What are Tin Whiskers?

Soldering PCB Surface Finishes 

What are Tin Whiskers in PCBs? What causes them? How can they be avoided?

1 Answer
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Editorial Team - PCB Directory

May 20, 2020

Tin Whiskers are conductive, very thin filament or needle-like structures that protrude from a PCB surface having Tin (Electroplated Tin) as final surface finish. Their incubation period is long and may grow from 1 mm to 10mm. They can cause short-circuits which can result in circuit or system failure.

Tin whiskers are conductive and carry high currents, which is not at all acceptable. The PCB industry is working towards minimizing the use of lead-based materials in plating. So, Tin-based alloys are being adopted on a large scale, but they come with some threats. One of them is Whiskering. With its advantages in solderability, manufacturing and compatibility with existing assembly methods, pure tin plating has been considered as a potentially simple and cost-effective alternative to lead-based materials.

All the factors responsible for ‘Tin Whiskers’ are not clear yet, but some theories suggest that they are the result of stress relieved, say compressive stress within the Tin plating. Some potential factors responsible for Whiskers could be residual stresses within the Tin plating, intermetallic formation (intrusion of substrate material into the Tin plating or Vice-versa), externally applied stress on the board, and CTE mismatches. Other factors like temperature, pressure, moisture, thermal cycles are also known to be responsible for the growth of tin whiskers.

Key Identifiers to validate Tin Whiskers

  • Shape: They may be straight, hooked, forked, or kinked in appearance.
  • Growth Time: They take several days to finally become visible or maybe a year in some cases.
  • Length: Their length may vary from a few mm to up to 10mm.

Is it possible to avoid Tin Whiskers?

It is tough to predict Tin Whisker growth however, this can be avoided using pure tin plating. The use of tin-alloy material for plating is recommended, even some military applications prohibit the use of pure tin plating. According to recent studies, alloying tin with a second metal mitigates the chances of whisker growth. Alloys of tin and lead are considered to be acceptable, where the alloy contains a minimum of 3% lead by weight. The whisker growth in a PCB can be partially prevented by following these measures:

For PCB:

  • Avoid tinning of traces (no tin – no tin whiskers)
  • Use electroless nickel immersion gold (ENIG), electroless silver or nickel as a surface finish.
  • Also, treat solder pads with ENIG, electroless silver or nickel.

For Components:

  • Use flat IC packages without bent terminal pins to avoid mechanical stress.
  • Use nickel, silver or gold-plated terminals.

For Soldering Process:

  • Use SnAgCu alloy as the solder.
  • If pure tin is used as the solder, use matt finish rather than gleam finish as it prevents whisker growth.
  • Expose the populated PCBs at 150 °C for about one hour right after the soldering as it eliminates internal stresses from the tin.
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