"Solder ball" formation in silicon alloy transistors

Abstract

The authors have found that formation of "solder balls" may not be due to any foreign matter in the package but instead be a characteristic of this type of device when: a) Local hot spots develop as the result; b) temperatures rise in excess of 23O°C,; c) during life testing or "burn-in." The devices which were extensively studied in this investigation were highpower silicon alloy-diffused transistors and high-reliability medium-power silicon alloy transistors. The five conclusions reached from this work are the following. 1) Balls are formed as a result of temperature in excess of 230°C (melting point of tin). 2) These temperatures are reached either by electrically induced hot spots or by external heating of the device. 3) Analysis of the balls has shown it to be 99% tin, which is the main ingredient of the emitter and base regions. 4) Electrical parameters initially are not degraded by formation of balls. 5) The balls are physically attached to the device until mechanically jolted loose. The ball problem has been found in this type of silicon alloy transistor and could be a principal mode of failure in this device. Balls have not been found in other types of silicon transistors, such as planar or mesa transistors.