HDI PCBs are the backbone of modern high-performance electronic devices such as smartphones, laptops, servers, GPUs, and many other devices in the automation aerospace and defence industries.
HDI PCB has such vast use in these industries because of its capability to assimilate a large number of components in a relatively low space thus to design and fabricate a successful prototype of the PCB engineers had to come up with various strategies both in the design process and in fabrication process.
Here we are going to take a look at 8 strategies whose implementation will produce HDI PCB prototypes that can meet the high-performance criteria of modern-day electronics.
What is HDI PCB?
HDI PCB is a type of PCB that has a higher routing density per unit area compared to traditional PCBs.
Due to the characteristics of HDI boards, they can carry more components per unit area, making them an ideal choice for compact high-performance electronic products such as smartphones, tablets, and medical equipment.
They typically have micropores, blind holes, buried holes inside and multi-layer designs to achieve compact and complex circuits.
The application of HDI technology can achieve faster signal transmission, reduce signal loss, and improve the overall performance of PCBs.
Many factors affect the HDI PCB prototype manufacturing and HDI PCB cost, so we should adopt some strategies to get a satisfactory finished product.
Strategies for HDI PCB Prototype Design and Fabrication
Strategy 1: Implement Microvias
A via generally refers to a hole in HDI PCB prototypes that mounts through-hole components and connects routing traces of different layers.
The size of the via is significant for the overall performance of the HDI PCB prototype if a via is too big there will be less space for routing and larger vias also cause parasitic capacitances microvias are the best solution to counter these issues.
Microvias of HDI PCB prototype have a diameter in the range of 50 to 150 micrometres (0.05 to 0.15 mm) vias of this small size have very little parasitic capacitance which means microvias work better at high speeds and their small size will also permit higher routing density.
Strategy 2: Using Via-in-pad Technology
Every via on the surface of the HDI PCB prototype requires additional space for its pad.
A pad connects the interior of the via to the traces on the surface so a pad even of a small size is going to consume space and if that via is connected to a component then the trace between the via and the component will hinder the path of other traces.
To deal with this issue Via -in-pad technology is used in the HDI PCB prototype fabrication. In Via -in-pad technology the vias are placed directly under the pad of the component.
Strategy 3: Fabricate with Low-loss Materials
HDI PCB prototypes have a high density of components placed on their surface that work at high frequencies thus maintaining signal integrity is very crucial. The best approach to maintain the signal integrity for better performance is to fabricate the HDI PCB prototype with low-loss materials.
Low-loss materials are those that have a lower dielectric constant and a lower dissipation factor compared to common PCB materials.
Rogers RO4000, Taconic RF-35, Isola I-Tera MT40, and Panasonic Megtron 6 are some of the low-loss materials that have a dielectric constant ranging between 2.2 – 3.66 and a dissipation factor of 0.0012 – 0.005. along with signal integrity heat generated by the components is also a reason for using low-loss materials because they are better suited for handling heat and unlike common HDI PCB prototype materials do not show warping and delamination.
Strategy 4: High-quality Copper Plating
Copper foil is the primary conductive material used in HDI PCB prototypes. To develop a successful prototype of HDI PCB it is necessary to ensure the proper thickness of the copper foil.
For HDI PCB prototypes it should be around 17-35 µm (0.5-1 oz/ft²) for external layers and internal layers should be around 12-17 µm (0.35-0.5 oz/ft²). Along with that to ensure a strong adhesion between the copper and the board, the copper-clad PCB board goes through a pre-treatment of its surface through a series of cleaning stages.
Here the copper-clad board is first scrubbed to remove all the imperfections from the surface of the board.
Then it is submerged into a chemical solution to dissolve any remaining contaminates such as oils and other residues that were not removed in scrubbing after this process micro etching is performed where the surface of the HDI PCB prototype boards is made slightly rough due to which a strong bonding is created between copper and the board.
Strategy 5: Sequential Lamination
Sequential lamination is a technique in HDI PCB prototype fabrication in which multiple layers of PCB are laminated together to form a single PCB. With sequential lamination, designers can create 3d designs of the circuits allowing them to achieve greater density and better connectivity in PCB.
With sequential lamination, designers can integrate various kinds of vias such as Blind, Buried, and Stacked Vias into the HDI PCB prototype Design. Since each layer of PCB is fabricated layer by layer fabrication of vias on every layer before lamination ensures perfect alignment and enhanced electrical performance.
Strategy 6: Use Layer Stack-up Design
Layer stack-up design is a process of arranging various layers of PCB mainly signal, power, and ground layer in a manner that the PCB is mechanically stable and has good thermal management along with optimal electrical performance.
In HDI PCB prototype stack-up configurations of 1+N+1, 2+N+2, or 3+N+3 types, are used where N is the insulation layer while “1” and “2” represent the number of copper layers. layer stack-up design improves signal integrity by carefully arranging signal layers concerning the ground layers.
The careful placement of signal power and ground layer also helps in minimizing crosstalk and eliminating noise. The stack-up design of the HDI PCB prototype also reduces the chances of localized heating by improving heat dissipation.
Better layer design not only enhances the overall PCB performance but also reduces the HDI PCB cost.
Strategy 7: Consider Impedance
Controlling impedance in HDI PCB is very important for high-speed applications common impedance value is 50 ohms. proper impedance control helps prevent the signal from getting reflected, degraded, or completely lost.
A thin signal trace has a higher impedance thus designers must carefully calculate the width of the trace to match the target impedance.
Another way of controlling the impedance is to use low-loss materials for HDI PCB prototype boards that have stable dielectric constants like Rogers and Taconic while standard materials like FR-4 can cause variations.
Strategy 8: Design for Manufacturability (DFM) Considerations
Design for manufacturability is one of the most important aspects that designers need to consider while developing HDI PCB prototype boards by applying DFM rules, designers can simplify the manufacturing process while optimizing the layout design, via placement, and selection of components to avoid issues that may arise during production.
DFM is necessary because it focuses on minimizing production costs while increasing yields along with enhanced reliability.
The IPC-2221 and IPC-2226 are the industry standards set by the IPC organization in which many rules for manufacturability in PCB and HDI PCB prototype design are been addressed along with IPC many PCB manufacturers publish their DFM to showcase their capabilities and requirements thus designers can design their HDI PCB prototypes according to these DFMs to maintain a balance between performance and manufacturability.