Reliability Improvement in Pulse Transformers

As part of the NASA Orbiting Geophysical Observatory (OGO) program, an investigation was made of failure mechanisms in miniature pulse transformers, a type commonly used for drive pulse coupling in low-power digital circuitry. The resulting corrective action methods are useful for reliability improvement in other similar magnetic devices and components. Failures were traced to breaks in small diameter magnet wire. Examination showed two possible causes of failure: (1) mechanical stressing of material during assembly and (2) metallurgical stressing of the fine copper wire during soldering. Preventive action was taken to eliminate accidental mechanical stress. To avoid metallurgical changes during soldering, a process specification was developed to prevent gross solution of the solder. This was accomplished by close control of soldering temperature, wider distribution of heat at the joint, and control of the time material is held at soldering temperature. Tests verified the effectiveness of these process improvements. No failures have been experienced in subsequent manufacturing.