Bump-on-trace structures with high assembly yield

A package includes first package component, which further includes a first metal trace at a surface of the first package component, with the first metal trace having a trace width measured in a direction perpendicular to a lengthwise direction of the first metal trace. The first package component further includes a second metal trace at the surface of the first package component. The first metal trace and the second metal trace are parallel to each other. A second package component is overlying the first package component, wherein the second package component includes a metal bump. A solder region bonds the metal bump to the first metal trace, wherein the solder region contacts a top surface and sidewalls of the first portion of the first metal trace. A ratio of a volume of the solder region to the trace width is between about 1,100 μm2 and about 1,300 μm2.