When selecting PCB materials, it is essential to strike a balance between design requirements, manufacturability, and cost. Simply put, design requirements encompass both electrical and structural reliability. Typically, material selection becomes particularly critical when designing very high-speed PCBs (operating at frequencies above 1 GHz). For example, the commonly used FR-4 material has a high dielectric loss (Df) at frequencies of several GHz, which may render it unsuitable.
For instance, a 10 Gb/s high-speed digital signal is a square wave, which can be viewed as a superposition of sine waves at different frequencies. Therefore, a 10 Gb/s signal contains many different frequency components: a fundamental signal at 5 GHz, a third-order harmonic at 15 GHz, a fifth-order harmonic at 25 GHz, a seventh-order harmonic at 35 GHz, and so on. Maintaining the integrity of digital signals and the steepness of their rising and falling edges is as critical as achieving low-loss, low-distortion transmission of RF and microwave signals (where the high-frequency harmonics of digital signals extend into the microwave band). Consequently, in many respects, the selection of PCB materials for high-speed digital circuits shares similar requirements with those for RF and microwave circuits.
In practical engineering applications, selecting high-frequency PCB materials may seem simple, but there are actually many factors to consider. Through this article, PCB design engineers and high-speed project managers will gain a better understanding of material characteristics and selection criteria, including electrical performance, thermal performance, and reliability. By utilizing layer stacking appropriately, they can design a product with high reliability and good manufacturability, optimizing all relevant factors.
The following sections outline the primary factors to consider when selecting suitable PCB materials:
1. Manufacturability:
For example, performance under multiple lamination cycles, temperature performance, CAF resistance, heat resistance, mechanical toughness (adhesion) (for reliability), and fire resistance rating;
2. Performance characteristics compatible with the product (electrical properties, performance stability, etc.):
Low loss, stable Dk/Df parameters, low dispersion, minimal variation with frequency and environmental changes, and tight tolerances on material thickness and adhesive content (good impedance control). If trace lengths are long, consider copper foil with low surface roughness. Additionally, high-speed circuit design requires simulation in the early stages, and the simulation results serve as a reference standard for the design.
3. Timely Availability of Materials:
Many high-frequency laminates have very long lead times, sometimes as long as 2–3 months. Aside from standard high-frequency laminates like RO4350, which are typically in stock, many high-frequency laminates require customers to place orders. Therefore, it is essential to communicate with the manufacturer well in advance to ensure materials are procured as early as possible;
4. Cost Considerations:
Consider the price sensitivity of the product—whether it is a consumer product or intended for applications in telecommunications, medical, industrial, or military sectors;
5. Compliance with Laws and Regulations:
Designs must comply with environmental regulations in different countries and meet requirements such as RoHS and halogen-free standards.
Among the factors listed above, the operating speed of high-speed digital circuits is the primary consideration in PCB selection. The higher the circuit speed, the lower the selected PCB’s Df value should be. PCB materials with medium to low loss are suitable for 10 Gb/s digital circuits; materials with lower loss are suitable for 25 Gb/s digital circuits; and ultra-low-loss materials are suitable for faster high-speed digital circuits, with speeds of 50 Gb/s or higher.
From the perspective of material Df:
PCB materials with a Df between 0.01 and 0.005 are suitable for digital circuits up to 10 Gb/s;
PCB materials with a Df between 0.005 and 0.003 are suitable for digital circuits up to 25 Gb/s;
PCB materials with a Df of 0.0015 or less are suitable for digital circuits at 50 Gb/s or even higher speeds.
Processing Methods:
1. Cutting: The protective film must be left intact during cutting to prevent scratches and indentations.
2. Drilling:
2.1 Use a brand-new drill bit (standard 130); stacking one sheet at a time is optimal, with a clamp pressure of 40 psi.
2.2 Use an aluminum sheet as the cover plate, then secure the PTFE board with a 1mm melamine spacer.
2.3 After drilling, use an air gun to blow dust out of the holes
2.4 Use the most stable drilling machine; drilling parameters (generally, the smaller the hole, the higher the drilling speed; the smaller the chip load, the lower the feed rate)
3. Hole Treatment
Plasma treatment or sodium naphthalene activation treatment facilitates hole metallization
4. PTH Copper Plating
4.1 After micro-etching (controlled to a micro-etch rate of 20 micro-inches), feed the board into the PTH line starting from the degreasing tank
4.2 If necessary, perform a second PTH pass; simply feed the board starting from the designated tank
5. Solder Mask Application
5.1 Pre-treatment: Use an acidic board wash; do not mechanically sand the board
5.2 After pretreatment, bake the board (90°C, 30 min), then apply and cure the green solder mask
5.3 Bake the board in three stages: 80°C, 100°C, and 150°C, each for 30 minutes (if oil residue is found on the substrate surface, rework is required: wash off the green solder mask and perform reactivation treatment again)
