Project Name
An Advanced Thermal Material for Metal-Core Printed Circuit (MCPCB) (ITS/120/06)
Project Summary
The designs of electronic devices and systems are being continuously improved by becoming smaller in size and faster in communication speed. The potential risk associated with these specific design improvements will be an increase in power density and, consequently, a greater risk of thermal problems and failures. In order to reduce this risk, improved thermal management techniques must be identified and researched.
It is envisaged that more effective thermal management can be achieved by using Metal Core Printed Circuit Board (MCPCB). However, the conventional dielectric material (FR4) is considered to be inappropriate for the manufacturer of MCPCB because of its low thermal conductivity. A polymer-matrix composite with ceramic fillers is suggested to be used as the dielectric material. Currently, there is no qualified MCPCB material supplier in Hong Kong producing MCPCB with ceramic-filler dielectric. Thus, there is a need for local MCPCB materials suppliers to acquire the know-how on the formulation and synthesizing techniques of such dielectric materials.
R&D methodology
The transport of heat in non-metal is caused by flow of phonons or lattice vibrational energy. The thermal resistance is caused by various types of phonon scattering processes, e.g. phonon-phonon scattering boundary scattering, and defect or impurity scattering. Therefore, in order to maximize the thermal conductivity in materials, these phonon scattering processes must be suppressed. Phonons travel in matter with the speed of sound. In theory, the scattering of phonons in composite material is mainly due to the existence of an interfacial thermal barrier from acoustic mismatch, or the damage of the surface layer between the filler and the matrix. These interfacial phonon scattering phenomena are similar to the scattering of light due to differences in refractive indices of the media.
There were two important parameters that regulated the conductivity in the composite, i.e. the number of conducting paths and the particle density along the heat-flow paths. The effect of the mode of sample preparation can lead to different particle distributions in the matrix which produces the variations in thermal conductivity.
In the project, three approaches to enhance the thermal conductivity will be investigated:
1. Maximizing the formation of highly thermally conductive networks
These can be done by first choosing the filler which should be able to form many heat-flow paths.
2. Minimizing the thermal resistance along the conductive paths.
This minimization can be done in three ways: (i) select particles with perfect lattice or crystal to suppress the scattering of phonons by lattice defects; (ii) use large-size particle to minimize the scattering of phonons due to the interfacial thermal barrier; (iii) Minimize the matrix resin layer between particles to reduce the thermal resistance due to resin itself while still keeping the mechanical properties.
3. Decreasing the thermal contact resistance at the filler-matrix interface by surface treatment.
Surface treatment on the filler improves the affinity between filler and the matrix; therey significantly increase the thermal conductivity of the composite.
This project would make use of the above guideline to synthesis a new breed of Metal Core Printed Circuit Board material. The new material would undergo a series of performance evaluation to ensure that the new material not only has improved performance on thermal conductivity, but also can meet the IPC specification.
Objectives and Benefits
The objectives of this project are:
1.To develop and formulate an appropriate synthesizing technique for a MCPCB material with good thermal performance;
2.To characterize the thermal, mechanical and electrical behaviors of the developed material, and make a comparison with existing MCPCB materials.
Deliverable(s)
1.Develop and formulate an appropriate synthesizing technique for the manufacturer of MCPCB with superior thermal conductivity;
2.Develop a technique for preparation of dielectric film using a casting method;
3.Promote and transfer the know-hows of MCPCB material fabrication technologies to PCB industry;
4.Setup a database and information link to support the Hong Kong PCB and substrate industries with a specific emphasis on the latest technological development of MCPCB materials.
Summary of Findings
A good thermal performance of the MCPCB material developed and formulated in this project was accomplished by maximising the formation of conductive paths and minimising the thermal barrier.
The former was done by using high thermal conductive filler which was proven in our study to have a positive effect on enhancing the thermal conductivity of traditional PCB materials, in particular, the micron-sized filler. Despite that the maximum percentage of the filler allowed to be added into the epoxy is low, (about 30%), as low as 13% of the micron-sized filler filled dielectric material is high enough to achieve the thermal conductivity that is larger than 1W/m.K without scarifying other material properties.
The latter was achieved by using the coupling agent. 1% of the coupling agent in respect to the weight of the filler is optimised enough to coat its surface and enhance its interaction with the epoxy matrix.
In addition, the thermal, mechanical and electrical behaviours of these filler-filled thermal conductive MCPCB materials are characterised. These materials outperform traditional PCB material in terms of thermal, mechanical and electrical properties. Meanwhile, it is cost-effective.
The only concern that arises from using this dielectric material is that the drilling and routing parameters of this dielectric are different from that of the typical FR4 due to the presence of the filler. Nevertheless, MCPCB always comes with a base metal plate to act as a heat spreader, therefore, fine adjustment on the drilling, routing and punching processes, and the selection of proper types of drill bits are inevitable when using this kind of material.
