Fabrication of a Multilayer PCB Utilising TC350 and FR408HR Laminates with Advanced Via and Edge Plating Techniques

Fabrication of a Multilayer PCB Utilising TC350 and FR408HR Laminates with Advanced Via and Edge Plating Techniques

Overview of the Fabricated Printed Circuit Board

A high-performance, 8-layer printed circuit board has been fabricated, designed to meet stringent thermal and electrical demands. The board's construction was specified with a mixed dielectric layup to optimise both signal integrity and thermal management. An overall finished thickness of 2.0 mm was achieved.

The layer stack-up was configured as follows:

All 8 copper layers were specified at 1 oz. (35µm) thickness. The physical dimensions of the fabricated panel were 99 mm x 83 mm. The surface finish applied was immersion gold over the exposed copper features. A green solder mask was used for electrical isolation, and white legend printing was added for component identification.

Further construction details are summarised in Table 1.

Key Board Specifications

Several advanced fabrication techniques were mandated to meet the design's performance goals. These included the integration of blind vias, the filling and capping of 0.2 mm vias, and the application of metal edge plating.

TC350 Laminate: Introduction and Application

TC350 is a ceramic-filled PTFE/woven fiberglass laminate, specifically engineered for microwave printed circuit boards. Its material properties are characterised by a stable dielectric constant and enhanced thermal conductivity, making it suitable for high-power, high-frequency applications. 

Typical Properties of TC350 Laminate

The incorporation of TC350 laminates in this PCB design was driven by its material attributes. These include its low signal loss at high frequencies and its effective heat dissipation, which are critical for the long-term reliability of the final assembly.

FR408HR Laminate: Introduction and Application

FR408HR is identified as a high-performance FR-4 resin system, noted for its maximum thermal performance and reliability in multilayer applications. The material is manufactured using a patented high-performance multifunctional resin system, reinforced with electrical grade glass fabric. This construction is reported to deliver improvements in Z-axis expansion and electrical bandwidth compared to standard materials. 

Typical Properties of FR408HR Laminate

The FR408HR material was selected for the inner layers of the stack-up. Its properties, such as UV blocking for AOI compatibility and controlled dielectric performance, are considered beneficial for the board's overall signal integrity and manufacturability.

Via Filling and Capping (Resin-Filled Vias with Electroplated Caps)

All vias with a diameter of 0.2 mm were specified to be filled and capped. This is a specialised process where the via holes are first plated through to create a conductive barrel. Subsequently, the hollow centre of the via is completely filled with a non-conductive epoxy resin. After the resin is cured, the surface is planarised, and a copper cap is electroplated over the filled via. This technique is employed to create a flat, solderable surface directly over the via, which is essential for component placement and to prevent solder wicking away from the pad during assembly.

The Function of Metal Edge Plating

The requirement for metal edge plating was also specified. This process involves plating the peripheral edges of the printed circuit board with a conductive material, typically copper, which is then connected to an internal layer, most commonly the ground plane. The primary functions of this feature are to enhance electromagnetic interference (EMI) shielding by containing radiation within the board and to improve thermal dissipation by providing a conductive path for heat to be transferred from the internal layers to the board's edge. It can also serve as a connection point for a grounding clip in the final assembly.