Project Number: C96-25

Failure of Plastic Encapsulated Microelectronics at Low Temperature

Point of Contact: Dr. Pat McCluskey email: mcclupa@calce.umd.edu Phone: (301) 405-5323 Fax: (301) 314-9269

Objective

Background

Approach

Work Accomplished

Objective

Characterize the effects that limit the use of plastic encapsulated microelectronics (PEMs) at low temperatures to -65° C, and identify potential failure mechanisms. Characterize delamination and cracking of PEMs in low temperature applications.

Background

Recently attention has focused on the use of electronics at highly elevated temperatures. This is a result of the potential benefits to be gained from the development of distributed control systems, smart sensors, and remote actuators, which can operate in high temperature environments. To address this issue, a report was generated this past year describing the generic materials-related limitations to the use of electronics at elevated temperatures up to 200° C.

Current systems requiring operation at -40° C and lower, usually use military grade, hermetically sealed ceramic packaged microelectronic circuits. However, the limited availability of these devices has prompted military and aerospace product developers to consider commercial or industrial grade PEMs for these applications. Advantages to be gained from switching to PEMs include small size, light weight, and low part cost. However, their reliability at low temperature has not been well characterized. In particular, delamination and cracking of the PEMs at low temperatures could lead to wire breakage as well as corrosion during subsequent high temperature exposure.

Approach

A design of experiments study will be conducted to determine the effect of cold temperature operation on delamination and cracking in representative PEMs. Packages will be examined before and after low temperature exposure with C-SAM. The effect of key operating parameters including but not limited to moisture level, minimum temperature, temperature ramp rate, and package geometry will be studied.

The materials-related limitations to the use of PEMs at temperatures as low as -65° C will be determined. Potential failure mechanisms will be identified.

Work Accomplished

A Design of Experiments Plan has been developed to determine the susceptibility of PEMs to delamination and cracking upon repeated exposure to low temperatures. This study in its current form will expose 84 lead PQFPs and 14 PDIPs with different moisture contents to different minimum temperatures at different ramp rates. Half the samples of each package type will be encapsulated with a 72% SiO₂ filled epoxy novalac which has a low fracture toughness, and half will be encapsulated with an 80% SiO₂ filled biphenyl compound with a high fracture toughness. Further information about the proposed design of experiments is available in "Design of Experiments for investigating delamination and cracking in plastic encapsulated microelectronics exposed to low temperatures."
Packages encapsulated with an 80% SiO₂ filled biphenyl compound and having a geometry which is highly susceptible to cracking, were exposed to low temperatures and examined for increased delamination and cracking as a result of the exposure. Cooling rates of 2°C/min and air quench (50°C/min) were used to lower the temperature to -55°C or -65°C. The effect of moisture on cracking was examined by baking some samples at 125°C for 24 hours prior to low temperature exposure and saturating others at 85°C, 85%RH for 168 hours prior to low temperature exposure. For results and details see "Delamination and Cracking in Plastic Encapsulated Microelectronics Exposed to Low Temperatures."
A capability study has been conducted on a typical device of complex functionality, a Motorola MC68332 microprocessor.

Ten devices were parametrically tested at -70°C, which is 15°C below the coldest desired operating temperature, and nine were parametrically tested at 150^oC which is 25^oC above the highest desired operating temperature.
75 devices were functionally tested at temperatures below -70°C to as low as -150°C to determine the distribution of the lowest possible operating temperatures. In this test the devices are turned on at -70°C checked for functionality and turned off. The temperature is then decreased 10°C and the devices are again turned on, checked for functionality and turned off. This is continued until the devices fail or reach -150°C. Functionality is defined as the operation of the watchdog timer circuit of the microprocessor. If the devices fail, they are warmed in 10°C increments and rechecked for functionality to determine if the cession of operation is a "hard" or "soft" failure. 25 devices are tested as received, 25 are tested after baking, and 25 are tested after moisture saturation. The results to date are provided in the summary of electrical test results .