Tin whiskers are slim metallic filaments that emanate from the surface of tin plating. As they are conductive, they can cause shorts across adjacent conductors which could result in failure.
Whiskers occurs due to the presence of compressive stress or so called “stress gradient”. This compressive stress drives the preferential diffusion of tin atoms. For whiskers to form and grow, there must be more things present, but in the absence of such stress, they will not occur.
The conditions of the surrounding environment (ambient temperature, thermal cycling, elevated humidity, and bending) simply initiates or accelerates the stresses present within the plating. It is the stress itself that should be modified, measured, and tracked over time to capture whisker behavior. The inability to capture and quantify the stresses present within the tin plating, and to clearly identify the sources of these stresses, will not prevent tin whiskers (not just mitigation), of whisker-induced failures.
The stresses that drive the tin whiskers usually are related to the following five sources:
1. Base metal intermetallic formation;
2. Base metal differences in coefficient of thermal expansion (CTE);
3. Bulk plating conditions;
4. Oxidation/Corrosion;
5. External pressure.
Whiskers occur when one or more of these sources induce stresses of a sufficient magnitude. Some of them can be fixed at the time of production or can evolve over time, which then results in an electronic circuit failure.
- Base metal intermetallic formation
Sn – Ni inter-diffusion rates are slower than Sn – Cu. The volumetric expansion of Tin over Nickel
will result in lower compressive stresses within the plating. The Sn and Ni intermetallic compound is
relatively thin, uniform, and is self-limiting due to the low dissolution rate of Ni in Sn. This morphology
does not create compressive stresses in the tin plating and actually induces a slight tensile stress.
To mitigate growth of tin whiskers, Aimtec uses tin plated Nickel pins for most of the products in its DC-
DC product line.
In a special note, Nickel under layer can help reduce compressive stress from other sources – but may
not be able to overcome it all.
Matte tin pin finishes, used in Aimtec products, “typically” have lower propensity for whisker growth.
They are “typically” lower stress finishes and have larger grain sizes.
- Base metal differences in coefficient of thermal expansion (CTE)
Compressive stresses can also arise when the base metal has a lower coefficient of thermal expansion (CTE) than tin plating and the plated component is subjected to repeated changes in temperature.
|
Material |
CTE |
|
Tin |
23 ppm/C |
|
Brass |
19 ppm/C |
|
Copper |
17 ppm/C |
|
Steel |
11-17 ppm/C |
|
Nickel |
13 ppm/C |
|
Bronze |
10 ppm/C |
|
Alloy 42 |
5 ppm/C |
Most of the commonly used based materials have a lower coefficient of thermal expansion (CTE) than the tin plating. Any repeat temperature change that increases the risk of tin whiskers formation will have to be under control over time of use.
- Bulk plating conditions
This condition is seen when using low carbon/organic content tin plating or when melting the tin.
- Oxidation/Corrosion
Because of significant differences in the diffusion rate of oxygen (O) through tin (Sn) grains and grain boundaries at room temperature, tin oxide (SnO2) will tend to grow preferentially into the grain boundaries. The volumetric expansion can result in large compressive stresses within the plating. Similarly, certain conditions can cause corrosion on the surface of the tin plating and the corrosion product will induce compressive stresses within the tin.
To prevent oxidation or corrosion, formal coating or potting are used as technics to minimize the impact of this tin whiskers formation source.
Almost the entire Aimtec product line consist of potted components and an oxidation would be possible only on the tin plated pins.
- External pressure
Common external pressure points within electronic products, include connectors (on-board and
press-fit), standoffs, card guides, washers/terminals, and separable shielding. Of particular concern is the contact pressure on tin plated flexible circuits. The high contact pressure areas are therefore the high stress points. The larger the stresses in the tin, the longer the whiskers must grow and these are the typical areas relieving stress.
Mitigations can fail and will fail if only one source of stress is addressed. For example, two of the most popular mitigations, nickel under plate and annealing, can resolve stresses due to intermetallic formation and they can also provide some mitigation to bulk plating stresses. However, they provide no assistance in regards to stresses that arise due to oxidation or external pressure points. It is believed that the inability to protect against all stresses is the reason for the tin whisker failures.
The new risk mitigation process proposal relies on two basic activities: a checklist reflecting the five sources of the tin whiskers and the related process control.
1. Are stresses due to intermetallic formation adequately controlled?
- Yes, through use of an appropriate under plate (nickel, brass and copper).
2. Are stresses due to differences in coefficient of thermal expansion adequately controlled?
- Yes, the coefficient of thermal expansion is greater than or equal to nickel (13 ppm).
3. Are stresses in the bulk plating adequately controlled?
- Yes, the plating is subject to reflow temperatures that melt the tin.
4. Are stresses due to oxidation or corrosion adequately controlled?
- Yes, the device is covered with potting material and pins will be soldered.
5. Are stresses due to external loads adequately controlled?
- Yes, the tin plating does not have separable mechanical load being applied.
Aimtec products containing lead-free plating (tin >95 percent which includes Sn or Sn alloys such as Sn-Ag-Cu and Sn-Cu), meet industry standards regarding tin whisker growth.