free alloy compositions are more prone to erode the wave machine s structures, such as the impeller, baffles, and pot walls. New wave-soldering machines address this problem through the use of alternative steel alloys and ceramic coatings. 

Quaternary alloys are composed of four elements (e.g., Sn-Ag-Bi-Cu) and pentanary alloys (e.g., Sn-Ag-Cu-In-Sb) are composed of five. The latter is least understood binary systems are best understood and easiest to formulate. Some of the small differences in compositions (0.5 wt percent) as seen in ternary, quaternary, and pentanary systems are difficult for solder vendors to control accurately during formulation. Also, in wave soldering, where the solder pot contents are in constant contact with metals from component leads and circuit board surface finishes, materials will be leached into the molten charge in the solder pot and will effect solder composition over the long term. Minor constituents of a solder may be depleted more quickly during wave soldering. 

Bismuth expands upon freezing, whereas tin contracts. A phenomenon called fillet lifting has been reported (see Hg. 45.2). It is mostly associated with bismnth ternary alloys such as Sn-Cu-Bi and Sn-Ag-Bi used in plated throu -hole (PTH) wave soldering, but has also been observed with Sn-Bi system. 

Zinc Solders. Zinc (Zn) alloys, favored by some Japanese companies, oxidize rapidly. Solder paste shelf-life has been an issue even when these alloys are refrigerated at very low temperatures. Dross formation in wave soldering has also been problematic. Zn alloys are also known to have some corrosion problems. 

When adjusted properly, the board meets the crest of the wave and disrupts the oxide skin. In doing so, the fluxed components and board are immersed in the flowing, oxide-free molten solder. If all steps are carried out properly, the solder alloys with the fluxed, oxide-free component leads, component pads, and PTH barrels. Upon exiting the wave solder machine, the assembly cools below the solder liquidus temperature and solder joints are formed. The more the system is used to solder boards, the faster the contamination and dross build-up. 

Sn drift Excessive dross on the surface of the solder can disrupt normal wave dynamics. In the case of Sn/Pb solder, Sn oxidizes more easily than Pb. The solder can become Sn depleted over the long term. This is known as Sn drift. The same is true of Sn-based Pb-free solder alloys, but since some alloys are nearly all Sn (SAC305 with 96.5 percent Sn by weight), this effect will be less dramatic. However, other Pb-free alloy perturbations such as dissolution of Cu become mnch more important. For example, in the case of the eutectic Sn-Cu alloy, the copper content is only 0.7 wt percent. Small changes in the copper content by means of copper dissolntion of copper pads on the board can have a dramatic effect on the melting temperatnre of the solder. 

The tin-copper alloy has been found to be inferior to SAC in terms of wettability, dross formation, and reliability under typical loading conditions however, its much lower cost as compared with SAC makes it an attractive alternative alloy for wave soldering, especially for cost sensitive products. Even though most manufacturers may prefer to use the same alloy for all of the solder interconnects on the entire board (including reflow and wave soldering), there are products in volume production today which use SAC for reflow and tin-copper for wave soldering on the same board. As such, methods for inspection, rework, and accelerated testing must be compatible with both alloys. Several variants of the tin-copper alloy have also been intro-... 

Complicating the picture further is the use of different solder alloys on the same board (such as SAC for reflow and tin-copper for wave soldering). Detailed discussions on accelerated reliability testing methodology can be found in Chapter 7. 

Lead-Free Solder Alloys. Because of the toxicity of lead and the concern that the lead in electronic products may end up in landfills, and ultimately in the water supply, the electronic industry is exploring alternative solder alloys that do not contain lead. These alternative solder alloys are typically composed of tin (Sn), with one, two, or three additives such as copper (Cu), silver (Ag), bismuth (Bi), antimony (Sb), zinc (Zn), or indium (In). Typical tin lead-free candidate solder alloys include Sn-Cu, Sn-Ag, Sn-Ag-Cu, Sn-Ag-Cu-Sb, Sn-In, and Sn-Cu-Bi-Sb. Some of these are more suitable for wave solder... 

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