Hermetic power packaging vs. PEMs for mil electronics? No power issues here
In the midst of the debate over hermetic packaging versus Plastic Encapsulated Microcircuits (PEMs) in mil electronics, Microsemi quietly yet confidently recently announced a full line of new and re-released military-level-upscreened PEMs incarnated (or reincarnated) as TVSs, MOSFETs, IGBTs, and rectifiers. Editor Sharon Hess recently caught up with Shane O’Donnell, Microsemi’s Hi-Rel Plastic Devices Program Manager, to find out why the company is focusing on PEMs rather than their rival, hermetic packaging.
Remind us about Microsemi – its technology focus and areas of expertise, locations, number of employees, and goal in the industry.
Microsemi recently announced a new line of plastic upscreened Plastic Encapsulated Microcircuits (PEMs). Which types of products are we talking about?
What was the impetus?
That’s quite a difference. Can you explain technically what the 5 types of upscreening tests on this new line entail?
M prefix in the TVS product range represents a controlled process, where all aspects of the device production and test are controlled by Microsemi. Customers are informed of any change in this process even if the form, fit, or function is not changed. Lot-norming ensures all parts are within a close distribution of the average, and 100 percent surge testing verifies the clamping voltage specifications in the datasheet are adhered to.
MA products provide the optimum COTS+ offering with minimal device mortality levels. The MA upscreening flow offers ten -55 °C to +150 °C temperature cycles followed by three surge tests, 24 hours of high-temperature reverse-bias testing, three-sigma lot-norm screening performed on standby current, and final electrical tests.
The MXL TVS devices undergo more temperature cycles, surge tests, and a longer reverse-bias test. The parts also receive PDA evaluation and delta calculations with conformance inspection based on MIL-PRF-19500 Group A. MX devices receive all of these tests in addition to Groups B and C conformance inspection based on MIL-PRF-19500 specifications. The MXL MOSFET, IGBT, and rectifier devices get a 24-hour stabilization bake followed by various temperature cycling and high-temperature tests depending on the part type. Final electrical tests are always performed prior to dispatch, ensuring the parts have remained within specification.
Plastic parts are known for moisture ingress. How does your upscreening processes thwart this specific danger?
O’DONNELL: For harsh environments, hermetic packages will always have a place in high-reliability applications. A hermetic package in the context of microelectronics implies an airtight seal that will keep moisture and other harmful gases from penetrating the sealed package. For PEMs, steps can be taken to reduce moisture ingress and tests can be added to ensure the barrier of the package. Defect-free passivation of the surface of the silicon die effectively blocks moisture access to microchip devices. Designing the lead frame for epoxy adhesion and lock minimizes the effects of moisture. Choosing the right epoxy can enhance the resistance to moisture penetration. Moisture penetration tests are performed to establish the Moisture Sensitivity Level (MSL) of each type of PEM. Standard qualification tests performed on upscreened PEMs include testing at 85 percent humidity/85 degree Celsius (85/85) and autoclave or pressure pot testing. Moisture measurements are then taken after testing to ensure package integrity.
What percentage of military-specific products does Microsemi test?
O’DONNELL: PEM screening includes the same rigorous 100 percent testing as is applied to our hermetic package devices.
For which types of military applications/systems might this new upscreened product line be suited? Why?
O’DONNELL: PEMs are used where size, weight, performance, and various grades of reliability are needed. Two big users are Unmanned Aerial Vehicles (UAVs) and Unmanned Ground Vehicles (UGVs). Missile systems are another example where size and weight play a critical role in the design. These vehicles and systems may be unmanned, but a malfunction at the wrong time can cost lives. Other applications include ground-based military systems such as radar, vehicle, and telecom systems.
Where and when will the new and re-released upscreened military-suitable PEMS be manufactured and upscreened?
O’DONNELL: Die fabrication and package assembly will be performed at onshore and offshore locations utilizing Microsemi Corporation facilities and contract manufacturing facilities. Screening locations are at the Microsemi facilities in Ennis, Ireland and Bend, Oregon. These products are available now.
Looking toward the future, what are some of the military-appropriate PEM technology trends immediately on the horizon, and why are they needed, technically speaking?
O’DONNELL: For a given device footprint on a circuit card, the engineering community continues to seek better performance, smaller size, and less cost. Discrete MOSFETs, TVSs, IGBTs, and rectifiers are used in power systems; to handle power for a given footprint, new surface-mount packages are being introduced such as the Plastic Large Area Device (PLAD) package. Until recently, only through-hole-type packages with multistacked die were available for high-surge applications. PLAD-type devices offer large die and an exposed copper bottom pad, contributing to an incredibly low package thermal resistance (< 0.2 degree C/W), which dissipates package heat without expensive cooling techniques. A smaller form of the PLAD-type package is currently under qualification at Microsemi.
OK, so wrapping up, which technologies will be needed in your sector of the defense arena in the next 5 to 10 years, and why?
O’DONNELL: With the cost of energy continuing to rise, saving power will continue to be important. The development of wide band gap materials such as SiC and GaN for diode and FET switching applications will be the emerging, maybe even disruptive technologies of the future replacing silicon sockets. Wide band gap materials offer near zero conduction losses in switching applications. For example, we took measurements on a 40 W point-of-load converter with a 3.3 V output using GaN MOSFET switches and did a comparison against silicon MOSFET switches in the same circuit. We found an 8.5 percent improvement in overall circuit efficiency when using the GaN MOSFET switches. Both SiC and GaN base material costs are expensive today; but as more applications emerge, cost will come down, allowing for greater industry adoption.
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