Driven by European Union directives, most commercial electronics manufacturers began delivering lead-free electronic components, assemblies, and equipment in 2006. As a result of a global movement away from using lead (Pb), component manufacturers are increasingly applying tin-rich finishes to the leads of their devices and soldering with lead(Pb)-free solders.
Unfortunately, this can create a risk of tin whisker formation that can result in electrical failures. Motivated by its unique requirements such as long service lifetimes, rugged operating environments, and high consequences of failure, the aerospace and defense industries must mitigate the detrimental effects of tin whisker formation when lead-free materials are used.
The present paper provides a status on the effort associated with a multi-year testing and modeling program that aims to assess and quantify tin whisker growth on lead-free manufactured assemblies. The tin whisker growth of tin finished parts soldered with SAC305 (Sn-3.0Ag-0.5Cu) solder alloy under high temperature/high humidity (85 degrees C/ 85 percent relative humidity) conditions were evaluated.
Significant whisker growth was observed from the SAC305 solder alloy, particularly in the fillet regions where it was less than 25 microns thick. Details of the sample inspection and whisker growth results are provided.
AS-RECEIVED PART FACTORS
The lead-free Sn plated parts that meet the typical quality levels in order to form a good solder joint can have microstructural characteristics that may increase whisker formation such as:
- Uneven Sn plating with very thin or skipped Sn plating in some locations
- Void and cracks
- Excessive contamination
WHISKER LENGTH
Large whisker growth was observed after exposure to isothermal 85 degrees C/85 percent relative humidity conditions after 1,000 and 4,000 hours. Lead material and contamination level were the most significant factors contributing to whisker growth, while electrical bias had less influence.
The following key points can be made:
- There is a high risk of whisker growth in electronic assemblies if Pb-free SAC305 solder is used.
- Long whisker growth was observed from the SAC305 solder, particularly in the fillet regions where the solder became thin.
- Lead-free soldered assemblies exposed to 1,000 hours of 85 degrees C/85 percent relative humidity conditions grew whiskers with sufficient length to fail the JESD201 [25] piece part acceptance requirements for class 2.
- Whisker growth occurred on SAC305 solder joints containing either the copper or the alloy 42 leaded components, but the alloy 42 leads exhibited a delay in long whisker growth.
- Testing durations longer than 1000 hours are needed to ensure whisker nucleation and growth in 85 degrees C/85 percent relative humidity conditions.
- Contaminated components in clean assemblies grow more long whiskers than clean components because of contamination entrapped in solder.
METALLUGICAL OBSERVATIONS
Optical microscopy, scanning electron microscopy in conjunction with cross-section examinations revealed that:
- The source of whisker growth stress was SAC305 solder oxidation and corrosion.
- Many whiskers were broken from the joints forming debris between the leads. Oxidation of whiskers is believed to cause embrittlement making the whiskers susceptible to fracture under mechanical loading conditions. Once a whisker has fractured, the growth has terminated.
