High-Speed Boards with Less-Expensive PCB Materials

Optimizing High-Speed Boards with Less-Expensive PCB Materials

Posted April 1, 2021

In PCB

The current belief in the industry is that traditional low-cost printed circuit board materials are not useful for producing next-generation high-speed designs. For instance, modern technological requirements for, say PCI 5.0, are one of the most challenging for board designers and manufacturers, High-Speed Boards with Less-Expensive PCB Materials.

At Rush PCB, we have examined the materials and methods involved in making high-speed boards, and we understand there are some ways to get around to using regular material to perform at high speeds.

We understand that next-generation materials make it easier to incorporate new technologies into the latest designs. However, there is a price to pay for the increase in performance and capabilities. In our experience, using exotic materials to make PCBs is much more expensive, and the increase may go as high as 100% or even more, depending on the specifics of the design.

Large PCB manufacturers can make this change as their designs and systems can use the exotic materials, while absorbing the extra costs they entail. However, other manufacturers may be bound by budgetary constraints and might find the transition more difficult to make.

Therefore, Rush PCB looks at the various factors involved and the different approaches available to designers for building PCBs for the next generation using low-cost current-generation materials.

The Cost Factor

Incorporating next-generation technology into designs inevitably leads to some increase in manufacturing costs. At Rush PCB, we manage several variables to keep the increase to as low as 30%.

Although we have outlined several steps in this direc

direc Additionally, while cost is an important factor, there are other factors that may be more important.

Variables in Design

We consider several variables when designing printed circuit boards and manufacturing them. However, we have determined that four of these variables merit a major focus to deliver the most benefits. These are:

Dielectric constant of the material
Surface roughness of the copper tracks
Oxidation process of the copper
Optimization of the stackup

We discuss each variable in detail, and list the options available for delivering cost-effective solutions.

Dielectric Constant of the Material

Traditional materials like FR-4 use E-glass reinforcements within an epoxy-based resin—this is a low-cost arrangement. Exotic materials with improved dielectric constant use PPE/PPO-based resin systems and these are more expensive. They also use low-Dk glass for improving the performance, increasing the material cost and the overall manufacturing cost.

Newer materials are now available that combine the PPE/PPO-based resin with epoxy. This produces boards capable of meeting performance requirements but at a marginal increase in costs.

Surface Roughness of Copper Tracks

High-speed signals need smooth copper surface to travel without hindrance. Skin-effect forces high-frequency signals to concentrate on the periphery of the conductor, where its roughness contributes to the impedance. However, it is not possible to use extra-smooth copper as other material like solder mask and surface finish will have difficulty adhering to the copper surface and might peel off.

Therefore, manufacturers need to use copper with some level of roughness. Where higher electrical performance is necessary, manufacturers use hyper very low profile (HPLV) copper with a smoother surface, but they must also use more expensive materials for solder mask and surface finish, ultimately pushing up the manufacturing cost.

A newer type of copper foil is available in the market. This foil, commonly known as RTF2, has a non-uniform roughness profile. RTF2 has a performance like HPLV, but its manufacturing costs are much lower. Manufacturers are also working on the next-generation of RTF2 that has almost the same performance as HPLV at a minimum cost increase.

Oxidation Process of Copper

While manufacturing PCBs, we ensure an optimal bonding between core and prepreg layers by imparting a surface oxidation process on the copper foil for promoting adhesion. While this process increases the surface roughness of the copper foil and increases adhesion, it also significantly affects the signal integrity.

Therefore, the process must obtain a balance between the optimum surface roughness of copper and the desired signal integrity. With a low surface roughness, there is the possibility of the board falling apart due to delamination. On the other hand, an aggressive oxidation process may nullify the use of RTF2 or HPLV copper by over-roughening the surface.

Rush PCB uses adhesion promoters and low-etch oxidation chemistry to reduce the requirement for improving the roughness of the copper surface for a better bonding. By lowering the requirement of surface oxidation on copper, the surface can maintain its smoothness, thereby improving the signal integrity.

Optimizing the Stackup

Optimizing the stackup has a dual functionality in the PCB design and manufacturing stages. Determining the stackup not only optimizes the performance, but also helps minimize the cost, mostly by making the PCB thinner.

At Rush PCB, we understand that it is possible to minimize losses by determining the right stackup, and more precisely, by adjusting the core and prepreg layer thickness options. For instance, for signal losses, a 5/6 stackup can outperform a 3/9 stackup by more than 15%, even when using the same copper profile and substrate. An x/y stackup has an x mil core thickness and a y mil prepreg thickness.

However, this approach may not be a panacea for all evils. Changing the stackup design may impact the routing density negatively and increase the noise coupling. However, carefully selecting the thickness of materials in the stackup can help towards improving the signal integrity and at the same time, also be a right step in the process of cost optimization.

Conclusion

Using less-expensive PCB materials for high-speed boards is a subject of great interest in the PCB industry, especially for those manufacturers who find it difficult to use exotic materials or next-generation materials in their regular setup and design. Rush PCB has extensive knowledge and experience in this area, and has links with industry experts and academicians working towards identifying and solving problems faced by PCB manufacturers.