Gideon Analytical Laboratories received several shorted PX6A07 silicon rectifiers for failure analysis. Two good devices were provided for reference and comparison. A semiconductor diode is a crystalline piece of semiconductor material with a p-n junction connected to two electrical terminals. It conducts in the forward direction and is blocked in the reverse direction.
The good diodes were tested for conformance to the manufacturer’s specification. The good diodes performed per specification or exceeded electrical parameters listed. The failed diodes were electrically dead shorts.
The diodes were decapsulated to expose the inside diode chip sandwiched between the copper anode and cathode lead frame. The die is soldered to the lead frame and has a silicone RTV surrounding the exposed die edges.
The first step is removing any residual silicone and inspecting the die edge for shorts, which may include solder reflow, melted silicon and cracked die. In the left picture, the hexagonal die has one side with extensive damage to the die. The right-hand picture has a sliver of silicon missing from the side.
This is a picture of the anode. In the middle is the copper contact and the ring is actually the contact via solder to the P+ region of the die. There is a glass oxide extending from the sides of the die inward to meet the P+ region. Under the glass oxide, there is a P-type guard ring(s), which do not invert when the P+ (anode) is powered. These isolated diffused P rings break down if the voltage is excessive (beyond the specification). The P rings prevent the voltage from extending laterally to the die edge.
Both have breakdown defects in the guard rings.
This die also has a breakdown in the guard ring area. The four rectifiers had damage in the guard ring area. This indicates the devices were exposed to excessive voltage.
Gideon Analytical Laboratories can determine the root cause using several techniques. If you have failures and want to know how it failed, call us. We are the “Problem Solvers of the Electronic Industry”.