ENVIRONMENTAL STRESS SCREENING

Environmental Testing of Plastic Encapsulated Microcircuits

by Joseph G. Federico, New Jersey Micro-Electronic Testing

For more than 10 years, the military and aerospace electronic component industries have undergone cutback initiatives instituted by the Department of Defense that diminish the manufacturing of hermetically sealed microcircuits (HSMs). As a result, these industries have reconsidered the use of commercial plastic encapsulated microcircuits (PEMs). These PEMs must be subjected to a series of reliability tests tailored to meet the functional performance requirements of their specifications.

A PEM is a microcircuit with the die and the lead frame encased in a solid plastic encapsulant as shown in Figure 1.1 Today, the automotive industry alone installs 2.7 million PEMs per day.2

As military and aerospace designs lean toward increased usage of PEMs, environmental laboratory tests such as component temperature evaluation, temperature cycling, thermal shock, stabilization bake, highly accelerated stress testing (HAST), autoclave, salt atmosphere, moisture resistance, and static and dynamic burn-in play a vital role in evaluating reliability performance.

In June 1994, Secretary of Defense William Perry released "Blueprint for Change," an acquisition reform initiative proposing a change in policy on military specifications and standards. The directive contained more than 80 recommendations, most addressing the electronic parts industry.3 Mr. Perry called for military program managers to use commercial parts and performance-based specifications for new systems as well as to eliminate costly and time-consuming military specifications and standards.4

Another factor behind the Perry directive is the change to commercial parts acquisition methods for military systems. This change was proposed as essential for the health of the domestic military-industrial base and, by extension, the health of our national security.5

Affected most significantly by this policy is the fate of HSMs produced according to military standards and specifications. Although HSMs are the preferred microcircuits of the military and its contractors, many are being phased out. However, program managers are not expected to rely completely on commercial microcircuits. Mr. Perry’s directive does not specify that commercial parts be used exclusively, but that commercial parts become the rule and military parts the exception.

The History of PEMs

PEMs originally were considered more susceptible to product failure because of a host of structural and material factors. These factors made PEMs seem unsuitable for the high-stress environments and the high-reliability nature of military applications. Specifically, plastic packages were thought less reliable than HSMs for two reasons:

PEMs used materials with wider variations in coefficients of thermal expansion (CTE), leading to temperature-related problems.
PEMs absorbed moisture that often permeated to the die and caused corrosion or flash to steam during heating to induce cracking or popcorning.6
These beliefs are not without merit. Before the 1980s, failures were common for PEMs due to moisture ingression, corrosion, cracking, and delamination. By the late 1980s, however, technological advances in the plastic material, molding processes, and die yields satisfactorily eliminated much of the early failure history.7

Examining Test Procedures

The following environmental tests—some new and some more common—currently are used to evaluate the environmental performance of PEMs.

Temperature Elevation

In general, the application of elevated temperatures to ICs accelerates chemical degradation due to an improper combination of materials during fabrication or contaminants within the package. For military applications, a general range of -55°C to +125°C is applied to evaluate performance functionality. Elevated temperatures also relieve residual mechanical stresses within metals of the circuit.

Temperature Cycling and Thermal Shock

Temperature cycling uses an air-to-air conditioning medium and may require several minutes to transfer between temperature mediums. For thermal shock, a liquid-to-liquid medium provides a severe temperature shock environment and does not need dwell time at room temperature when transferring between temperature extremes.

For both applications, the military typically requires temperatures ranging from -65°C to +150°C. Failures accelerated by temperature cycling or thermal shock are the following:

Bad bonds.

Thermal mismatch of materials such as die-to-package interfaces.
Lid-seal anomalies on hermetically sealed packages.
Inadequately or improperly cured plastic packages or material such as epoxy die attach.
Cracked dies or substrate mounting.

Stabilization Bake

Stabilization bake is performed on electrically unbiased PEMs while subjected to an environmental temperature. This procedure accelerates failure mechanisms such as metallization defects, corrosion, surface instabilities and contaminants, package defects due to thermal mismatches of materials, outgassing of internal materials, and plating defects. A typical stabilization bake is conducted at +125°C in accordance with MIL-STD-883 Method 1008 Condition B.

Highly Accelerated Stress Testing

HAST is a pressurized moisture-resistance test that forces moisture through the plastic encapsulation while exposing the test sample to a static electrical bias under typical operating voltage and current loads. A typical application includes a temperature range from +105°C to +140°C, relative humidity of 85%, and a vapor pressure ranging from 17.6 to 44.5 psia in duration from 25 to 200 hours in accordance with JEDEC Standard No: 22-A100.

HAST generally identifies failure mechanisms such as packaging defects, passivation, and metallization weaknesses. Test may be performed either unbiased or power cycled.

Salt Atmosphere

Salt testing evaluates external plating and corrosion to simulate the effects of seacoast atmosphere. Salt conditioning normally is performed at 95 ±5°F, and the duration of exposure can range from 24 to 240 hours as indicated in MIL-STD-883E Method 1009.

Burn-In

Burn-in is the artificial aging of the electronic component to improve acceptability and lower the failure rate.

In static burn-in, a DC bias is applied at an elevated temperature (powered and loaded for maximum power dissipation either forward or reverse) to as many device junctions as possible. This environmental process assists in identifying ionic contamination, inversion, channeling, oxide defects, metallization defects, and thermally activated surface defects.

A dynamic burn-in process serves the same objective and identifies the same anomalous conditions as static. However, during dynamic burn-in, voltage pulses or sinusoidal voltages are applied to the inputs of the device, and the outputs are measured in terms of time or instantaneous voltages.

The burn-in temperature is +125°C in accordance with MIL-STD-883 Method 1015 Conditions A-E. A more common practice applies temperatures that do not exceed the maximum ambient operating temperature of the device to decrease the artificial aging in evaluating defects.

Moisture-Resistance Testing/Moisture-Induced Stress Sensitivity

Moisture-resistance testing subjects the components to a high-heat and high-humidity environment. This test identifies devices sensitive to moisture-induced stress so they can be properly packaged, stored, and handled to avoid mechanical damage. Typical application temperatures for this process are at 85°C/85% RH as indicated in JEDEC Standard JESD22-A112.

Autoclave

The autoclave, or the accelerated moisture-resistance test, uses severe conditions of pressure, humidity, and temperature to accelerate the penetration of moisture through the external seal to evaluate the device’s moisture resistance. Typical temperature applications are +121 ±1°C and 100% RH at a vapor pressure of 15 ±1psig as indicated in JEDEC Standard JESD22-A102-B. Dwell durations can range from 24 to 336 hours.

Conclusion

Environmental reliability tests on PEMs continue to be an integral part of military and aerospace evaluations. They also illustrate how environmental testing plays a vital role in assuring that PEMs can be used in harsh environments.

PEMs will continue to play a vital role in these industries because they are stronger in construction, smaller in size, lighter in weight, less brittle, and less expensive than ceramic. Also, the solid construction can easily withstand mechanical shock, vibration, and centrifugal forces.

Taking full advantage of these benefits requires that military designers and engineers be aware of the risks so they can take appropriate precautionary actions. In the end, these environmental reliability tests will be essential in providing designers and engineers with the information necessary to keep up with ever-improving nonmilitary electronics technology.

References

Hakim, E., Presentation on PEMs, Army Research Laboratory, July 1993.
Watson, G.F., "Plastic-Packaged ICs in Military Equipment," IEEE Spectrum, February 1991.
"Perry Releases Plan to Streamline DoD Purchasing Practices," News Release: Office of Assistant Secretary of Defense (Public Affairs), No. 390-94, Washington, DC, 1994, Contact: 703-697-3189.
Rayner, B., "Perry Scraps MIL-Specs," Military & Aerospace Electronics, August 1994.
Kinsella, M.E., and Vincen, P.M., "Military Products form Commercial Lines," IEEE Aerospace and Electronic Systems Magazine, 10(9), September 1995.
Condra, L.W., O’Rear, S., Freedman, L., Pecht, M., and Barker, D., "Comparison of Plastic and Hermetic Microcircuits Under Temperature Humidity Bias," IEEE Transactions on Components, Hybirds, and Manufacturing Technology, 15(5), October 1992.
Nguyen, L.T., Lo, R.H.Y., Chen, A.S., and Belani, J.G., "Molding Compound Trends in a Denser Packaging World: Qualification Tests and Reliability Concerns," IEEE Transactions on Reliability, 42(4), December 1993.
Acknowledgement

Casasnova, J.W., The Navy F/A-18 Program and Plastic Encapsulated Microcircuits.

About the Author

Joseph G. Federico is the director of engineering and operations at New Jersey Micro-Electronic Testing and has more than 20 years of environmental laboratory testing experience. He has received various laboratory inspection certification titles from the Department of Defense as well as a bachelor's and an associate's of science degrees in electronics engineering technology from Fairleigh Dickinson University and the Metropolitan Technical Institute. New Jersey Micro-Electronic Testing, 1240 Main Ave., Clifton, NJ 07011, (973) 546-5393.

Copyright 2000 Nelson Publishing Inc.