Influences on crack initiation in conductor tracks on three-dimensional thermoplastic substrates

Advanced mechatronic products are distinguished by a high functional density in miniaturized design. Due to the versatile possibilities to apply circuitry and functional structures directly on spatial substrates, the MID technology (Molded Interconnect Devices/Mechatronic Integrated Devices) provides enormous potential for multi-functional mechatronic solutions in various areas of application. The laser direct structuring process (LDS), as the most widely used production method in the field of MID, has become well established in the market in recent years. This process is based on a structured exposure and activation of a special additive embedded in an injection-molded thermoplastic substrate by the use of a laser beam. The resulting metallic seeds and micro-rough structures on the substrate surface enable a subsequent selective metallization in an external electroless metallization process. A decisive factor for the quality and reliability of LDS-MID is the crack tendency of the metallized conductor tracks. Cracks in the metallization primarily result from mechanical and thermomechanical stresses of the device caused by environmental influences during use as well as by stressfull conditions during production processes. The thermomechanical stress of MID is particularly effected by the differences in thermal expansion of the thermoplastic substrate materials and the metallization. The paper presents results of comprehensive investigations on the crack initiation in LDS conductor tracks. The influences of specific properties caused by geometric characteristics and injection-molding-related conditions on the initiation of cracks were investigated using specially designed three-dimensional specimens. These specimens consider MID-typical surfaces, such as concave and convex shapes, edges, weld lines, or ejector markings. For the investigations, specimens were produced in a standard variant with typical production parameters as well as in a variant with reduced metallization roughness using less laser power and a CO
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snow-jet cleaning after the structuring process. The cracking behaviour of the conductor tracks was evaluated under stress conditions through temperature shock (1000 cycles, -40 °C / 125 °C) and damp heat tests (1000 hours, 85 °C / 85 % reI. humidity) by monitoring the electrical resistance of applied meander-shaped test circuits. An online measurement during the tests enables the detection of cracks from the recorded resistance profiles and provides information on the time of crack formation and the prevailing conditions. Furthermore, damaging effects, which occur only temporarily during the periods of stress, can be detected. The results of the investigations clearly show significant influences on the crack tendency of LDS circuit tracks under stressfull environmental conditions and thus provide helpful hints to improve the reliability of LDS-MID. A considerable increased crack susceptibility can be recognized at specimens exposed to a reflow soldering process prior to the environmental tests. Regarding the properties of the circuit tracks, layers with reduced roughness clearly show a lower risk of cracking than layers with a higher roughness. A further decisive factor concerning the crack tendency is the substrate shape. Conductor tracks on convex surfaces increase the risk of cracking remarkably, whereas concave circuit tracks reduce the risk of failures.