"Hermetically sealed" implies "air tight", and is usually taken to mean "liquid proof" as well.
What's the difference between an epoxy diode and a glass diode?The terms "epoxy" and "glass" refer to how a diode p-n junction is passivated. Without passivation, and because the distances involved are so short, any applied voltage across a p-n junction would cause it to arc in an unpassivated device.
Passivation is required to prevent internal arcing. In a glass passivated diode, commonly referred to as 'glass diode', or 'glass-body diode', a glass slurry is applied to the diode junctions. The device is then fired at a high temperature to solidify the glass. During the firing process, the glass shrinks and bonds to the surface of the silicon wafers. Once the glass is fired, the diode is hermetically sealed and passivated. It can survive a high reverse voltage without arcing.
An epoxy diode uses a specially adapted varnish capable of withstanding high temperatures and high voltage to passivate the diode p-n junctions. The varnish is applied to the p-n junctions. Once the varnish has dried and cured, the assembly is encapsulated in a rigid epoxy. At that point, the diode can be tested, and a high reverse voltage applied.
Hermetically Sealed AdvantagesThe primary advantage of using a hermetically sealed diode is that it is impervious to liquids and gases. If it wasn't, chances are a penetrating liquid or gas would short out the diode junctions and the diode would not be able to block reverse voltages.
When installed on a printed circuit board (PCB) that is immersed in cleaning solvents, a hermetically sealed diode offers a significant advantage over a non-hermetic device. Unsealed components often require a bake-out after the cleaning process. Sometimes even a bake-out, where the PCB is subjected to dry heat at temperatures high enough to drive off all liquid solvents, is not enough to salvage an assembly. Solvents are used to clean excess solder or flux residue from printed circuit boards. A hermetically sealed device will not absorb the solvent.
Why is a Hermetic Seal important?In the case of a diode, the glass passivation also serves to protect the junctions. If the diode absorbed liquid, and it penetrated deep enough into the device, the liquid could short out the junction so that when a reverse voltage was applied, the diode would not block it. The diode would appear to be shorted.
|K-Body High Voltage Diode|
DisadvantagesThe main disadvantage of a hermetic device is they generally have lower voltage and current ratings than a comparable epoxy encapsulated diode. The K50UF, 5kV diode is rated at 2.2A compared to the 1N6517 5kV diode rated at 1.0A. Several factors are at work, but K-body diodes can get the heat out faster than an equivalent glass-body diode, which means they can handle higher reverse voltages and forward currents.
A second disadvantage of epoxy type diodes such the K-body type, is cost. Epoxy diodes with a varnish passivation are typically more expensive. That does not apply to epoxy over-molded glass-passivated diodes such as the SMF6533, or SXF6525. The SMF6533 and SXF6535 take a hermetically sealed diode and over-encapsulate it. The advantage of these devices is they are surface mount, easily pick-and-placed, and the diode is hermetically sealed.
Choosing the right diode for your application is critical to the success of your project. Different diodes offer different advantages. If you're not sure which diode is right for you, give us a call.