Microelectronic Technologies

COF-TECHNOLOGY BASED ON ADHESIVELESS ALUMINUM-POLYIMIDE DIELECTRICS

Using flexible boards and flexible cables based on copper-polyimide foil dielectrics in the designs of modern electronic assemblies and printed circuit assemblies made it possible to basically solve problem of minimizing volume of modern semiconductor devices in various branches of science and technology. Implement the possibility of a 3-D layout of electronic modules in general and detector modules in particular. To provide possibility of their assembly without “dead space” with high energy and spatial resolution using COF technology (English Chip-on-Flex – a chip on a flexible board). However, the use of both adhesive-containing and adhesiveless copper-foiled polyimide films did not make it possible to fully realize the advantages of COF technology when assembling detector modules. Some problems inherent in traditional COF technology based on copper-polyimide foil dielectrics have remained unresolved.

In this case, to ensure a reliable corrosion-free connection with aluminum contact pads of microcircuits and sensors on copper conductors of flexible boards and cables, it is necessary to apply additional layers of nickel and gold, which complicates and makes the process of forming flexible interconnection elements more expensive. In addition, ultrasonic welding of flexible cables with sensors and microcircuits using aluminum wire limits the possibility of reducing the overall dimensions (installation area) and volume of detector modules and requires increased accuracy and caution when assembling products in order to prevent deformation of aluminum wire interconnections and short between them.

From this point of view, the most optimal option for further improvement of the COF assembly technology in electronic devices is the use of adhesiveless aluminum – polyimide lacquer-foil dielectrics, that is, “aluminum” COF technology. Adhesiveless aluminum – polyimide lacquer -foil dielectrics have all advantages that adhesiveless copper-polyimide materials also possess. However, a number of their additional advantages compared to copper-polyimide foil dielectrics make it possible to significantly expand the capabilities of COF technology at the present stage.

The main advantages of aluminum-polyimide lacquer-foil dielectrics:

• aluminum has high corrosion resistance;
• aluminum has a radiation length almost 6 times greater than the radiation length of copper (X_0Al = 8,9 cm, X_0Cu = 1,43 cm);
• • despite fact that aluminum, compared to copper, has lower mechanical strength (σ_рА1 = (10-40)  ; σ_рCu = (16-45)), lower thermal conductivity (λ_ТА1 = 218 W/(m·K); λ_ТCu = 385 W/(m·К)), electrical resistivity about 1.6 times greater than the electrical resistivity of copper (ρ_А1 = 0,028 μΩm; ρ_Cu = 0,017 μΩm), it is important that aluminum is almost 3,5 times lighter than copper (γ_А1 = 2,7 g/cm³, γ_Cu = 8,92 g/cm³) and more than 3.6 times cheaper (as of November 2021);
• due to the low density of aluminum, a high electrical conductivity per unit mass is ensured (that is, with the same resistance value and the same length, aluminum conductors are almost twice as light as copper conductors, despite the larger cross section).

Thus, interconnection elements based on aluminum-polyimide lacquer-foil dielectrics make it possible to further minimize the mass of a substance in the detection volume and the dimensions of detector modules, which is especially promising for detector systems with a high density of information channels.

“Aluminum” COF assembly technology is easily adapted to existing industrial automated ultrasonic welding equipment for aluminum flat leads. This ensures high quality and reliability of welded joints due to the fact that homogeneous materials are welded (aluminum contact pads of electronic components and aluminum conductors of interconnection elements).

In addition, interconnection elements based on adhesiveless aluminum-polyimide dielectrics can significantly improve the capacitive characteristics of electronic devices. Due to another positive property of aluminum – low yield strength (σ_0.2 = 2) – it is possible during ultrasonic welding to carry out direct through connections of ribbon aluminum leads of boards with contact pads of sensors through deep (up to 100 μm) “wells” in polyimide. This provides a decrease in the output capacitances of the sensors by 5÷7 times, which significantly increases sensitivity and resolution of detecting systems.

For the first time for European experiments in high-energy physics, an improved “aluminum” COF assembly technology was proposed and successfully implemented in the ALICE (CERN) experiment by specialists who currently represent the LLC “Research and production enterprise “LTU”. For this experiment, more than 50 thousand different components based on aluminum – polyimide adhesiveless lacquer foil dielectrics were developed and manufactured. To assemble the detector modules, with the participation of Kharkiv specialists, assembly sites were created, where a significant part of the strip and all drift detector modules for the ALICE experiment were assembled, which ensured high quality and reliability of the manufactured products.

In 2018-2019 LLC “Research and production enterprise», LTU” developed, manufactured and delivered to CERN sets of flexible boards for the outer detector layer based on ALPIDE 4 monolithic active pixel sensors (MAPS) of the ALICE ITS Upgrade experiment.

The “aluminum” Chip on flex (COF) technology of LLC “Research and production enterprise “LTU” is promising for successful application in almost all areas of special instrumentation, including: for individual dosimetry, matrix introscopy, medical and industrial tomography, for physical experiments , as well as for use in equipment for military and space applications.