Reliability in transistors is being greatly enhanced by the combined use of automatic testing systems, and data-processing and computer techniques.
In the evolution of automated testing, two systems have been introduced—one specifically designed for high-frequency transistors, and the other for power transistors. The former is called Mascot (Motorola Automatic Sequential Computer Operated Tester), and the latter Prompt (Parameter Recorder of Minuteman Power Transistors).
According to Motorola, Mascot can process with one operator a total of 56,000 transistors in a two-shift day. This processing includes testing, selection, and sorting of transistors with frequencies as high as 300 mc, and with switching times as low as a few nanoseconds. The heart of the system is an IBM 1620 computer with a transistorized core memory and a 20,000-decimal-digit capacity. So far, says the company, Mascot has processed more than 3,000,000 transistors without serious operational problems.
In processing uhf transistors, Mascot uses six principal subsystems (see the figure):
- A mechanical transfer system that carries the transistors to as many as 40 test modules.
- Test modules with circuitry testing various parameters of the transistor.
- Analog-digital converters.
- A line scanner that sends digitized outputs to the IBM 1620.
- A core memory that compares test results to specifications.
- A sorting mechanism that classifies transistors.
The most time-consuming operation required by Mascot is the placement of the transistor on the transfer board. After this, Mascot speeds up considerably, completing all mechanical test functions in 0.6 sec. The connecting leads of each module are gold plated to ensure good low-resistance contacts with the three leads of the transistor.
Mascot monitors its own operation in that an erroneous reading will stop the testing operation. The computer will then type out the number of the test module that is out of calibration.
The other automated system, Prompt, was developed by Transitron Electronic Corp. for life testing of power transistors used in Minuteman. If preprogrammed test systems are used, Prompt can process 1800 transistors an hour. This rating applies only to low-level devices. Usually, Prompt will operate at a rate of 180 to 240 transistors per hour. It must be emphasized that these rates are relative to the life test being given.
Five environmental and power cabinets are used in Prompt, each containing 12 power-distribution frames and 12 temperature-control plates. Temperature values are constant for each cabinet, while power conditions vary for each temperature-control plate. In this manner, Prompt can test, simultaneously, transistors with different temperature values and 60 power conditions programmed into the cabinets.
Before testing, various transistor parameters are measured and stored on tape in Prompt's data-processing equipment. The transistors are then mounted onto the temperature-control plates. Power levels vary from 1 to 60 W in each plate and are controlled by the power-distribution frame, which introduces varying currents and voltages to the temperature plates. Transistors are permanently mounted to the temperature-control plate, thus minimizing heat-sink temperature gradients. (Electronic Design, Dec. 20, 1961, p. 12)