Laminates for mmWave PCBs: Different Solutions for All-RF Multilayer, Hybrid Multilayer, and Double-Sided PCBs

#mmWave #automotive #phase #array #radar #PTFE #copper #foil #insertion #loss

At high mmWave frequencies all antenna features are smaller than even before. This means that the RF base material for antenna printed circuit boards (PCBs) have to have tighter tolerances at lowest loss. Different PCB solutions require different RF laminates: hybrid multilayers (RF laminate + FR4), all-RF multilayers, and double-sided PCBs. From electrical and mechanical reliability points of view, ceramic filled PTFE base materials provide a leading edge over thermoset resin system base materials.

The high consistency of dielectric constant and loss factor of the thermoplastic material PTFE over the required operating temperature range at resonance frequency of the antennas, has been recognized by the RF and microwave industry for a long time. This technical presentation discusses the main reasons why such types of base materials are being used. All-RF multilayers are the main PCB type for phased array radar PCBs. There, the thin RF laminate must also show a very high dimensional stability in order to get combined with a suitable lowest loss prepreg, which is provided by engineered selection of suitable glass fabrics.

Hybrid multilayer PCBs are widely used for 77/79 GHz automotive radar PCBs. Tested electrical and reliability data confirm that very thin ceramic-filled non-reinforced PTFE laminates, are most suitable in order to meet the requirements of next generation ADAS sensors. Double-sided PCBs for mmWave systems usually consist of relatively thin (10 mil / 0.25 mm) thick laminates. There, the combination of PTFE resin, woven glass fabric, ceramic filler systems and ultra-low profile copper foil provide lowest loss at highest dimensional stability. The market introduction of an ultra-low profile ED copper foil provides an even improved insertion loss over rolled annealed copper foil, in addition to its lower cost. The higher the frequency, the better the value. Only PTFE laminates result in high enough copper peel strength even at repeated rework cycles.

Almost all PCB manufacturers capable of making mmWave PCBs have processing experience of PTFE laminates; and several of these also use PTFE laminates together with lowest loss RF prepregs.