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January 1997

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From:
"ddhillma" <[log in to unmask]>
Date:
Fri, 17 Jan 97 10:26:44 cst
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     Hi Gordon -
     
     Do you really put things in the freezer? I have used a refrigerator as 
     a method for extending solder paste use life but you have to take 
     precautions for condensation, volatiles loss,etc. I would like to see 
     the data that shows a tin/lead oxide growth is significantly different 
     at 23 C than at 0 C - I have doubts that all of that effort/logistics 
     of using a freezer would be beneficial. I agree with your comments 
     except for one section:
     
     "A tin-bearing finish quickly develops a protective oxide coating of a 
     few dozen nanometers (a few hundred atomic layers).There is no further 
     oxidation unless the finish is porous, no matter how long the storage. 
      The thickness will be the same if the part is stored in your desk 
     drawer, your garage at home, or in dry nitrogen."
     
     If we were talking about nickel or aluminum I'd agree but tin finishes 
     form an initial SnO structure and then over time convert to SnO2 oxide 
     structure. There have been several studies using wet chemistry, Auger, 
     and SERA methods demonstrating that the oxide changes and is not 
     "stable". Even in nitrogen storage the oxide grows (especially if your 
     nitrogen gas purity is poor) - it grows slowly put it does grow. The 
     studies have also shown that solderability of your board or component 
     is dependent your flux's ability to breakdown the type of surface 
     oxide present - i.e. SnO2 gives many flux chemistries fits whereas SnO 
     most fluxes handle easily. The problem many of us face in industry is 
     that the flux-oxide type interaction gets complicated by the thickness 
     of the solder coating - just as you pointed out. 
     
     Potatoes in solder, finishes in freezers, solder paste in 
     refrigerators, Joy in waterwashes - we should have all taken home 
     economics in school!
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
     
     


______________________________ Reply Separator _________________________________
Subject: RE: GEN: Shelf life for components...
Author:  [log in to unmask] at ccmgw1
Date:    1/16/97 11:04 PM


     G K Bhat asks about a "standard shelf life" for components, presumably 
     referring to solderability.  Since most components have a tin-bearing 
     finish, that is what is discussed here.
     
     While it is often asserted that solderability deteriorates with 
     storage, (and sometimes it really does), it is important to understand 
     that solderability _need_ not deteriorate, and parts that are twenty 
     years old will solder just fine, even without an elaborate (expensive) 
     storage environment.  In some cases where old parts are found not to 
     solder well, they weren't very solderable when they went into storage, 
     and no one noticed.
     
     To the best of my understanding, there are two conditions that cause 
     loss of solderability during storage:
     
     1. A finish that is very thin.  (See my recent TechNet posting on 
     immersion tin coatings.)  This is typically seen on such low-cost 
     items as terminals, for which the price of the finish is a significant 
     fraction of the cost of the part.
     
     2. A finish that is porous.  Plating salts remain at the base of the 
     pores and in a humid environment promote corrosion.  While in princi- 
     ple it would be possible to prevent this mechanism by storing in a dry 
     environment, a better prevention is not to accept parts that have a 
     porous coating.  Porosity is a characteristic of plating that is thin 
     or not properly controlled.  Reflow of a plated coating, or dipping in 
     solder, removes porosity.  Note that it is entirely possible for a 
     plated finish to not be porous, and in the case of a rectangular lead 
     (see below), there is a benefit to a plated finish.
     
     About ten years ago, steam aging was added to the solderability test 
     methods (IPC and military).  Manufacturers of military integrated 
     circuits found that to pass the test with steam aging, they had to dip 
     the leads in solder after burn-in. They were reluctant to do that, 
     because it was inconvenient to perform a manufacturing operation after 
     electrical test, but they did it anyway, and these parts went, in just 
     a couple of years, from the ones with the worst solderability to the 
     ones with the best.  The solder provides a pore-free finish that 
     remains solderable indefinitely without special storage provisions. (I 
     believe that the steam aging reveals a porous finish by accelerating 
     the corrosion to such an extent that the solderability is lost.)
     
     Incidentally, the issue of "oxidation" needs to be addressed, since 
     the term is so often used in discussing loss of solderability.  A 
     tin-bearing finish quickly develops a protective oxide coating of a 
     few dozen nanometers (a few hundred atomic layers).  There is no 
     further oxidation unless the finish is porous, no matter how long the 
     storage.  The thickness will be the same if the part is stored in your 
     desk drawer, your garage at home, or in dry nitrogen.
     
     The only case I know of where a part which has passed the solderabil- 
     ity test after steam aging fails it later is on rectangular solder- 
     dipped leads.  Because of the geometry, the amount of solder protec- 
     tion along the four sharp edges that bound the major and minor flats 
     is minimal.  Also, it is hard enough to judge solder coverage along 
     the minor flats (typically only about 5 mils (125 micrometers) wide, 
     let alone the edges themselves, so the people performing dip-and-look 
     solderability testing for the component manufacturers may well not 
     notice lack of solder coverage in these regions.  The wetting balance 
     is also insensitive to such cases because the area affected is only a 
     small part of the total area tested.  (A plated (unreflowed) finish 
     covers the lead much more uniformly than does a reflowed or dipped 
     finish, and for a small-diameter round lead, may be thicker than is 
     achievable by dipping.)
     
     There are three ways I have been able to think of for dealing with 
     this possible local loss of solderability:
     
     1. Store the parts in a freezer.
     
     2. Test again just before soldering and redip if necessary.
     
     3. Alter the acceptance requirements for the solder connection so that 
     wetting to the sharp edges is unneeded.  Since, as just stated, the 
     area affected by the loss of solderability is small, the loss of 
     strength due to lack of wetting to this area is also small.
     
     Gordon Davy
     
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