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
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
