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 *************************************************************************** * 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] * *************************************************************************** *************************************************************************** * 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] * ***************************************************************************