Temperature humidity bias (THB) test is widely used to evaluate the moisture resistance of non-hermetic packages in semiconductor industry. During THB test, one kind of 90FBGA was found severely burned and the evidence has been completely destroyed. This brings a great challenge to failure analysis. In this paper, a double daisy chain structure substrate was designed to reproduce the short and burn failure. The substrate layout design, bias and high humidity environment were proved to be the three key factors inducing dendrite growth and burn failure. A “corner-missing” phenomenon inspected through nano-focus X-ray was reported and it could finally verify the theory of electrochemical migration. The countermeasure to prevent burn failure was proposed to the designers. The insulation property degradation due to THB test was evaluated.