HDI PRINTED CIRCUIT BOARDS

PRINCIPLE KINDS OF VIAS IN HDI PCBS

As described above, vias are the conductive holes that connect and allow electrical (or thermal) signals to be transmitted through the multiple layers of an HDI PCB. Vias have different names depending on their position and their construction:

1. Through-Hole Vias: are copper-plated holes or cylinders that traverse the entire printed circuit board from top to bottom. They form conductive paths that connect all of a board’s layers. Through-hole vias are the least costly vias to fabricate.
2. Blind vias: are copper-plated holes or cylinders that connect the top or bottom layers of a PCB to one or more of its inner layers. Unlike through-hole vias, blind vias do not go through the entire board thus they are only visible from one side of the board.
3. Buried Vias: are copper-plated holes or cylinders that connect two or more of the inner layers of a board but do not penetrate either outer layer of the board. Buried vias thus are not visible from either side of a board.
4. Microvias: are particularly tiny vias that interconnect two adjacent layers. They have a diameter less than 150 microns, making them the smallest and most space efficient type of via. Microvias are rarely found in traditional PCBs but are common in HDI PCBs. Microvias may be through-hole, blind or buried.

ADVANTAGES OF HDI PCBS OVER TRADITIONAL PCBS

Signal Integrity
Microvias in an HDI board have a significantly improve signal integrity. And, because components are placed closer to one another than in traditional PCBs, more versatile routing is possible, which results in faster signal transmission with greater quality and reduced power consumption. Also, microvias do not introduce parasitic capacitance and inductance problems, thus reducing crosstalk and noise coupling issues.

Cost-effective
Because of the lower number of required layers, smaller dimensions, and the greater number of components, HDI printed circuit boards can function as an all-in-one circuit board rather than requiring multiple boards as with traditional PCBs. When compared to purchasing several traditional PCBs for different programs, an HDI PCB will provide equivalent or superior functionality by means of a smaller board that costs less to produce.

Reliability
Microvias are far more reliable than through-holes due to the usage of a lower aspect ratio (the ratio of the thickness of the board to the diameter of the hole. While HDIs typically use thinner boards than traditional PCBs, the use of stacked vias give the board exceptional strength and resistance power by serviing as a strong shield against adverse environmental conditions. HDI printed circuit boards will perform reliably for much longer than most traditional PCBs.

Faster time to Market
Because of the versatility allowed for component, pad, and trace placement, the reduced number of components required, the usage of vias, and the increased electrical performance, the design and testing process for HDI PCBs is much less time-consuming that for traditional PCBs.

Power/Efficiency
HDI PCBs create a more convenient and effective routing that enables the deployment of several transistors for improved electronics performance while reducing power consumption without sacrificing electrical functionalities.

Reduced Inductance, Capacitance, and Electro-Magnetic Interference
Signal radiation and reflection are major issues with larger vias since they behave as high frequency radiating antennas, especially when a signal resonates in a stub. Microvias are smaller than ordinary vias, hence they have lower emission and absorption cross-sections, and there will be no stubs due to sequential build-up. Because HDI PCBs are built with thinner dielectrics, microvias are closer to ground planes and have better shielding. Finally, because there are no dog-bone pads with traces, filled and capped microvias put directly in component solder lands minimize inductance in the distribution network. All the above elements contribute to decreasing both radio frequency and electro-magnetic interference.

Increased Density of Connections
Microvias-in-pad save space by creating connections inside the pads of surface-mounted technologies (SMTs), in addition to providing more dense connections between layers and with smaller traces. Furthermore, high circuit density boards are required to enable components to be packed into smaller places and allow fine-pitch components to be used. Microvias-in-pad will fit inside the pads of even the finest pitch packages, particularly BGAs, and they will be far more beneficial in breakout channels. Using microvias can enhance escape path width, potentially allowing you to require fewer layers to breakout a BGA, allowing designers to design and install additional components on both sides of the PCB. Other notable improvements in performance include increased thermal dissipation, more stable voltage rail, reduced parasitic loads, and lighter weights than non-HDI printed circuit boards.