High Temperature Applications for IC Plastic Encapsulated Packages

Recently, the trend has been toward higher device operating temperatures. Manufacturers are no longer satisfied with the 0�C-to-70� C commercial range, but want products that are rated to 110�C or 125�C or greater temperature. This desire is driven by many factors, a prime one being automotive needs, where the operating temperature ?>under the hood?> may reach 200�C in the near future. However, such changes must be balanced with need for good reliability and without jeopardizing rated product life.

In the case of plastic packages, this concern extends to its elevated temperature stability, both of the device and of the package, especially with respect to the epoxy molding compound. Compounds have made enormous improvements in reliability behavior over the years, under both moist and dry conditions, but still have some inherent shortcomings. For one, epoxy molding compounds are required to meet the Underwriter's Laboratories Flammability Rating of 94V-0. To meet that rating, bromine and antimony are often incorporated as flame retardants. Unfortunately, these elements can act as catalysts to accelerate the intcrmetalllic growth between the gold ball bond and the aluminium bond pad on the silicon chip. The intermetallic growth itself is not detrimental, but the secondary effects of Kirkendall voiding and thermal stresses between the interrnetallic layers are. This phenomenon is typically seen during high temperature storage life reliability testing.

Also, many epoxy cresol novalac-based corn pounds have glass transition temperatures of around 150�C, but the biphenyl-based compounds recently introduced may range as low as 120�C. The question then becomes whether accelerated tests run at very high temperatures accurately reflect field failures. Resin breakdown and other effects manifest at these temperatures may never be seen in the field, yet these tests are relied upon to indicate good reliability. Another issue to be resolved is finding the upper operating or ambient temperature limit for commercial products in plastic packages, and how to predict reliability and product life. A possible resolution to this concern is multi-functional-based molding compounds. Their glass transition temperatures are typically greater than 190�C, insuring that even accelerated tests or high ambient operating temperatures should not affect the composition or the compound.

Since the molding compound itself cannot be eliminated from plastic packages, ohviously, additional measures can be tried to meet the criteria. To insure reliable, long-lived products may also require changes in device design, thermal management. and bond pad metallizations and assembly processing, as well as improvements to the molding compound, to reach the criteria of reliable operation at elevated temperatures. Many aspects must be considered to insure adequate reliability while meeting the customers' requirements.

In this study, comparisons were made between different compound chernistries-vcrcsol novolac, biphenyl, and multifunctional- using the High Temperature Storage Life Test, at 175�C and at 200�C. Static Operating Life Testing at 175�C was also performed on the multi-functional compound. Failure analysis was performed on a sample of the tested units. However, with the testing temperatures above the glass transition temperatures, over-testing may be an issue and must be considered.