Date: Fri, 15 Nov 1996 13:32:00 EDT From: "Foster, Donald C." <[log in to unmask]> Subject: Re: ASSY: BGA SOLDERABILITY To: 'Technet' <[log in to unmask]> MIME-version: 1.0 Content-type: TEXT/PLAIN; CHARSET=US-ASCII Posting-date: Tue, 19 May 1998 00:00:00 EDT Importance: normal A1-type: MAIL No bites on this one yet so I'll give it a try. I don't think anyone has got a good handle on this one yet, however the solderability is much more robust than leaded components so the lack of solderability spec hasn't cause major heartburn yet(?). This issue also depends on whether you are using all eutectic balls or dual alloy structure (CBGA). With all eutectic solder balls, any oxidation (even very thick ones) will be broken up and displaced when the joint collapses during reflow, even if the flux doesn't have enough activity to reduce all the oxide. And obviously none of the standard methods used today would work. And I don't think anyone is ready to use ellipsometry methods to measure sub-micron oxide thickness' yet and I would have to guess the effectiveness would not be that great. The only risk is if a large non-wetting oxide particle is trapped at the Cu pad interface which will act as a starting crack reducing the fatigue life of the joint. My past experience (at another employer) with dual alloy structures indicates they are very robust also. When we were attempting to define solderability specs, we tried using steam aging to produce poorly solderable parts. We had a very difficult time trying to produce unsolderable parts. At the time, we were looking a tensile pull methods for evaluating solderability, but this proved to be an unreliable method due to the large scatter inherent in the method and the lack of producing parts with known poor solderability. (The tensile pull method we looked at mimicked the board attach process using WS-605 flux, pretty active stuff). We never came up with a spec. I did have one experience that was very bizarre which lead to a high temp tensile pull method that allowed examination of the 90/10 ball , eutectic interface without the use of metallurgical x-sectioning or C-SAM methods. We noticed a growth of contaminant on the solder balls that was a result of tray outgassing (due to improper processing of the tray) and a catalytic interaction of the ball surface. It took about 3 weeks to grow the material to a point in which it cause solderability problems. The main problem was poor wetting and outgassing which caused excessive voiding. If you solder to the ball using a tensile pull stud and pull at temps above 100C at low strain rates, you can fracture at the ball / eutectic interface. One can directly see the level of voiding in the eutectic part of the joint and can see non-wetted parts of the solder ball. I left this employer before this work was finished, but I don't think it ever developed into a spec. Don Foster Symbios Logic [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. * ***************************************************************************