A Novel Printed Circuit Board Laminate for High Frequency Electronics

Abstract

A novel PCB material that is advantaged at the 10-100+ GHz range has been developed. It consists of a 3 layer laminate, nominally with a total thickness of 140 microns, and individual laminates can be stacked to make multilayer boards. The outer two layers of the laminate consist of the polymer Topas, 50 microns thick and the middle layer is Corning’s ribbon alumina, 40 microns thick. For brevity, the laminate is called “TAT”. The permittivities of the Topas and alumina are 2.5 and 10 respectively, and that of the 50/40/50 TAT laminate is 3.2 in a direction transverse to the plane of the laminate. The individual loss tangents are about 0.0002, and the loss tangent of the TAT laminate has about the same value. Thus, TAT has significantly lower loss with a lower effective permittivity than other PCB materials currently available. These dielectric properties have been measured experimentally at 10 and 15 GHz. The permittivity and loss tangents can also be predicted accurately using extremely simple analytical formulae. This makes predicting the properties of TAT structures with different dimensions very easy. A TAT strip line has been modeled and found to have an effective permittivity of 2.3. The machining of TAT is somewhat incompatible with conventional PCB shop practice. However, laser drilling and cutting are perfectly practicable. In-house processes have been successfully developed, and an external vendor has also developed these capabilities. Copper sheet adheres strongly to the TAT, and the conductivity of the resulting traces is high. Boards for a 77 GHz radar demonstrate the manufacturability of TAT. Mechanical testing of TAT has been carried out, and the measured flexural modulus and stress at failure compare well with calculations. Environmental tests at 80 °C and 80% humidity indicate that TAT is quite robust. Moreover, it is quite possible that TAT itself may satisfy the UL94 criteria, since the alumina may act as a flame retardant. The feasibility of loading Topas with ceramic or fluoropolymer powders to reduce flammability has been investigated, and it has been found that the addition of these powders causes negligible dielectric loss. In summary, laminates of this type are plausible extreme low loss candidates for PCBs for very advanced applications such as 5G and 6G devices.