A plurality of test pads are formed on a first substrate or a second substrate. A plurality of first solder joints are reserved on a first surface of the first substrate, and each of the first solder joints is coupled to at least a test pad or to another first solder joint through at least a first trace. A plurality of second solder joints are reserved on a second surface of the second substrate. Each of the second solder joints is coupled to at least a test pad or to another second solder joint through at least a second trace. A plurality of dummy solder balls are formed between the first solder joints and the second solder joints. Probes are coupled to the test pads to measure circuit characteristics between the test pads.