Rob Trudell - GSI TransZorb
TranZorb – Transient Voltage Suppressors
What, Why & How?
by Mel Clark
General Semiconductor makes millions of Tranzorb Transient Voltage Suppressors (TVS) every year. Have you ever wondered what they are, what they do and how they are used? This article along with those appearing in subsequent issues of the Curve Tracer will help to answer these questions for you.
First, what is a TransZorb TVS and how is it made? It is a two leaded solid state silicon device used for clipping voltage spikes that would otherwise damage electronic equipment, especially microchips. At the very heart of this device is the silicon die, where the action takes place. The rest of the part is packaging for protection and electrical contacts.
The die is made of high purity silicon that is “grown” in specialized crystal cylinders at our GenSil plant in Texas. There crystals, two to three inches in diameter, are sliced with a diamond saw into thin discs we call wafers. These wafers are shipped to Tempe for further processing.
Here the wafers are placed in white hot furnaces where phosphoric is diffused into one side and boron into the other. This forms a PN junction We now have the basic TransZorb TVS. By adjusting the impurity levels in the wafer, the voltage rating of the TransZorb TVS can be predetermined. After diffusion, the wafers are metallized for solderability, then diced into one of several sizes on a precision saw. Sizes range from a little longer than a pinhead to almost the size of a dime depending on the power rating. We cal these elements die. The die is next soldered between two metal discs. A TransZorb TVS at this stage of assembly is a cell. This is done in a special furnace we call the DAP. The soldering at this stage and subsequent operations is done in a high pure atmosphere of nitrogen with a small amount of hydrogen.
After solder, the cells are etched to remove the mechanical damage on the silicon left by the sawing operation, then tested on a curve tracer for a “sharp” knee which indicates a clean, stable surface.
The cells are then soldered between leads for plastic packages or on a metal disc with leads attached for the metal package. After a second test to assure “sharp knees,” the exposed silicon edge of the assembly is coated with a silicon elastomer for electrical stability. Then the unit is encapsulated.
TransZorb TVS devices destined for plastic encapsulation go to the mold room where they are placed on racks and then into mold pens which cover each cell with a plastic cylinder.
The metal package parts have a cap with an insulated tube welded to the disc assembly and a wire is welded to the tube. This package provides hermetic sealing, (protection from severe moisture exposure) and is also mechanically rugged. Most metal packaged devices go into military electronic equipment.
After molding, the plastic parts are fed into a large machine which applies a thin coat of solder on its leads for easy assembly by our customers. the metal packages are tin plated.
Now they are ready for final test. The parts are hopper-fed into special high speed testers designed and built in General Semiconductor’s Instrumentation Labs. Each part is individually tested with the operation performed at a rate of more than 3,000 units per hour.
After testing the parts are often marked and go directly to one of our customers or to stock. Some parts are stocked unmarked if there is no immediate need. this gives us the flexibility of marking the device with a customer part number as many of our products require special marking.
Now that you have a better understanding of what a TransZorb TVS is and how it is made, the next article will describe what they do and why they are needed.
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