While it is difficult to speculate as to why voltage data is not being reported, one possible explanation is that the manufacturers are only considering the effects of temperature on operating life and are holding the voltage constant at some level during their accelerated life tests. It may be believed that temperature has the biggest impact on operating life as long as the other parameters remain within the manufacturer’s specifications. If you believe that voltage is critical to your applications, you may want to contact the component manufacturers to determine the applied voltage stress ratios during the life tests. You can then adjust your FIT data up or down based upon your application using the inverse power law for the voltage acceleration.