An improvement of joint reliability between Sn-yAg solder and Ni\u2014P under-bump metallic layers is achieved by cobalt addition. In one arrangement, the method can include the steps of receiving a first call from a caller, receiving a subsequent second call from the caller, and if the second call meets a predefined parameter, increasing an alert level to provide an indication or urgency or to locate the communication unit. The optical sensor unit includes a light-emitting element to generate light output and a light-receiving element to collect light outputted from the light-emitting element. The device is useful in a variety of applications, and most notably, nucleic acid based diagnostic applications and de novo sequencing applications.