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Tin Whiskers (Page 2 of 2)
A common and widely-held explanation for the formation of whiskers states that the phenomenon is a stress-relief mechanism. According to this theory, the tin layer or deposit becomes subject to internal residual stresses once the tin plating process is completed. These residual stresses are reduced by whisker formation. The origin of these internal stresses is discussed in the next paragraphs.
As soon as Sn is deposited over the copper leadframe, an oxide layer starts to form over the deposited Sn layer. At the same time, Cu atoms from the substrate start to diffuse into the Sn layer, forming Sn-Cu intermetallics. Since the oxide layer over the Sn coating impedes outward movement of the Sn atoms, this process of Sn-Cu intermetallic formation builds up internal compressive stresses within the Sn layer as more Cu atoms diffuse into the same volume of the Sn coating.
Eventually the stress becomes too large that excess Sn material begins
to extrude at the weakest points of the oxide layer. This protrusion
originates as an Sn nodule over the oxide, which eventually grows into
an Sn filament or 'whisker', as it is more commonly referred to.
Figure 2. Photo of a 'bend' whisker (left) and a 'nodule' whisker (right); source: http://nepp.nasa.gov
This process has been known to be driven by the following factors: 1) application of internal and external (compressive) stresses, e.g., trimming and forming of the leads ; 2) thickness of the coating; 3) structure of the crystal; 4) substrate used; 5) temperature; and 6) humidity. Thus, the presence of contaminants does not directly induce the formation of whiskers, unless it results in a change to any of the factors above.
The risks
posed by whisker formation generally fall under four categories: 1)
stable short circuits in low-current circuits (low voltage, high
impedance); 2) transient short circuits; 3) metal vapor arcing, wherein
the whisker is turned into a highly-conductive plasma for several
seconds (if the conditions for sustaining the metal vapor arc are met),
consuming adjacent materials as it conducts hundreds of amperes; and 4)
debris or contamination.
Tin whisker formation may be avoided by not using pure tin in the lead finish process. Adding about 3% of Pb by weight to Sn greatly reduces the occurrence of whiskers, while using 5% Pb virtually eliminates it. Then again, the industry is moving away from the use of Pb, so other Pb-free lead finish alternatives must be explored. Please see the article "A Pb-free Semiconductor Industry" for more on this.
The industry has yet to come up with a single standard for acceptance testing of lots in relation to whisker formation. Recently, however, the National Electronics Manufacturing Initiative (NEMI) has offered the electronics industry a revised set of proposed recommendations for "tin whisker acceptance test requirements and acceptance criteria for evaluating devices with tin finishes."
The revised acceptance test requirements have already been submitted to both IPC and JEDEC for approval and subsequent release as a formal standard or guideline for tin whisker acceptance testing for the industry.
See also: Lead Finish; Pb-free Manufacturing
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