Lead-free 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. 

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)... 

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... 

The implementation of lead-free wave soldering brings with it several decisions. Whether to purchase new equipment, which flux to implement, and which solder alloy to use can have a significant impact on the throughput of your operation and, ultimately, your bottom line. As always, no one can tell you what is best for your business. 

Similar to traditional Sn-Pb wave soldering, metals dissolve relatively quickly during lead-free wave soldering operations. The rate of alloy or metal dissolution of exposed pads on PCBs or component leads depends upon their composition, solder composition, bath temperature, and the flow velocity of the wave solder. The rate of dissolution of a specific metal is lower if this metal is already present in the lead-free solder bath. Of particular concern is the dissolution of copper from circuit boards when exposed to lead-free solder waves. 

FIG. 52 A real-time monitor based on DTA techniques was developed in conjunction with Malcom Industries to automatically monitor the impurity levels in a lead-free wave soldering bath, a) The monitor indicates the b) initial melting point of the solder bath alloy and c) lower melting point due to lead (Pb) contamination. (Courtesy of Panasonic and Malcom Industries.)... 

Matsushita Electric Industrial introduced Pb-free wave soldering to mass production of videocassette recorders (VCR) in 1999. The single-sided PWB assembled with Sn-Cu eutectic solder for a VCR is shown in Figure 12. In August 2000, Nissan Motors released a lead-free soldered, keyless entry component produced by wave soldering with a Sn-Ag-Cu alloy. 

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. 

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. 

Lower-budget operations and/or those that have a relatively new wave machine that is not depreciated fully may want to continue using their current wave-soldering equipment after lead-free soldering has been implemented. However, if you are using an SnAgCu alloy, baffles, pumps, and... 

For companies planning to use existing wave-solder equipment, it is important to understand that because lead-free alloys are less dense than tin/ lead alloys, the hardware found in older wave-soldering equipment will sink to... 

It is generally agreed upon that none of the lead-free binary systems investigated is likely to be a consensus replacement for eutectic Sn Pb in assembly reflow applications, although this may be the case for wave soldering. Viable candidates typically consist of a small quantity of a third or fourth element added to a lead-free binary alloy compared to the relatively uncomplicated eutectic Sn-Pb solder. 

Lead-free manufacturing does not only pertain to solder pastes and solder bars for wave soldering, but also as a substitute for lead-bearing finishes utilized for component leads and solder bumps. Lead-free alloys are also used to pretin and protect the copper features on PWBs against oxidation. 

In addition, four and five-component lead-free solder alloys, many of them patented, will present manufacturing problems because it will be very difficult to maintain the exact alloy composition in wave soldering machines. Solder powder manufacture for these complex solder alloys will also be complicated. 

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