Tin Whiskers

The drive to eliminate lead (pb) from electronics has resulted in an interest in the use of pure tin (sn) finishes as an economical lead-free (pb-free) plating option. the re-emergence of tin-plating has renewed concern over the threat of failure due to tin whickering. What are Tin Whiskers?

Whiskers are thin fibers of tin that grow apparently spontaneously from electroplated tin surfaces. They are elongated single crystals of pure tin that have been reported to grow to more than 10mm (250 mils) in length. Tin whiskers can cause short circuit and have caused several satellites, missiles, heart pacemakers and a nuclear power station to fail. Whiskers grow spontaneously without an applied electric field of moisture (unlike dendrites).

Of cause, the best strategy to prevent tin whisker induced failure of electronic hardware is to avoid using pure tin plating on any of the parts used in the construction of the electronic hardware. Electroplated tin coating are used on most components terminations to aid soldering and to provide corrosion resistance, only long whiskers cause failures.

Tin whiskers are a real threat, however, their caused are understood. To avoid Tin Whiskers, avoid components from sources you do not trust, Do not use SnCu plating but SnAg is OK. if custom parts are to be plated with tin for corrosion resistance – Use nickel barrier layer and matte tin.

Hot solder dip (HSD) is the process of dipping the components terminations in molten solder. It used as a final finish, and replacement finish. The hot solder dip process can reduce the tin whisker opportunity if you found the caused is from the components plating.

Hot Solder Dip

Hot solder dip typically involves five steps: (a) flux, (b)preheat, (c) solder, (d) cool down, and (e) clean

A. Flux: The flux performs two important functions in the solder dip process. First, it removes oxides from the solder and the surface of the components. Second, it displaces oxygen, thereby preventing re-oxidation of the surfaces.

B. Preheat: Preheat serves three functions. First, it removes the volatiles from the flux. This is especially important for water-based fluxes that would otherwise cause splattering. Second, it activates the flux, and third, it minimizes thermal shock. Fluxes used for solder dip become very active at elevated temperatures. However, above 150°C, most of these fluxes break down prematurely and do not function properly during the solder step. There are several types of preheat methods, the most common of which are hot air (forced convection) and infrared. The preferred method is forced convection where heat is applied evenly to all surfaces.

C. Solder: In the solder step, most solderable finishes (e.g., tin, gold, etc.) are replaced with the alloy composition of the solder pot. There are two types of solder pots: dynamic and static. In the former, an oxide-free standing wave is produced by using a solder pump. In the static solder pot, some provision is made to remove the solder dross (oxides). A solder temperature that is too low can lead to poor wetting and artifacts related to increased surface tension such as icicles, bridging, and inconsistent thickness. On the other hand, a temperature that is too high can lead to thermal damage or excessive thermal shock.

D. Cool Down: Cool down is used to reduce the component temperature prior to clean, to prevent thermal shock. Cooling can be accomplished in several ways, either by natural convection, forced air-cooling, or a combination of both methods.

E. Clean: Cleaning is used to remove residual flux from the component that could cause corrosion. Hot deionized water is typically utilized.

HSD process and boundary conditions used in the model for Case 3.

Step Temperature of Exposure Duration (sec)

Mode of Heat Transfer Preheat 150°C 4 Uniform forced convection heat transfer in air.

Solder 245°C 3 Conduction heat transfer for the edge being solder dipped (leads and the edge of the package) and natural convection heat transfer (at 30°C) for the balance of the component.

Clean 60°C 10 Uniform conduction heat transfer in DI water.

Tel: 310-715-1898

steve@boardassembly.com

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