Polymeric packaging of high power semiconductor devices: Material selection & reliability assessment

For power thyristor devices used in high voltage direct current (HVDC) schemes, hermetic packages are still being used despite plastic packaging having made successful progress towards replacing them in various high reliability applications, e.g. aerospace and military. Although hermetic technologies have demonstrated an excellent history of reliability and performance, they offer several drawbacks, if used for the thyristor devices intended for future HVDC transmission schemes. This is because future HVDC schemes will be required to have higher current/voltage ratings e.g. to meet the increasing energy demands from large and rapidly developing countries, such as China, Brazil, India, etc., and such systems will require thyristors based on larger semiconductor wafer diameters. Consequently, this will drive the packages to be bigger, more fragile and expensive. It is expected that by switching from the present ceramic housings to a polymer material, the device will be more robust, cheaper and lighter than the current hermetic configuration. However, such a shift also provides many challenges in terms of materials selection, design and manufacturing. Potential issues include polymer permeability to moisture, delamination, voiding, crack formation which could all lead to thyristor failure. To assess whether the performance and reliability of a polymer housing can be comparable to current ceramic packages, a polymeric package demonstrator was developed and tested. Along with electrical and thermal modelling studies performed to study the package behaviour, different activities, such as identifying appropriate material candidates and design concepts for the housing, and reliability of the manufactured polymer housing, were also carried out to develop the housing. It is these activities that are discussed in this paper. For instance, from the material selection study, polymers, such as polyimide (PI) and epoxy, were identified as being ideal candidates for the high operating temperature and good electrical performance required from the housing. From the reliability study which comprised of temperature cycling accelerated life tests, the manufactured polymer housing was concluded to be reliable.