How to make HDI Rigid-Flex Board with Blind Microvias

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HDI or High-Density Interconnect circuit boards are printed circuit boards with a higher wiring density per unit area than the conventional board. With an average of 120 to 160 pins per square inch, HDI PCB occupies minimal space and enables versatile routing. HDI PCBs improve electrical performance and reduce the weight and size of electric and telecommunication devices.

Laptops, personal computers, mobile phones, MP3 players, gaming consoles, and all other end-user portable devices use HDI PCB technology.

Rigid-flex PCBs are a hybrid of rigid and flexible circuits. They can resist hundreds of thousands of flex cycles without failure.

Advantages of HDI PCBs

The HDI PCBs allow the following advantages:
• Utility
• Improved Signal Quality
• Reliability
• Cost-effective
• Easy Installation

What Are Microvias?

According to IPC, “a microvia is a blind structure (as plated) with a maximum aspect ratio of 1:1. It terminates on or penetrates a target land, with a total depth (X) of less than or equal to 0.25 mm [0.00984 in] measured from the structure’s capture land foil to the target land.”

The conductive holes for forming electrical connections between HDI layers microvias are used for making high-density interconnect PCBs. They are an essential component of our electronic devices, laptops, smartphones, etc. Microvias are the reason behind the lightweight and small-sized hand-held devices nowadays.

PCB manufacturers use laser drilling to drill holes for microvias. Sever layers are drilled separately and then stacked on top of each other during the manufacturing process.

Blind Vs Buried Vs Stacked Microvias

Microvias can further be divided into blind, buried, and stacked microvias.

Blind Vias only connect the topmost layer of a PCB with one or more inner layers they can easily reach without going through the entire board. Blind vias provide the easiest path for enhancing the wiring density by routing signal tracks from the outer layer of a PCB to the inner layer.

Buried Vias connect the inner layers and do not pass through to reach the outer layer. The consecutive laminations technique is used for creating the buried vias.
Stacked Viascan be blind or buried. They incorporate electroplated copper for connecting the high-density layers to ensure a reliable transfer of signals through PCB layers. When an essential blind goes beyond the 1:1 aspect ratio and phases out the sequential lamination, stacked vias provide that extra capacity.

Why Select Blind Microvias?

Blind microvias offer the following advantages:

Space and Cost

As established earlier, microvias save space. This automatically converts to low costs. The less space the vias requires, the smaller the board requirements and the cheaper your project will become.

Blind microvias require an easy fabrication than regular blind and buried vias.

Reduced EMI

Microvias also help in decreasing electromagnetic interference (EMI). The EMI occurs as an electromagnetic field's aftereffect to an electrical circuit. They can disrupt the operational ability of an electronic device. Due to low EMI risks associated with microvias, they are helpful for circuits that are susceptible to EMI, like the ones used in high-frequency applications.

Applications with high-speed circuits face signal radiation issues due to reflection in vias. The reduced side of vias leads to a reduction in radiation capacity, which means: less EMI.

Thermal and Electrical Conductivity

Copper-filled vias have multiple benefits relevant to thermal and electrical conductivity. The increased thermal conductivity disperses the heat from the board, improving the board's life and decreasing the risks of defects. The other material used in place of cotton is gold, but it has lower electrical conductivity than copper. So, copper-filled microvias are the preferred choice for high-voltage applications and those needing strong currents to move from one side of the board to the other.

Manufacturing Process of HDI Rigid-Flex pcb Board with Blind Microvias

• The first step is to drill the microvia holes and pour epoxy resin with copper or pure copper into these holes for connecting the PCB surface to the internal-layer circuits. This copper helps join the connectors or components on the board and enables the circuit connection between layers. The copper is deposited until the via gets closed.

• To ensure more copper gets deposited in the via and avoid voids in the microvia holes, manufacturers add some additives. These organic additives lead to conformal deposition of copper into the blind microvia. This conformal plating controls the coating characteristics, improves the aspect ratio, and impedes the solution and mass transport. Some examples of these organic electrolyte additives are a starter, grain refiner, and leveler.

• Another way to fill copper into a blind microvia is the bottom-up method. In this method, copper is deposited on the capture pad. This results in a low dimple with the least amount of copper plated on the surface.

• While filling copper into the via hole, it is vital to keep the copper layer within the via, even. The outer layer does not have to be very thick because the added weight will deposit excessive copper on the traces, defects, and increased costs.

• Pure copper causes voids to form; hence a mixture of epoxy resin and copper is the preferred option for filling blind microvias. To avoid the voids and contaminants in the copper, manufacturers leave a space at the top of the filling.

• Sometimes, when the holes are filled, it creates 2 blind microvias. For this process, manufacturers begin with conformal plating and, halfway through the process, switch to aggressive pulse plating. Doing this creates an X-shaped cross in the hole due to the formation of 2 copper triangles on the wall of each hole. This shape is known as 2 blind microvias on both sides of the PCB.

• The latest research and advancements have led to more accessible methods of filling blind microvias without causing defects and excessive copper deposition. Blind microvia filling electrolytes have low chloride ions, sulfuric acid concentrations, and high copper concentrations.

Final Consideration

If you're in the market for HDI rigid-flex PCBs with copper-filled microvias, the experts at will make the process smoother. We ensure that your orders are developed according to the exact specifications and delivered on time.




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