I find it interesting that in the article I didn't notice any comments about
physical stresses such as component lead forming or residual stresses. I
thought I'd read somewhere that they increased the propensity for tin
whiskers.

KennyB

-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask]] 
Sent: Thursday, June 12, 2008 12:21 PM
To: [log in to unmask]
Subject: Re: [TN] Tin Whiskers - how long? Size matters!

Hi Graham!

Below is a cut and paste from:

http://nepp.nasa.gov/WHISKER/background/index.htm#q5

Steve

What are the Commonly Reported Characteristics of Tin Whiskers?

The vast disparity in the observations reported by different experimenters
is evidence of the complications associated with understanding and
controlling tin whiskers.  The following list is intended to provide a very
basic overview of some of the observed characteristics of tin whiskers.

 

Shapes:  Whiskers may be straight, kinked, hooked or forked. Their outer
surfaces are often grooved.  Some growths may form as nodules or pyramidal
structures. 

Incubation (Dormancy) Period: Experimenters report the incubation period may
range from days to years.  This attribute of whisker growth is particularly
concerning because meaningful experiments to determine the propensity for a
particular process to form whiskers may need to span very long periods of
time. 

Growth Rate: Growth rates from 0.03 to 9 mm/yr have been reported.  Growth
is highly variable and is likely to be determined by a complex relationship
of factors including plating chemistry, plating thickness, substrate
materials, grain structure and environmental storage conditions. 

Whisker Length:  Whiskers as long as a few millimeters are not uncommon with
some experimenters observing whiskers as long as 10 mm (400 mils) in length.


Whisker Diameter:  Typical diameters are a few microns with some reports as
large as 10 um 

Environmental Factors:  There is a great deal of contradictory information
regarding environmental factors that might affect whisker formation.
Several organizations are attempting to devise accelerated test methods to
determine a particular plating process's propensity to form tin whiskers.
However, to date, there are no accepted test methods for evaluating whisker
propensity.  Indeed, much of the experimental data compiled to date has
produced somewhat contradictory findings regarding which factors accelerate
(or retard) whisker growth.  Temperature: Some experimenters report that
ambient temperatures of approximately 50°C are optimal for whisker
formation, while others observe that room temperatures (22°C to 25°C) grow
whiskers faster.  Reportedly, whisker growth ceases at temperatures above
150°C
 
 Pressure:  Whiskers will grow in vacuum as well as earth based atmospheric
pressure.
 
 Moisture: Some observe that whiskers form more readily in high humidity
(85% RH) whereas others report moisture is not a contributing factor
 
 Thermal Cycling:  Some experimenters report that thermal cycling increases
the growth rate of whiskers, but others report no effect due to thermal
cycling.
 
 Electric Field:  Whiskers grow spontaneously without requiring an applied
electric field to encourage their growth.  Some recent observations of tin
whisker induced field problems in the commercial sector seem to suggest that
an electric field could stimulate whisker growth, but more analysis is
required to confirm these effects (if any).  GSFC has demonstrated that
whiskers can bend due to the forces of electrostatic attraction thus
increasing the likelihood of tin whisker shorts (ref. to GSFC experiment
#4).
 


Whisker Prone Processes:  There is tremendous debate in the industry
regarding which plating processes are prone to whisker formation.  Most of
the literature agrees that "pure tin" electroplated surfaces (especially
those that employ brighteners in the plating process) are the most
susceptible to whisker formation.  There are also reports that tin-lead
plating can also grow whiskers; however, such whiskers are generally
reported to be less than 50um long. 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: Thursday, June 12, 2008 2:13 PM
To: [log in to unmask]
Subject: [TN] Tin Whiskers - how long? Size matters!

Good day TechNet
A tin whisker question.  

If (hypothetically, as this thankfully hasn't been built yet) a pure tin
finish part is used, what sort of length whisker could happen?

What we are looking at is this: a part we want to use is only available with
a pure tin finish on copper.  Matte tin, not bright.  So if we use it, and a
tin whisker grows, how long might it grow?  If the part is (for example)
10mm away from adjacent parts, and a realistic maximum whisker length is
less than that, it obviously mitigates (YES, but does not entirely remove)
the risk.

The only value I've seen is in a Dave Hillman/Doug Pauls presentation from
Apex 05, wherein it is stated that they "can be 9.5mm long".  Dave /  Doug,
any comment on this?  Can you cite a source?  Does matte vs bright tin make
a difference?

regards,
 
Graham Collins
Halifax Production Engineering
(902) 873-2000 ext. 6215

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