When you hear the term NWO, you might think of late 1990’s professional wrestling. However, in the world of soldering, NWO has its own unique meaning.
NWO stands for “Non-Wet Open.” An “open” occurs when solder has not effected a physical connection/bond between two items (pads, leads, terminations, etc.). “Non-wetting” occurs when the conditions (materials, atmosphere, process, metallizations, etc.) are not suitable to effect a true solder bond. An open caused by non-wetting is a non-wet open.
This logical thinking leads down the right path, but does not tell the whole story, of what this particular term actually represents. It is probably easiest to start by thinking about another, more commonly known defect: Head-in-Pillow (HIP). With the HIP defect, solder paste is printed onto the PCB pads where a BGA component will be placed. If the BGA warps (smile warp) during reflow and pulls the ball away from the reflowing paste, it is possible that both ball and paste become so oxidized before coming back together (frown warp) that the solders are unable to coalesce into one another. The ball sits on top of the reflowed, oxidized solder paste as a head would lie on a pillow.
Now let’s consider a different scenario. If the component warped and lifted the sphere, and, if the paste went with it, leaving the board pad free of solder, the paste and the solder ball would coalesce during reflow since they are already in contact. Once the component warps back (frown warp), the solder would not be able to wet to the pad – and an open joint would be created. If this joint is pulled apart, the pad will look like clean copper and the component will have a large sphere on it. I mention the crisp copper pad because this defect only happens with OSP coatings. It is not seen with other finishes such as HASL or ENIG.
In summary, a NWO happens between BGA-type components and OSP-coated pads. When the component warps (smile warp), it pulls the solder sphere away from the board pad and takes the paste with it. When the component frown warps, the solder is unable to wet to the pad, leaving an open joint.
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