Embodiments described herein provide techniques for testing a semiconductor package by using a diode to couple a test pad to a contact pad. In one scenario, a package comprises a die stack comprising one or more dies and a molding compound encapsulating the die stack. In this package, a substrate is over the molding compound. Also, a test pad and a contact pad are on a surface of the substrate. The contact pad is coupled to the die stack. A diode couples the test pad to the contact pad. In one example, the test pad is coupled to a P side of the diode's P-N junction and the contact pad is coupled to an N side of the diode's P-N junction. In operation, current can flow from the test pad through the contact pad (and the die stack), but current cannot flow from the contact pad through the test pad.