The described embodiments of mechanisms of forming a package on package (PoP) structure involve bonding with connectors with non-solder metal balls to a packaging substrate. The non-solder metal balls may include a solder coating layer. The connectors with non-solder metal balls can maintain substantially the shape of the connectors and control the height of the bonding structures between upper and lower packages. The connectors with non-solder metal balls are also less likely to result in bridging between connectors or disconnection (or cold joint) of bonded connectors. As a result, the pitch of the connectors with non-solder metal balls can be kept small.