Ready to make 5G/IoT projects? Read this article for an overview of designing HDI PCBs for 5G and IoT
PCB design and production have evolved throughout the years. From simple single-sided or multilayer boards, to highly sophisticated High-Density (HDI) PCBs. Each technological advancement in printed circuit boards can be used across various technologies. One such technology is 5G or IoT. UET PCB (https://uetpcb.com/hdi-pcbs/) specialises in these HDI boards tailored for 5G and IoT with optimised RF and digital performance.
Why is HDI needed for 5G/IoT?
5G devices operate in the extremely high frequency range (Sub-6 GHz to 28-39 GHz mmWave). There are various design considerations when working with these frequencies because:
- A higher operating frequency results in greater loss or attenuation in certain PCB materials.
- The layout tolerance decreases as the signal wavelength decreases.
- Ordinary through-holes can produce “stubs” that can act like antennas.
- Crosstalk and EMI increase with operating frequency.
- 5G antennas are ultra-sensitive to traces, vias, and copper pour designs
Starting a 5G PCB Design
You should define your PCB stackup before starting your HDI PCB design. Along with this, define your chosen PCB material.
Sub-GHz PCB Materials
If you are working in the Sub-6 GHz range (3 – 6 GHz), you can use high-performance PCB laminates such as:
- Megtron 6 (from Panasonic)
- I-Speed (from Isola)
- Rogers 4350B
- Rogers 403C
However, if you are on a tight budget, you can go with:
- FR4 or High‑Tg FR‑4
These are significantly cheaper than the materials mentioned previously. A high-Tg FR-4 material is simply an FR-4 with a high glass transition temperature (Tg). You can use these materials if your performance margins allow it.
mmWave PCB Materials
If you are working in the mmWave (24-40+ GHz), you’ll definitely need a higher frequency PCB laminate, such as:
- Rogers 5880
- Rogers 4350B
- PTFE (Teflon-based laminates)
- LCP (Liquid Crystal Polymer)
- Hybrid Stackups
Once you define your PCB Materials, you can proceed to define your HDI PCB stack-up.
How to Determine the Right HDI PCB Stackup
It’s best to determine the optimal HDI PCB stackup before aiming for the highest possible routing density on your boards.
Determine Operating Frequency and PCB Material
First, determine your project’s operating frequency, which should also reflect what PCB materials to use. As mentioned before, there are different materials you can use for Sub-GHz and mmWave. Additionally, you’ll need to know the manufacturing process you intend to use for that PCB.
Low or High Density Routing
The routing density indicates which PCB stackup to use. Take a look at your components, especially the BGAs. BGA pitches of 0.8mm – 1mm don’t need HDI, while pitches below 0.65mm need them. Examples of low pitches include 0.5mm, 0.4mm, and 0.3mm. Finer pitches may require microvia or via-in-pad technology.
Determine Power Distribution
Having enough power, ground, and plane layers is critical to having stable power delivery. This process will affect your layer count and distribution. Additionally, grounding layers help determine the characteristics of impedance-controlled lines and signal line shielding. Overall, these layers will definitely affect your routing density.
Determine your Stackup along with your Design Needs
Your project says a lot about the HDI stackup you need. For example, if you’re into entry-level RF modules or IoT sensor products, you can go with simpler stackups. When moving to 5G modules and WiFi 6 or 7 chipsets, you’ll need a finer PCB signal pitch and advanced via structure. Moving into 5G modems and advanced IoT gateways demands more sophisticated via structures, higher routing density, and ultra‑short interconnect paths. Below is a table you can use as a reference:
| HDI PCB Stackup | Kind of Project | Stackup Advantage/Disadvantage |
| 1-N-1 | Entry-level RF, IoT sensors, Consumer electronics | Low cost, simpler fabrication |
| 2-N-2 | Mid-range RF, 5G Modules, Wifi 6/7 chipsets | Balanced cost, complex fabrication |
| 3-N-3 or 4-N-4 | High-end RF, 5G modems, IoT gateways | High cost, long lead time, complex fabrication |
| Any Layer | High-end, Servers, Aerospace, Medical, Military applications | High cost, lower yield, long lead time, complex fabrication |
How to Achieve Higher-Density Routing
Use Microvia Technology
Microvias allow you to get through tight spots on your HDI PCB while also helping eliminate noise-generating stubs. The kind of microvia you can use depends on your manufacturing process. Ideally, you’ll be able to generate higher-density routing using laser-drilled microvias.
One core technology to use is stacking micro vias. You’ll be able to transition from one layer to another without taking up much vertical space. You can also stagger your microvias, although this can take more space, but it can be more reliable. Staggering mirovias are less prone to cracking, thermal cycling failure and delamination.
Using Via in Pad
Via-in-pad offers a via already on the BGA pads. This characteristic may require a copper filling and a planarization process. It’s easier to fan out a via in a pad, as there’s no need for stacking or staggering vias. It’s recommended to use via-in-pad when routing ultra-fine pitch BGAs cleanly. However, there are downsides to using via-in-pad, such as solder wicking, voids and cracks, and reliability concerns.
Use Fine Line/Space Pitch Traces
Laser Direct Imaging (LDI) can help you achieve trace widths as low as 75um. If you need finer lines, fabricating using semi-additive copper plating (SAP/mSAP) can help your traces go down to 25 um. Note that you may need a controlled etching process when using these advanced manufacturing techniques.
Strategic Design Process
The PCB engineer should take charge of developing a strategic design process for designing the PCB. Proper placement of critical components, such as BGAs and RF components/modules, should be at the top of the list. Carefully planning the fan-out or escape routing of BGAs enables complete net routing of your board; otherwise, you may end up with trapped nets or traces. Additionally, all controlled-impedance lines should be carefully managed using signal integrity tools and strategies.
Conclusion
You’ll need careful PCB material selection, advanced fabrication processes, and a well-designed PCB stackup to achieve higher-density wiring for your 5G and IoT PCB projects. Along with this, careful component layout and routing techniques are a must to avoid unnecessary complications and delays to reach your 100% wiring. UET PCB, with its more than 15 years of experience in PCB manufacturing and assembly, can help you with all your 5G and IoT manufacturing needs.