LEAD-FREE
FREQUENTLY ASKED QUESTIONS

Hand Soldering with Lead-free Alloys

What lead-free solder alloys and fluxes can be used for hand soldering?

Popular lead-free wire solders in use today are Tin-Silver-Copper (melting point 217-221C), Tin-Silver( melting point 221C), and Tin-Copper( melting point 227 C). All three alloys are available with no-clean, water washable or rosin systems and can be drawn down to the finest wire diameters. These alloys have been used for hand assembly on lead-free products and are compatible with lead-free alloys.

Do lead-free solder alloys require soldering irons with hotter tips?

Hand soldering with lead-free solder wire doesn’t necessarily require higher soldering temperatures and tip temperatures from 700-800 F will enable adequate soldering. Operators will notice the wetting will be slightly slower than traditional Sn63 solder, and an incremental increase in contact time may be required to achieve good results. The solder joint finish will look different also the slightly duller finish is however typical of the lead-free solders mentioned above. One consequence of using lead-free solders with high tin content is tip erosion, iron tips may require replacement more regularly.

What are the main concerns associated with reworking lead-free BGAs?

BGA components can see higher temperatures during the de-soldering and soldering process, tin-silver-copper has a melting point of 217-221 C. Excessive localized heating can cause board damage and in the case of component attachment, damage the BGA’s reliability. Excessive heating should be avoided. There are excellent BGA reworking equipment designed for lead-free soldering which prevent this by directing a controlled amount of localized air or nitrogen underneath the component, coupled with good bottom side heating excesses are avoided.

What soldering fluxes can be used for reworking lead-free solder joints?

Lead-free soldering doesn’t differ from Sn63 soldering. Fluxes are available in the no-clean, water washable and rosin formats to suit any hand soldering and rework process. Water washable flux types due to their higher concentrations of activators will solder more effectively, no-clean fluxes are traditionally made with weak organic acids and solder more slowly, and are more prone to de-activation when exposed to excessive heating.

Will there be more smoke or fumes when soldering with lead-free solders?

There are new fluxes for lead-free soldering, designed with heat stable flux systems. These fluxes do not decompose at the slightly higher temperatures that could be associated in a lead-free process.

Is a nitrogen atmosphere needed for hand soldering?

The use of nitrogen-assisted rework is not a necessity if a flux designed for lead-free soldering is used. A good solder manufacturer will insure the flux chemistry remains active at the higher soldering temperatures. The fluxes for solder wire use and flux gels used in rework will have stable activators and resins designed for the specific alloy and the process temperatures they will be used in. Nitrogen use will however, reduce oxidation, enabling the use of less active fluxes and lesser quantities of flux for adequate soldering.

How can I develop a good lead-free hand-soldering process, which will ease the operation?

In a recent study, which appeared in the Lead-free Update by TechSearch International in December 2004, hand-soldering was found to be more problematic to implement when compared to lead-free wave soldering and SMT.

The reason could be that hand-soldering is more operator dependant than reflow and wave soldering but also the surface tension in lead-free solders is slightly higher. Wetting or spread is also a little slower when compared to 63/37.

To reduce operator issues and reduced wetting proper optimization of the soldering process is key. To avoid issues use a flux content of 2-3% by weight in the solder wire, use a solder tip temperature of 700-800º F. Also Tin-Silver-Copper (SAC) solder will flow more readily than Tin-Copper (SnCu) solder.

The main issues encountered with lead-free hand-soldering are cold solder joints, poor wetting and de-wetting. These can be avoided.