Reworking Lead-free Solder in PCB Assembly

Estimated reading time: 4 minutes

While lead-free solder rework is more difficult because of the lack of wetting and wickability, successful rework and assembly methods have been developed with lead-free solders for all types of components.

By Dongkai Shangguan, Ph.D.

Rework is an important part of the volume manufacturing process for lead-free PCB assembly, especially true during the initial transition period when every part of the supply chain goes through the learning curve. However, the issue of rework persists throughout the product lifecycle due to the need for warranty repair.

Lead-free solder rework has been found to be more difficult because the lead-free solder alloys typically do not wet or wick as easily as the Sn/Pb solder; a clear example is seen with QFPs. In spite of these differences, successful rework methods (both manual and semiautomatic) have been developed with lead-free solders (Sn/Ag/Cu or Sn/Ag) for all types of components, such as discrete components, area-array packages, etc., using flux gels, flux pens, wire-core solder, etc. Most of the rework equipment for Sn/Pb can still be used for the lead-free solder. The soldering parameters must be adjusted to accommodate the higher melting temperature and lesser wettability of the lead-free solder. The other precautions for Sn/Pb rework (such as board baking when needed) still apply to lead-free rework. Studies have shown that reliable lead-free solder joints, with proper grain structures and intermetallic formation, can be produced using appropriate rework processes.

Care must be taken to minimize any potential negative impact of the rework process on the reliability of the solder interconnects, the components and the PCB. As the soldering temperature increases, the Z-axis coefficient of thermal expansion (CTE) mismatch between the laminate material, the glass fiber and the Cu will exert greater stresses on the Cu, potentially causing failures by cracking the Cu of the plated vias. This is a rather complex issue because it depends on numerous variables such as the PCB's layer count and thickness, the laminate material, the rework temperature profile, the Cu distribution, the via geometry (such as aspect ratio), etc. Much work is needed to determine under what conditions alternative laminate materials (such as high Tg, low CTE) to the traditional FR4 may be needed for lead-free soldering. This is not to say that lower-cost materials (such as CEM, FR2, etc.) cannot be used with lead-free soldering. In fact, such applications do exist in volume manufacturing, and the situation must be examined on a case-by-case basis. The impact of rework on the adhesion of the solder pads and masks must be evaluated carefully as well.

Similarly, rework's impact on component reliability also must be studied carefully as well. Warpage and delamination are some of the potential issues. The recently released IPC-020B standard indicates that rework is an important consideration for higher temperature component ratings for lead-free soldering.

Electrochemical reliability is another important issue to be considered. When flux residues are dissolved in moisture condensation on the board, electrochemical reactions will take place between conductor traces under an electrical bias, causing surface insulation resistance (SIR) reduction. If electromigration and dendritic growth take place, even more fatal failures can occur due to the formation of "shorts" between the conductor traces. The electrochemical reliability is determined by the resistance of the flux residue to electromigration and dendritic growth in no-clean applications. As such, SIR tests, which are based on IPC (per TM-650 2.6.3.3) or Telecordia tests, must be performed to ensure that the rework flux and any products of reaction between the reflow/wavesolder flux and the rework flux do not pose a risk for electromigration and dendritic growth for no-clean applications.

The issue of "component mixing" warrants special concern, especially during the transition period. Preliminary studies on the impact of lead in lead-free solder on long-term reliability indicate that the impact varies with the amount of lead in the solder joint, and the impact may be the greatest when the amount of lead is within some intermediate range because of the formation of segregated phases (e.g., coarse lead grains) in the last-to-solidify interdendritic tin grain boundaries, where cracks may initiate and propagate under cyclic loading. For example, it has been shown that 2 to 5 percent lead can be detrimental to the fatigue life of lead-free solder, but it probably is no worse than the Sn/Pb solder. As such, if a leaded board is to be repaired with lead-free solder, from the solder point of view, the reliability of mixed lead-free solder and Sn/Pb solder probably would not be inferior to the Sn/Pb solder. However, the temperature impact on the components (especially plastic package parts) would be a concern. On the other hand, repairing a lead-free soldered board with Sn/Pb solder would create solder joints that are not as reliable as the lead-free solder joints on the rest of the board.

In terms of logistics, during the transition period it is key that solder irons and materials (wirecore solder, flux gels, etc.) for lead-free soldering be marked clearly. Operators must be trained in lead-free rework processes and inspection. More rework and higher materials cost will have some direct impact on cost.

Dongkai Shangguan, Ph.D., is the director of Advanced Process Technology at Flextronics. He can be contacted at Dongkai.Shangguan@flextronics.com.