Failure Analysis and Reliability of Thermal Printing Devices

Silicon thermal printing devices present, very real challenges to both the failure analyst and reliability engineer. These devices are unusual in several ways. They are intended for operation of the active areas at temperatures above those normally experienced silicon devices. Thermal conductivity plays an inordinately significant role in the operation and particularly the lifetime of the device. This fact makes analysis and control of the materials thermally surrounding electronically active areas as important as the active areas themselves. The typical structurel is illustrated in figure 1 which shows the individual printing element or mesa which is used as a heating element employing current flow through the bulk resistivity of the mesa to generate heat. The mesa heat is then dissipated through the paper and through the epoxy tb the ceramic substrate. The mesas may be arranged in matrices or lines as illustrated in figure 2. In addition to the thermal and electrical characteristics of the device, device lifetime is ultimately limited by mechanical wear, i.e. abrasion against the paper. Thus the devices cannot be viewed in an electronic vacuum but must be viewed as a thermo-mechanical electronic system which puts an added stress on normal reliability and failure analysis functions.