Abstract
Thermosonic wire bonding is a well-established process. However, when working on advanced substrate materials and the associated required metallization processes to realize innovative applications, multiple factors impede the straightforward utilization of the known process. Most prominently, the surface roughness was investigated regarding bond quality in the past. The practical application of wire bonding on difficult-to-bond substrates showed inhomogeneous results regarding this quality characteristic. This study describes investigations on the correlation among the surface roughness, profile peak density and bonding quality of Au wire bonds on thermoplastic and thermoset-based substrates used for high-frequency (HF) applications and other high-end applications. FR4 PCB (printed circuit board flame resitant class 4) were used as references and compared to HF-PCBs based on thermoset substrates with glass fabric and ceramic filler as well as technical thermoplastic materials qualified for laser direct structuring (LDS), namely LCP (liquid crystal polymer), PEEK (polyether ether ketone) and PTFE (polytetrafluoroethylene). These LDS materials for HF applications were metallized using autocatalytic metal deposition to enable three-dimensional structuring, eventually. For that purpose, bond parameters were investigated on the mentioned test substrates and compared with state-of-the-art wire bonding on FR4 substrates as used for HF applications. Due to the challenges of the limited thermal conductivity and softening of such materials under thermal load, the surface temperatures were matched up by thermography and the adaptation of thermal input. Pull tests were carried out to determine the bond quality with regard to surface roughness. Furthermore, strategies to increase reliability by the stitch-on-ball method were successfully applied.
Highlights
Wire bonding is a key technology for making electrical connections
One of the technologies utilizing 3D space are molded interconnect devices (MID), which mostly require a pure autocatalytic copper metallization that differs to the standard metallization used in PCB technology in terms of metallization thickness, grain morphology, roughness or adhesion to the substrate [5]
In order to investigate the correlation of surface and bond quality of innovative 3Dcapable laser direct structuring (LDS)-MID materials used for high-frequency applications and made of liquid crystal polymer (LCP) and PEEK substrates were chosen and compared to state-of-the-art FR4 and HF substrates
Summary
Wire bonding is a key technology for making electrical connections. The technology is established and widely researched on printed circuit boards such as FR4 [1,2]. With many new innovative materials and metallization techniques, on the other hand, the suitability of wire bonding is unexplored for a wide selection of materials and manufacturing parameters, so far To optimize those properties of the substrates for HF, relatively soft thermoplastic materials (e.g., PTFE) [3] as well as ceramics or modified thermoset formulations are often employed, which differ significantly from the typical FR4 in terms of surface quality, roughness and hardness of the metallization layers and the underlying substrate properties [4]. In HF applications, this utilization of the third dimension opens up new possibilities, e.g., for the development of antennas with improved transmission characteristics and miniaturized geometries, especially for mm wave application [6,7] To exploit this potential and to avoid the losses of interconnection paths through an IC (integrated circuit) package, the direct attachment of monolithic microwave integrated circuits (MMIC) on substrates is often demanded. Wire bonding on 3D-capable substrates has come into focus for many new applications
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