Electronic control components, which perform logical and signal conditioning functions, are complex devices characterized by large die areas and a large number of small contact pads. Control modules that operate at high temperatures(>175°C) for extended periods of time, such as the Engine Control Units (ECUs) that optimize the performance of automobile engines, present unique design challenges for packaging engineers. Materials used in the fabrication of reliable low temperature control electronics degrade rapidly at high temperatures. Thermal mismatches between traditional organic printed circuit boards and semiconductor dies produce high thermomechanical strains when subjected to extreme temperature swings, leading to premature failure. To ensure the reliability of high temperature control electronics, appropriate packaging materials must be selected.
Key Parameters and Requirements
A study conducted at L’École Polytechnique Fédérale de Lausanne (EPFL) in Lausanne, Switzerland, investigated alternative materials for use in high temperature control electronics packaging.1 Among the materials tested by the research team were adhesives used for die-to-substrate assembly. The substrate was constructed using low-temperature co-fired ceramic (LTCC) technology, which is stable at temperatures above 200°C, and the underside of the die consisted of bare silicon. Requirements for the die-to-substrate bond included reasonable bond strength and a suitable path for thermal dissipation.
Master Bond Supreme 10HT was one of the adhesives tested in the study. Several test vehicle samples were manufactured for use in high temperature storage and temperature cycling shear tests. Each sample was constructed by dispensing the Supreme 10HT epoxy through a syringe onto the underside of five blank silicon dies, which were then placed onto a test substrate using a component placer. The adhesive bonds were cured for 90 minutes at 120°C (248°F) to complete each sample. One die from each sample was sheared using a shear tester prior to subjecting the samples to heat.
Five samples were subjected to 210°C (410°F) temperatures in a process oven. All samples were removed after 24 hours, and one die from each sample was shear tested. The process of heating, removing, and shearing the samples was repeated for intervals of 250 and 1000 hours. Results showed that the strength of the Supreme 10HT samples increased progressively after 24 hours and again after 250 hours. Although the strength of the bond decreased after 1000 hours of heat exposure, it still exceeded the minimum 2.4 Kg force threshold stipulated by MIL-STD-883H method 2019.8.
Temperature cycling tests
An additional five samples were subjected to repeated temperature cycling between 20°C and 180°C. Shear tests were performed on one die from each sample after both 10 and 100 cycles. The Supreme 10HT samples showed no measurable degradation in bond strength after either 10 or 100 cycles, even under a maximum strain of 200μm.