Digital microfluidic biochips (DMFBs)-an emerging technology that implements bioassays through manipulation of discrete fluid droplets-are vulnerable to actuation tampering attacks, where a malicious adversary modifies control signals for the purposes of manipulating results or causing denial-of-service. Such attacks leverage the highly programmable nature of DMFBs. However, practical DMFBs often employ a technique called pin mapping to reduce control pin count while simultaneously reducing the degrees of freedom available for droplet manipulation. Attempts to control specific electrodes as part of an attack cannot be made without inadvertently actuating other electrodes on-chip, which makes the tampering evident. This paper explores this tamperresistance property of pin mapping in detail. We derive relevant security metrics, evaluate the tamper-resistance of several existing pin mapping algorithms, and propose a new security-aware pin mapper with superior tamper-resistance as compared to prior work.