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.

In a "pure" production environment, Tin-lead (SnPb) solders would be used only on Tin-lead (SnPb) finished hardware. Lead-Free solders would be used on Lead-Free finished hardware. If these soldering alloys get cross-contaminated with Lead (Pb), the resulting Pb contamination of the rework and repair solder joint can cause unreliable solder joints, as evidenced by premature failures of Pb contaminated Lead-free solder joints in temperature cycling.

Unfortunately, as illustrated in table 1, there will be a period of transition, where Tin-lead (SnPb) solders will be used to solder Lead-Free finished components. It is conceivable that Lead-Free solders will be used to solder Tin-lead (SnPb) finished components.

Lead-Free Solders will require a longer dwell time – the time the soldering iron was in contact with the hardware – to promote adequate heat transfer during the soldering process.

Due to the higher soldering temperatures, the soldering iron has to be removed quicker for Lead-Free than for Tin-lead (SnPb).

The higher soldering temperature requires that the soldering iron must remain clean and coated with the solder alloy. Lead-Free solders are more sensitive to the effects of a dirty soldering iron.

The higher soldering temperatures can result in the soldering iron tip becoming oxidized if not cleaned and coated.

Do not use solder tips that have soldered Tin-lead (SnPb) solders. The Lead (Pb) in the solder will contaminate the Lead-Free Solder joint.

Here are some suggestions for using Lead-Free Solders in a rework and repair mode, as shown in figure 1:

• The preheat temperatures will be higher. With SMT rework and repair operations, the ramp times will be longer.

• For hand soldering, SnPb solder tips are set ~ 280 °C. For Lead-Free Solders, the solder tip can be set as high as 350 °C.

• Due to the higher solder tip temperatures, the solder tip has to remain on the solder joint longer to promote heat transfer. However, the solder tip must not remain on the pad to avoid overheating the underlying epoxy of the board and pad lifting.

• As previously noted, the operator must remove the solder tip quickly after soldering to avoid damaging the solder joint and the board pads. The solder pads are more sensitive to heat with Lead-Free Solders.

• For leaded devices, techniques like mini wave hand soldering may have to be modified for Lead-Free solders due to the higher temperatures and solderability differences.

• Work closely with your solder and solder paste manufacturers. For the same Lead-Free solder alloy, there will be differences between solder manufacturers.

• More active fluxes should be used to improve solderability. However, this may result in more aggressive cleaning operations.

• Tip tinning and cleaning becomes more important due to the higher Lead-Free Soldering temperatures.

• Choose the proper solder tip size carefully.

• Lead-Free Solders requires an operator to be more careful and patient with the process.

Multiple thermal profiles may be required. For example, one thermal profile may be required to remove a component with Tin-lead (SnPb) solder, and another thermal profile may be required to solder that same component with a Lead-Free solder.

Special attention needs to be paid to the very common occurrence of BGAs which need to be replaced, particularly if a BGA having SnPb solder balls needs to be replaced with one having Lead-free solder balls using either SnPb or Lead-free soldering alloy. This rework/repair is termed BGA Repair for Mixed Assemblies, and should be approached cautiously to maximize reliability of the repaired assembly.

It is recommended not to use the higher temperature thermal profile to remove the component, for risking damaging the assembly via thermal shock.

Pad dressing, the cleaning of the pads prior to soldering the new component, is an important step. It is critical to remove all Tin-lead (SnPb) solder from the pads, to prevent Lead (Pb) contamination.

The Lead-Free Soldering rework equipment should be capable of reaching the soldering temperatures for the various Lead-Free solders. Remember, for Tin-lead (SnPb) solders, the peak reflow soldering temperature is between 220 °C and 230 °C. For some Lead-Free Solder alloys, the peak reflow soldering temperature can be as high as 240 °C to 260 °C, depending upon the alloy and the solder paste manufacturer. As table 2 depicts, the rework and repair processing parameters are significantly different between Tin-lead (SnPb) and Lead-Free solders.