Lead-Free Hand Soldering - Ending the Nightmares

This article is based on an original publication by Kester.

As facilities continue their transition to lead-free assembly, hand soldering remains a necessary process. Unfortunately it has been known to cause more problems than lead-free SMT or wave soldering combined, and these complications are typically more than simple material issues.

Successful lead-free soldering requires a firm grasp of basic concepts, as well as proper training and experience. Suddenly making the switch to lead-free solder can lead to line stoppages, operator complaints, decreased reliability and poor joint quality.

The following FAQs illustrate some of the key issues encountered during the lead-free soldering process.

Which alloys and fluxes are compatible with lead-free hand soldering?

One drawback of lead-free solder is its lack of availability in wire form, as certain alloys such as tin--bismuth are not easily drawn into wire. The most widely used alloys are tin-silver-copper (SAC) and tin-copper (SnCu). Wire solders in these alloys designed for hand assembly are widely available.

From a performance perspective, the primary difference between SAC and SnCu solders is their melting temperatures, which are 217°C and 227°C respectively. As SAC wets more readily, it offers better flow than SnCu based solders. Both are offered in water-washable, no-clean and rosin-based flux options. No-clean wires are by far the most commonly used, accounting for more than 85% of overall wire usage in the US, with water-washable solders accounting for less than 15% and rosin-based less than 5%.

What are the main variables in selecting lead-free solder wire?

The primary factor in determining wetting performance is flux content of the wire. For instance lead-free solders including SAC, SnCu or high-temperature SnSb wet less rapidly than 63/37 in a comparable environment.

While leaded solder wire may contain as little as 1% flux, lead-free solders should contain at least 2%. If wetting is slow a 3% flux wire can be used, but this may leave more residue and limit cosmetic appeal for no-clean tasks. It is essential that the flux used is designed for lead-free applications to ensure it can withstand high temperatures.

The IPC classification must also be considered in solder wire selection. The most reliable options meet ROL0 classifications and are rosin-based, halide-free and low-activity. Lead-free alternatives have a greater tendency towards high activity to offset reduced wetting, which may be detrimental in some cases.

More compatible with lead-free soldering applications, water-washable fluxes are higher activity and often classed as ORH1. However, ionic contamination testing should be performed to ensure residues can be entirely removed. A clean process change may be necessary if ionic contaminants linger, such as increased cycle time or water temperature.

Wetting balance tests have demonstrated that, when used with the same flux, SAC offers greater performance characteristics than SnCu solders - including those with additives such as nickel or cobalt - in time to reach maximum wetting.

How does lead-free hand assembly impact cosmetics?

At the same activation levels, lead-free solder typically flows slightly slower than 63/37, with marginally bigger contact angles, less defined feathering and less reflective solder joints. Shrinkage effects may also be a factor in certain cases.

 

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