Tin Whisker Testing
Tin whiskers pose a serious problem for electronics of all types. When used as a finish material for electronic components, pure tin can spontaneously grow conductive whiskers. These structures can form electrical paths, affecting the operation of the subject device. The scientific community has yet to agree on the reason for tin whisker growth. Some theories suggest that tin whiskers might occur due to a mechanism of stress relief, especially “compressive” stress, within the tin plating.
Tin Whiskers and Lead Material
Historically, most electronics were soldered with lead solder. Lead (3% minimum lead required) was added to solder as it melted at low temperature. Lead also seemed to prevent solder from growing whiskers. However, due to the RoHS directive’s ban on the use of lead in solder, most manufacturers have had to switch to lead-free solder. Unfortunately, this has increased the risk of tin whisker growth.
Tin Whisker Mitigation Strategies
There is no single strategy to mitigate tin whiskers. Any mitigation strategy will depend upon the particular device. Some of the mitigation concepts are:
- Using conformal coating
- Avoiding tin-plated parts
- Selecting matte or low-stress tin finish
How is Tin Whisker Testing Performed?
Tin whisker testing is one way to determine the product’s susceptibility to growing whiskers or if any mitigation strategy is successful. Samples are subjected to three main environmental exposures for a specified period. During the exposure time, laboratory personnel periodically inspect the samples for any tin whisker growth. The three exposures are:
- Temperature cycling (see example below)
- Ambient temperature-humidity storage
- High temperature-humidity storage
NTS is fully equipped to perform tin whisker testing per JEDEC, iNEMI and any customer-specific requirements. We have many temperature-cycling and humidity chambers for various exposures. Our Scanning Electron Microscope (SEM) and Optical Stereo Microscope assist us in screening and detailed tin whisker inspection.
Screening and Detailed Inspections
NTS performs a “screening” inspection at the beginning of testing as well as at each designated time/cycle interval. This inspection will determing the potential presence of tin whiskers on the product. We use an optical stereomicroscope which provides up to 115 times magnification to perform the inspection. If no tin whiskers are observed during this initial screening, we typically will not recommend further testing. This provides a cost savings for our clients. When tin whiskers are detected during the screening, we will perform a detailed inspection.
For the “detailed” inspection, the Scanning Electronic Microscope (SEM) allows the area of interest to be examined at extremely high magnifications with high resolution and detailed depth of field. Surface structures, anomalies, and contamination can be detected and isolated for further analysis.
NTS certifies that the test equipment used during its testing complies with the calibration test purposes of ISO 10012-1, ANSI/NCSL Z540-1-1994, and MIL-STD-45662A and that the data provided in its reports are accurate within the tolerance limitation of the equipment used.