Tin Whiskers

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• Tin whiskers arise from stresses in the plating and are known to occur on tin plated or tin-alloy plated parts.
  
  
• After plating, an incubation period of several thousand hours may be required before any whiskers begin to grow. The length of this incubation period depends on the lead finish thickness, lead finish grain structure, and on the base metal composition.
  
  
• Tin whiskers are a reliability concern throughout the industry due to the fact that they may bridge the gap between leads causing electrical shorting.
  
  
• As a whisker mitigation measure, National anneals all of its tin plated, leadframe-based packages for 1 hour at 150° C within 24 hours of plating. This is the industry-accepted method for controlling whisker growth.
  
  
• Since thin plating is known to be more susceptible to whisker growth, National maintains a minimum tin plating thickness of 8 microns, with a nominal thickness of 12 microns.
  
  
• iNEMI proposed a whisker test method in 2003. Since that time JEDEC has developed a test method which is based largely on the iNEMI proposal. This JEDEC test method passed ballot and was released in May 2005 as JESD22A121. The JEDEC acceptance criteria JESD201 was released in March 2006.
  
  
• National started whisker testing in mid-2004, prior to the arrival of the JEDEC test method.  The same basic tests were followed as the following chart shows:

• The following table summarizes National's whisker test results based on the iNEMI proposal:
  
  

• For the 60° C/93% RH and the TMCL tests, the reflowed legs are the most critical. This is because the parts would not likely experience these extreme conditions until after being mounted on the PCB.
  
  
• The current iNEMI recommendation for maximum allowable whisker length is 50 microns.
  
  
• Tamac 2 lead frames are only used for TO-220 and TO-263 lead frames. The smallest gap between leads for any of the TO-220 or TO-263 packages is 457um for the TO-263 9L package.  Since 69um (the measured whisker length at 4000 hr 60° C/93%RH, following 2x reflow) is significantly less than 50% of the gap between leads, whiskers are not expected to be a problem on this package.
  
  
• In terms of lead-free requirements, the current RoHS Directive exempts leadframe packages with pitches less than or equal to 0.65 mm.
  
  
• More recently, the following data was collected per the JEDEC requirements given in JESD201:
  
  

Whisker Test Results per JESD201

Packaging | Lead-Free Status | Impact to Customers | Tin Whiskers
Green Compliance | Industry Consortia | Questions & Answers | Literature