Static has more than one definition
Static has more than one definition. There is static charge which gives rise to electrostatic discharge (ESD) issues, especially regarding MOSFETs and IGBTs and then there is the verbal static to the effect that ESD issues are not important. (Yes, I know about radio static, but that's part of a different subject.)
I was witness to the second static definition not too long ago when I saw a handful of MOSFETs get unceremoniously dumped into a plastic storage drawer by someone who insisted that no harm could be done because those MOSFETs were high power devices.
Yep! Right here in the twenty-first century, after all the articles that have been written about ESD issues, I was confronted by an ESD denier. If you're the one whom I mean, read on and take note!
Figure 1 The ESD issue
Just for verbal convenience, I will refer to "FETs," but bear in mind that we are discussing IGBTs as well. In all cases, even if the device under consideration is rated for mucho high voltage, mucho high amperage, and mucho high wattage, the gate element of the device is delicate and must not be abused.
Proper anti-static handling with proper anti-static tools and equipment is a must. Much has been written on this topic so I will not repeat all of that here, but there is one particular mishandling issue that I've not seen written up that we will now examine.
Figure 2 Lead alteration
Trying to alter the shapes of the package leads is a cordial invitation to ESD damage in itself and additionally, it does metal damage to the package leads which may later fracture.
Figure 3 The wrong way (left) and the proper way (right)
The left-hand sketch above shows package leads that have been bent to accommodate their connections to a circuit board. Also, underneath the slightly upended FET package, there is a thin but very harmful separation of the package from its associated heat sink.
The right-hand sketch shows how I recommend that things be done. The package leads have not been bent and there is an aluminum block (in blue) that is used to rigidly and firmly mount the package in order to avoid getting a separation gap between the two metal surfaces.
To better understand that gap closure issue, please see "Wasted Heat Sinking Area."
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
- Designing in ESD ruggedness
- Understanding and comparing the differences in ESD testing
- ESD guidelines for touch-sensing applications
- Thermal design: Get the heat out of the electronics