Hand soldering remains an important element of electronics manufacturing. First-piece soldering, rework, and repair operations are performed using manual soldering irons, as shown in Figure 2-1. With the introduction of lead free solder alloys, operators performing hand soldering will have to become familiar with the differences between tin lead (SnPb) and lead free solders. The lead free soldering process requires more care and patience by the operator.

Material issues
Most lead free alloys have a higher melting point than SnPb. The melting point of eutectic SnPb solder is 183°C. Other than tin bismuth (SnBi), most of the popular lead free solder alloys have a melting temperature between 215°C and 227°C.

The components and board will be more sensitive to heat-related soldering damage with lead free solders than with SnPb solders. It has been shown that components will increase their moisture resistance sensitivity by two levels, based on IPC-J-STD-020 testing specifications. The higher lead free soldering temperatures can result in an increase in board delamination, measling, and blistering. For example, measling bridging over 50% of the span between conductors is a defect, in accordance to IPC J-STD-001. Board failure descriptions can be found in the IPC J-STD-001D, Appendix C and in the IPC A-610D documents. To prevent these types of failures, it is recommended that the components and boards be baked prior to soldering to remove any absorbed moisture.

Board finishes should not be a factor in performing hand soldering. However, OSP board finishes are prone to developing a halo around the solder joint. Because lead free solders do not wet as well, this halo effect is more prominent with lead free than with SnPb solders.

Component manufacturers are beginning to provide their hardware with lead free alternative finishes. Some of the available finishes include:

• Tin - Sn
• Tin Silver - SnAg
• Tin Copper - SnCu
• Tin Silver Copper - SnAgCu
• Nickel - Ni
• Nickel Palladium - NiPd
• Palladium - Pd

For a specific component, manufacturers will not provide multiple component finishes. Market demand will determine the finish of choice. The EMPF found no process incompatibilities and no reliability issues between the various lead free solders and lead free board finishes. These findings are consistent with other lead free soldering industrial studies.

However, as the technology matures, there are possible incompatibilities between lead free component finishes and SnPb solders. This is known as “backwards compatibility”. For example, tin-finished components represent a major reliability concern to some electronics manufacturers, because they are susceptible to tin whiskers, which can cause electrical system failures.

Depending upon the size of the board and the board’s thermal mass, operators may wish to use a digitally-controlled hot plate to preheat the board prior to soldering. This practice is performed with SnPb solders and should continue with lead free solders. The higher soldering temperatures necessitate using a digitally-controlled hot plate. Preheating the board will reduce the thermal gradients within the board during the hand soldering process.

Hand soldering process issues
Because of the higher melting temperatures of lead free solder alloys, the solder tip must be set to a higher temperature. For example, when using the lead free solder alloy, tin silver copper (SAC-305), the EMPF found that the solder tip had to be set to 343°C / 650°F, as opposed to 315°C / 600°F for SnPb.

Due to the higher soldering temperatures, the iron must be kept clean and fully coated with the solder alloy; otherwise, oxidation of the soldering iron tip can occur. Lead free solders are more sensitive to the effects of a dirty soldering iron.

Also due to the higher soldering temperatures, the soldering iron must be removed more quickly. Lead free solders and SnPb solders have different cooling rates and cooling characteristics. Icicles will be created if the soldering iron is removed too slowly. The size and frequency of solder icicles is dependent upon the alloy used and the soldering iron temperature setting. Quick removal of the solder iron also prevents disturbance of the solder joint and pads on the board. Lifted pads and fillets are more prominent with lead free solders.

To avoid contamination of solder joints, solder tips which are used with lead free alloys must be kept separate from those used with SnPb. There is some evidence that lead free solders joints contaminated with Pb are not as reliable as uncontaminated lead free solder joints.

To ensure adequate heat transfer, operators must select an appropriate solder tip and dwell time. A solder tip which is too small will not provide sufficient heat transfer. In addition, a longer dwell time (the time the soldering iron is in contact with the hardware) is required.

Lead free solders do not wet as well as SnPb solders. To improve solderability, an operator may increase the solder tip temperature or use a more active solder flux. Increasing the solder tip temperature may damage the hardware if done indiscriminately. If a more active flux is used, more aggressive cleaning processes will be required. Active fluxes leave residues on hardware, which will promote dendritic growth and poor adhesion of conformal coating.

The resulting lead free solder joints are dull and exhibit a grainy surface, as depicted in Figure 2-2. The J-STD-001 and IPC-A-610 standards allow for solder joints that have dull, matte, or grainy appearances, provided that such appearance is normal for the materials and processes involved. Operators will have to adjust their visual criteria for an acceptable solder joint to allow for these differences.

Despite the variations between lead free and lead-based hand soldering, it is possible to meet IPC Class 3 requirements. The Lead Free Component Focus Group and the JG-PP / JCAA Lead Free Soldering Program determined that it is feasible to meet IPC Class 3 requirements with respect to solder fillet quality, wetting, and solder pad coverage.

The EMPF offers a lead free soldering training course to all who are interested in developing and implementing lead free soldering processes. Lead free hand soldering, wave soldering, SMT reflow soldering, rework and repair, and inspection are all demonstrated to participants in this hands-on course. Please contact the EMPF Helpline at (610) 362-1320 for further information.

For more information concering Lead Free processes and surrounding issues, please stop by ACI's new Lead Free Manufacturing Page to download articles contributed to ACI by some of the industry's most knowledgable individuals and organizations, as well as material generated by ACI, and documents on the legislation surrounding the Lead Free issue.