I have an assembly that uses several different BGA packages. During troubleshooting we have found that some of the balls in the corners of the BGA's are separated from the underside of the package. The PWA is approx 10x10 and the components vary from the edge to 1.5 from the edges. They have been installed into the end item then removed several times for updates. Right now I am leaning in the direction that these failures are being caused by the flexing of the PWA during the installation/removal from the end item. Has anyone else seen this? I am thinking that maybe we need to design stiffeners onto the PWA? How about under fill, would that help?