Pb-free rework differences and practices: no lead means using four heating zones and one cooling zone for reflow
Good assembly practice calls for retrained staff and new profiles. Maintaining high quality rework is more challenging because the PCB and components adjacent to the targeted rework component are subjected to multiple cycles of higher temperatures. To maintain the integrity of the PCB laminate, the maximum preheat temperature is set approximately 10[degrees]C below the Tg of the PCB material. Higher thermal preheat temperature minimizes the potential thermal distortion and shock to the PCB during reflow. Thick boards–0.093″ or more–require additional heat, have greater [DELTA]T across assemblies and may require slower reflow oven line speed. As overall board size increases, the process window tends to shrink.
Solder pastes. The rework process is the same no matter the paste. It begins with a good thermal profile, removal of failed components, cleaning and preparing the site, and removing rust or solder residues. Next, the component is replaced with new flux and solder, reflowed and inspected.
Several major differences emerge here between Pb and Pb-free rework. These differences introduce a host of challenges and newer or altered practices. The higher temperatures of Pb-free rework mandate tighter processes, better thermal profiles and greater precision. Otherwise, a number of problems can occur. For example, too much heat could create a blowhole on a BGA ball, but too little heat could lead to BGA balls separating from the PCB substrate. Excessive thermal stress could cause micro-cracks on a BGA and PCB (see Figure 1 online).
Hundreds of Pb-free alloys are available. But there is no direct drop-in replacement for SnPb solder; nothing is as effective, costs the same and is nontoxic. Compared with SnPb, every Pb-free replacement alloy is inferior in some cost or performance aspect. Table 1 compares three popular Pb-free solder pastes.
SnAgCu (SAC) alloys are widely regarded as the most suitable choice in the near future for Pb-free soldering. The liquidus temperature of SAC alloys is 217[degrees] to 220[degrees]C, about 34[degrees]C above the melting point of eutectic SnPb. This higher melting range requires peak temperatures in the range of 235[degrees] to 245[degrees]C to achieve wetting and wicking. Lower peak temperatures (such as 229[degrees]C) can be used for SAC solders. Adding copper to SnAg reduces the melting point, improves wetting and increases long-term reliability. Also, SAC is not as sensitive to lead contamination as Bi-containing alloys and can be used with SnPb-plated components with no major problems.