These Pb-free Manufacturing Guidelines are compiled from both the hands-on experience of manufacturing, reworking, and repairing electronic systems hardware using lead-free processing at the EMPF (Electronic Manufacturing Productivity Facility), which is a COE (Center of Excellence) for U.S. Navy ManTech. This is a living document, representing benchmark presently used Pb-free electronics processing. Processes will be updated as new developments and techniques become available.

Figure 1. Example of Hand Soldering

Lead-free solder alloys generally have higher melting points. Therefore, the solder tip must be set to a higher temperature. For example, when using the lead-free solder alloy, tin silver copper (SAC-305), it was determined that the solder tip had to be set to 343°C / 650°F, as opposed to 315°C / 600°F for tin-lead (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.

The soldering iron must be removed more quickly. Lead-free solders and tin-lead (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.

Figure 2. Examples of Lead-Free Hand Solder Joints. Note the icicle formed from an improper hand soldering operation.

To avoid contamination of solder joints, solder tips which are used with Lead-Free alloys must be kept separate from those used with tin-lead (SnPb). There is some evidence from the JG-PP Lead-Free Soldering study that lead-free solder joints contaminated with lead (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.

Increasing the solder tip temperature may damage the hardware if done indiscriminately.

Lead-free solders do not wet as well as Tin-lead (SnPb) solders. To improve solderability, an operator may increase the solder tip temperature or use a more active solder flux.

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.

It is recommended that Printed Wiring Assemblies (PWA) be preheated between 100 ºC to 120 ºC (Figure 3). These temperatures will be dependent upon the board material, board thickness, and configuration. Preheating the boards before soldering offers several advantages:

• Prevent board and component thermal shock

• Reduces the amount of thermal gradients in the board

• Prevent pad lifting by reducing the solder tip contact time

• Reduces soldering dwell time

• Improve wetting during the soldering process by having the board at an elevated temperature during the soldering process

• Reduces the effect of coefficient of thermal expansion (CTE) mismatch between components and boards

There are 3 types of thermal preheating processes:

• Conduction – Used on ceramic and single sided boards

• Convection – Hot air heats the board

• Radiant / Infrared (IR) – The most expensive preheating process

Figure 3. Example of PWA being preheated.

The resulting Lead-Free solder joints are dull and exhibit a grainy surface. 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.