Lead-free solder degradation

Roughly, for every 10 °C (18 °F) increase in temperature, the moisture sensitivity level (MSL) degrades by one level. As such, new molding compound may have to be used for components intended for lead-free soldering in order to maintain the same MSL. Components... 

For lead-free solder, Dutta (Ref 16) has developed a creep model which takes into consideration strain-enhanced coarsening of inter-metallic particles under feermomechanical cycling. This model captures material degradation... 

The wettability and solderability of lead-free solder candidates must be of sufficient adequacy to form solder joints whose reliability is not degraded relative to eutectic Sn-Pb joints due to wettability-related issues. 

A minimum temperature of 230°C is required to ensure adequate wetting and spreading by the Pb-free solder on circuit board pads as well as on component leads and terminations. Temperatures exceeding 245°C increase the likelihood of thermal damage to larger, plastic-molded packages (e.g., BGA and QFP devices). When temperatures exceed 260°C, there is the potential for thermal degradation of passive chip components (chip capacitors, indnctors, or filters) as well as to circuit board structures (e.g., vias) and laminate materials. In the case of Sn-Pb eutectic solder that melts at 183°C, the minimum process temperature of 215°C ensures adequate solderability, which is 15°C lower than that of a nominal Pb-free process. The available Sn-Pb process window is 215°C to 260°C, or a AT equal to 45°C, rather than the AT of 30°C for the Sn-Ag-Cu Pb-free solders (T di = 217°C). 

Figure 24 shows the effect of storage of CSP assemblies with lead-free and tin-lead solders. There was degradation in the drop test results after 100 and 250 h of storage at 125 °C (257 °F) prior to the drop test. 

The next phase will address eliminating lead from board finishes—the protective coatings applied to termination pads on printed wiring boards to protect metal conductors from degradation (e.g., oxidation, corrosion) and remain solder-wettable. Finishes are applied in a number of ways, including dipping into a molten metal bath (e.g., tin, solder), electroless plating, etc. Alternative finishes must, of course, be compatible with the lead-free alloy selected in Step 1. 

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