TECHNET Archives

January 1997

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
[log in to unmask] (DAVY.J.G-)
Date:
Mon, 20 Jan 1997 17:06:41 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (104 lines)
     Hi, Dave!
     
     I was interested to read about the studies you and Dr. Tench have done 
     on tin oxide, and hope to be able to read more after I can get my 
     hands on the publications you cited, but don't keep me in suspense - 
     I'd still like to know whether my initial statement that tin oxide 
     grows a very thin protective coating that essentially then stops 
     growing is right or wrong.  The thicknesses cited by Klein Wassink are 
     1.5 nm (nanometers) immediately, 2 nm after a week, 3 nm after a year, 
     and 6 nm after 20 years.  Even in boiling water the thickness after 4 
     hours is only 4.5 nm.  Using as a rule of thumb three atoms per nm, 
     that means that the oxide layer on any given tin surface that hasn't 
     been heated is 5 to 15 atomic layers.  At such dimensions, it is a 
     little difficult to distinguish between a surface layer of SnO2 and of 
     SnO with chemisorbed oxygen.
     
     If your studies have shown that the native oxide on tin or solder at 
     room temperature is much thicker than this, and that it continues to 
     grow, could you please provide some numbers?  A simple citation of 
     thickness vs. time would be sufficient to allow comparison with what 
     I've quoted.  Also, do you have data for the role of water vapor at 
     room temperature on the oxide growth rate?
     
     Incidentally, let me mention that a great way to study oxidation of 
     tin and solder would be by ellipsometry - a technique very familiar to 
     people who measure oxides on silicon for device fabrication.  Ellipso- 
     metry uses elliptically polarized light and measures changes in the 
     angle of polarization of the reflected light.  It does require a 
     mirror-smooth surface (irregularities smaller than the wavelength of 
     the light being used), but it is able to detect sub-monolayer cover- 
     age, and to distinguish types of oxide.
     
     I don't understand why putting something in a freezer to slow down a 
     reaction rate should sound strange.  Actually, what seems strange to 
     me is to continue to put things in dry nitrogen without getting solid 
     evidence that that helps.  Surely the cost of all that nitrogen and 
     the storage containers must be substantial.  (It also seems strange to 
     me to do vacuum baking as if the vacuum could somehow suck water 
     molecules out of solids, but maybe that's getting too far afield.)
     
     Your comment about using SERA to study what happens in a freezer has 
     me concerned that you may be confusing two different things.  My com- 
     ments about the benefits of the freezer had to do not with the rate of 
     oxide growth (remember, I claimed that the oxide doesn't grow measur- 
     ably even at room temperature and hence that there is no benefit to 
     storage in dry nitrogen), but the rate of reaction between tin and 
     copper.  I don't see how you could use SERA to study that.  If you are 
     talking about the freezer retarding the conversion of SnO to SnO2, 
     then there is no doubt that it will, just because reducing temperature 
     reduces reaction rates in general.
     
     There are two other issues that need to be considered here: 1) tin 
     whiskers and 2) tin pest.
     
     1.  Based on what I've read, I'd have to conclude that immersion tin 
     could develop whiskers, but need not.  My thanks to Yisrael Leshman 
     for passing along information via TechNet on a new immersion tin that, 
     reportedly, does not form whiskers and does preserve solderability for 
     longer than people have usually experienced.  There is another ques- 
     tion to be dealt with, though, and that is, what happens if tin whis- 
     kers do form on a printed board that then gets soldered?  Certainly if 
     the entire assembly is heated above 232 C (tin's melting temperature), 
     the whiskers will all melt (to form tin balls?), and there will be no 
     residual stresses to drive further whisker formation, so how much 
     concern is justified?  (Incidentally, in order for a wave- or hand- 
     soldered through-hole assembly to develop a complete fillet of the 
     type that people expect, the destination side of the board must rise 
     at least above 183 C (solder won't wet to a surface that is below its 
     melting temperature), and practically, it must get substantially 
     hotter than that, so if whiskers were even a theoretical risk, it 
     wouldn't take much of a process adjustment to ensure that it got above 
     232 C.)
     
     2.  My thanks to Joseph Haimovich for raising the issue of tin pest as 
     a potential problem of putting a tin-plated board in the freezer.  Tin 
     pest is the conversion from a metallic crystal structure of tin 
     (white) to a nonmetallic, powdery form (gray).  According again to 
     Klein Wassink (p.148, 2d edition), "the transformation may occur after 
     a long incubation period at temperatures below 13 C, the rate of 
     transformation being highest at approximately -30 C."  I don't know 
     what the term "long incubation period" means, and I don't know if 
     anyone has data on tin pest forming with electroless tin on copper.  
     If someone were interested, it obviously wouldn't cost very much to 
     find out, although it would take a while, and of course, the most that 
     anyone could say would be, "It didn't happen during my investigation." 
     They wouldn't be able to say, "It never occurs" (and this applies as 
     well to studies of whisker formation).  If it did occur, it seems 
     certain that that would spell the end of any protection of solderabil- 
     ity which the tin would provide.
     
     Gordon Davy

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************



ATOM RSS1 RSS2