Tin Whiskers: Avoid Stubble Trouble

Jan. 11, 2007
A Q&A session on how to deal with the phenomenon of tin whisker growth.

The tin whisker phenomenon is a failure mode associated with all electronic devices that use a number of low melting-point elements (e.g., Sn, Cd, In) in operations such as soldering. Recognized many years ago, the problem was minimized by adding lead, which of course is now quite rightly identified as a hazardous and consequently banned substance.

This article addresses questions concerning tin whiskers, and, in particular, Vicor’s work to accommodate the no-lead requirement. This work resulted in the adoption of strategies that mitigate the impact of the tin whisker phenomenon on Vicor power products, when tin finishes are used.

Why are tin whiskers causing such concern?

Tin whiskers are a concern because they are conductive, can carry high current, and because Pb, the material that has been used for the last 50 years to limit whisker growth, is being removed from use in most electronic devices.

Where do whiskers grow?

Whiskers will grow from copper- containing substrates finished with most low melting-point elements, including Sn, Cd, In, Zn, and Sb. New research shows that whiskers will even grow from Sn-Pb surfaces, but the length of those whiskers is much shorter due to the presence of Pb.

Will whiskers grow from any copper substrate coated with other RoHS-compliant solderable finishes?

Whiskers do not grow on solderable Au, Pd, or Ag finishes.

What conditions will whisker growth occur on Sn finishes?

Certain conditions promote whisker growth, including:

  • Thin plating layers
  • Residual stress within plating
  • Intermetallic compound formation
  • Diffusion (enhanced by temp)
  • External stress (compressive)
  • CTE mismatch
  • Surface oxide
  • Electrical bias

What role does Pb play in preventing tin whisker growth?

The best explanations from the literature state that Pb slows the diffusion of Sn in the matrix and allows for rapid stress relaxation. Since diffusion is needed for whisker growth, slowing the diffusion rate of Sn in the matrix or along grain boundaries by introducing Pb will slow or prevent continuous whisker growth.

Besides using Pb, are there other elements that can be used as effectively as Pb?

No other elements have been shown to be as effective as Pb. Some researchers have tried Bi and Ag, but with mixed results. The use of SnBi, SnAg, or SnCu platings in products is being avoided. However, increasing the grain size of the plating or controlling the grain orientation has been shown to slow the rate of diffusion.

If a Pb substitute can’t be used with confidence or good economies, what’s the best method a plater can use to limit whisker growth on Sn finishes?

The plater can use advanced Sn plating processes that produce platings with unique properties, such as large grains or preferred grain orientation, barrier layers, or post plating treatments.

Why are thin platings most susceptible to whisker growth?

Thin platings will develop greater compressive stresses than thick platings.

Why do intermetallic compounds (IMCs) make for whisker growth?

Non-uniform growth of CuxSny IMCs increases the local compressive stress in the rest of the tin plating. Conversely, NiSn IMCs produce a tensile stress and will reduce the stress on the Sn plating.

Is it true that whisker growth will be highly accelerated by lead forming operations due to compressive stresses?

Recent studies by lead-frame manufacturers and semiconductor packagers have found no increase in Sn whisker growth when using preferred Sn chemistries. Furthermore, because Sn recrystallizes at room temperature, mechanically worked grains will relieve stress and grow, thus eliminating the risk of whiskering.

Why are large grains effective in reducing whiskering?

Whiskering is a diffusion-controlled process, and diffusion rates are very grain-boundary (GB) sensitive—the greater the number of GB, the faster the diffusion rate and whisker growth rate. IMCs also form preferentially along grain boundaries. Increasing the grain size of the plating will reduce the number of grain boundaries, slowing the diffusion rate. In addition, fewer GB mean fewer places for non-uniform IMC growth and reduced stress in the system.

What experience does Vicor have in using components that are finished with 100% Sn plating or SnAg(Cu) solder dipping?

Vicor has been using ceramic components with 100% Sn finishes for more than three years without whisker-related field returns and semiconductor plastic-molded packages with 100% Sn leads for more than two years. Leads on magnetic components finished with SnAg have been in production use for three years without field returns.

What whisker testing has Vicor performed on 100% Sn finishes?

Vicor conducted initial trials on 100% matte Sn in 2003 before extensive data was available. In that testing, large grained 100% matte Sn plating on Cu lead frames resists whiskering even when the plating is put in compression and at elevated and humid environments. Bright Sn was unacceptable with whisker growth, reaching 700 μm observed on samples stored at ambient temperatures. Compressed matte Sn whisker growth was 10-20 μm.

What has Vicor seen when performing grain size examination?

Simple SEM examination has shown that not all matte Sn plating is the same. It may look matte, but may not be whiskerresistant due to the presence of small grains or other properties. Figures 1, 2, and 3 show original SEM images that were taken at 4000X revealing grains of either 1-2 μm (less desirable) or >6 m (preferred).

Will Vicor use Sn plating on pc boards?

No. Vicor PCBs currently using ENIG (electroless nickel immersion gold) will continue that way. SnPb hot-air solder leveling (HASL) product will be finished with either ENIG or Immersion Ag or HASL (SAC Alloy).

Other than using matte Sn chemistry or reflowed Sn, or reflowing Sn plating in the fabrication process, does Vicor use any other techniques to mitigate or control whisker growth?

Yes. Where possible, Vicor uses Ni barrier layers under Sn plating. A minimum of 2 μm is requested based on iNEMI and plating chemical supplier recommendations. Sn plating is annealed after plating for 1 hour at 150°C to promote controlled IMC formation and reduced Snwhisker risks. Vicor pins are subjected to Pb-free reflow during surface-mount operations, further reducing the risk of whiskering.

Other than using matte Sn chemistry, or Ni barrier underplate, or post plate treatments, or reflowing Sn plating in the fabrication process, does Vicor use any other techniques to control whisker growth?

Yes, conformal coat. Conformal coatings or epoxies have been suggested by many as a way to reduce whisker risks. Vicor already uses a type of thick conformal coating in its brick products as part of its thermal-management strategy. This silicone-based polymer is 90%-92% filled ceramic particles, which adds further resistance to whisker growth.

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