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Page 54


www.us-tech.com


March, 2021


The Effects of Reflow Soldering in Nitrogen on Common Defects


By the Staff of Rehm Thermal Systems I


n modern electronics manufacturing, it is fairly common to solder in a nitrogen atmosphere, as opposed to air. However,


there is still a large number of manufactur- ers who are not aware of the specific benefits (and challenges) of soldering in nitrogen as it relates to specific defects, such as solder balling/beading, voiding, head-in-pillow defects, and many others.


Nitrogen Atmosphere The atmosphere surrounding the


soldered components can accelerate var- ious defects if not carefully selected, but when done right, can reduce or entirely eliminate them. The Vision series of reflow soldering machines from Rehm Thermal Systems offer a nitrogen atmos- phere as an option.


Beading is the term for solder balls or beads that form when grains of the sol- der paste get under the component and then squeeze out from beneath the chip during reflow. Rehm studied this effect and every


tion of oxide films on the surface of the solder grains. This then reduces the chance that a single grain can make its way underneath the component.


Voiding is the formation of pockets within the solder during reflow, which remain after the process is finished. Better wetting, along with an inert atmosphere can help to expel


quately protected by the flux. If the protec- tive flux has disappeared from the paste grain, due to “bleeding” in the preheating area (hot slump), it oxidizes and will then remelt, but not fuse with the other elements of the paste. The collection of these oxidized masses is what resembles a bunch of grapes. Using an inert atmosphere enlarges the process window, but does not completely eliminate the influence of the flux.


Head-in-Pillow (HiP) defects are caused by an oxide layer on a BGA ball that prevents the grains of solder paste from fusing. Typically, twisting and warp- ing of the BGA or substrate causes paste deposits to separate from the ball. Then, the lack of flux activation causes an oxide layer to form on the ball, preventing it from fusing completely with the paste, even though it has been dipped back into the melted paste deposit. Soldering in an inert atmosphere,


one of the solder pastes examined showed reduced beading after reflow solder- ing in a nitrogen atmosphere. The reason for this appears to be better


Graping, a term for bunches of solder particles that failed to fuse together during reflow.


these gases more efficiently when the solder is liquified. These results cannot be general- ized to all joints, however.


and faster fusion of the individual solder grains in the paste into a uniform volume. The absence of oxygen prevents the forma-


Graping is an issue when tiny deposits of solder appear not to be reflowed. As the grain size of solder decreases, they must be ade-


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like nitrogen, can prevent this separa- tion. A little residual activation by the flux must still be present, however. Here, the inert atmosphere protects the


flux, as there are fewer oxides.


Wetting conditions under nitrogen usually lead to better spreading of the solder and the prevention of wetting defects. While this is often the case, wetting can actually be too Continued on next page


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