Bendability of Flip-Chip Attachment on Screen Printed Interconnections
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Silkkipainetuille johtimille toteutetun flip-chip-liitoksen taivutettavuus
The world is heading towards the IoE (Internet of Everything) where everything will be connected to each other. New flexible, light-weight and low-cost electronic devices are needed to add intelligence everywhere in our surroundings. Conventional silicon-based manufacturing is not the best solution because silicon is mechanically rigid and expensive. One way to manufacture these devices cost-effectively in a very large scale is by roll-to-roll screen printing on flexible substrates. However, due to the low calculation performance of current printed electronics, silicon ICs are still needed to act as brains of the devices. Many studies about chip-on-flex or chip-on-film (COF) attachments are available but information about the integration of a silicon chip directly on a screen printed substrate is needed. This thesis investigates bare chip attachment on printed flexible circuitry and evaluates its subsequent level of bendability. The chip was attached on the high-density rotary screen printed circuitry using a flip-chip technique with anisotropic conductive adhesives (ACP and ACF). A selection of chips was stud bumped with gold. Chips without bumps were more challenging to bond due to the surface roughness of the screen printed lines and a small marginal of the suitable bonding pressure. A chip should be exactly parallel with the substrate while bonding so that the pressure is correct on all pads. After finding the suitable bonding parameters, approximately 90 % of the ACP bonded and 96 % of the ACF bonded interconnections worked without bumps. Stud bumping increased the yield almost to 100 % and decreased the contact resistances approximately 75% making the contacts more reliable. Calendering was tested for printed lines to increase their uniformity and decrease the pad height deviation by heating and pressing them with high force. Calendering reduced the line heights by approximately 1 μm and decreased the surface roughness, but following this process there still existed at least a 2 μm variation in the line heights (nominal line height 5 μm). Bending reliability of the chip attachments on flexible plastic substrates was determined using a self-built bending test set-up which bends the sample between two rigid plates. All chip attachments studied withstood at least a 2.5 cm bending radius. The main results of this thesis were to demonstrate bare die integration on screen printed circuitry and to show its suitability for flexible hybrid electronic applications. Still further development of the bonding process and materials are needed to achieve more reliable long-term solutions.