An HDI PCB is a multilayer circuit board with microvia diameter within 5mil (0.127mm), line space/width of inner and outer circuit layers within 4mil (0.10mm), and PCB pad diameter within 0.35mm. For HDI PCBs, microvias can be single microvias, staggered vias, stacked vias, and skip vias, and because of microvias, HDI PCBs are also known as microvia PCBs. HDI PCB features blind microvias, fine traces, and sequential lamination manufacturing.

HDI PCBs are usually classified by the HDI builds, and there are the 1+N+1, 2+N+2, 3+N+3, 4+N+4, and 5+N+5. In the HDI PCB's outer layers, microvias usually form the more expensive stacked vias or cheaper staggered vias.

Lamination process and the different types of HDI PCB stack-ups

HDI PCBs are multilayer boards that are constructed with densely routed layers and the boards are held together through a lamination process. These layers are electrically interconnected using different types of vias. The process of lamination begins with the etching of the inner copper layers. Later they are separated by partially cured laminates and stacked like a book with layers of prepreg on the top and bottom. The HDI PCB stack-up is then pressed and heated enough to liquify the prepreg. These liquified prepregs cool down and stick the layers together.

For blind and buried vias stack-ups the HDI PCB will undergo several numbers of sequential laminations. The more the number of laminations, the costlier will be the board. To increase routing density, designers increase the number of layers, producing a complex stack-up. Manufacturers use sequential lamination processes to fabricate such complex designs.

Some of the common types of stack-ups are:

HDI builds means how many times of laser drilling for microvias. 1+N+1 is the simplest HDI build, and it means the HDI PCB's microvias are single vias that need only one laser drilling.

2+N+2 means the HDI PCB needs twice laser drilling, and both the upper and bottom outer layers have 2 PCB layers. Unlike 1+N+1, 2+N+2 has two cases. One is that the microvias of the two adjacent outer layers overlap, and the two microvia form a stacked via. The other case is that the microvias of the two adjacent outer layers do not overlap, and they form a staggered via. As we know, it requires extremely high precision to locate and pile up two microvias, so stacked via HDI PCBs are more expensive than staggered via HID PCBs.

3+N+3 means the HDI PCBs need three times of laser drilling and both the upper and bottom outer layers have 3 PCB layers. And so on. The more times of laser drilling, the higher requirements for HDI PCB manufacturing technologies and more costs. 1+N+1, 2+N+2, 3+N+3, and 4+N+4 HDI PCBs are used commonly. 5+N+5 HDI PCBs are also in use but less. For example, iPhone 6 uses the 5+N+5 HDI PCB as its mainboard.

Advantages of HDI PCB

• HDI PCB provides designers with the freedom to design and place more components on both sides of the PCB. This is due to the higher wiring density with finer track arrangements on PCBs.
• With the use of microvias and via-in-pad technology, the components in an HDI PCB are densely packed with versatile routing which results in faster transmission of the signal and better signal quality.
• The HDI PCB boards allow you to pack all functions in one board rather than using several boards as in standard PCBs. This results in reducing the size and overall costs compared to the traditional PCBs.
• The HDI PCB boards are highly reliable due to the implementation of stacked vias which make these boards a super shield against extreme environmental conditions. Hence the boards are highly reliable.
• Laser drilling produces smaller holes and improves the thermal properties of the board.