An Introduction to high density interconnect (HDI) PCBs

1.What are HDI PCBs?

      High density interconnect (HDI) PCBs, just as the name suggests, are printed circuit boards packed with more components and denser wirings in a smaller area. One of the primary features that differs HDI PCBs from conventional ones is that HDI PCBs use microvias with no more than 150 um in hole diameter to achieve high density wiring. Microvias, blind / buried vias, or vias on pads, are contributing factors that make it possible to miniaturize HDI designs.

2.Why HDI PCBs?

      Due to the technologies of microvia and sequential lamination, HDI boards have the following features:
Laser-drilled microvias combined with blind / buried vias are used.
High density wiring reduces the space between components.
HDI PCBs which feature higher density are thinner, smaller than traditional PCBs.
With the increasing demand for hi-tech electronic devices, which require high frequencies, portability, and high performance, these features make HDI PCBs competitive in the following fashions:
Improved signal integrity – with shrunken distance between devices and signals as well as shortened via stubs, the pick-up noise, signal distortion, and loss of signal strength during transmission diminish significantly, which in turn enhances signal integrity for better performance.
Better reliability – the depth to diameter aspect ratio of traditional PCB holes is usually 10:1, while that of microvias is as small as 1:1. This largely improves transmission conductivity and ensures better reliability.
Cost effectiveness – the miniaturization of HDI PCBs also means that HDI PCBs can make the best use of board areas and decrease the quantity of materials used. This feature may also ease the design effort to achieve high performance so as to shorten the test and verification cycles. All of these contribute to making HDI designs cost effective.

3.HDI PCB Stack-ups

      Depending on the times that sequential lamination process is applied to attach HDI layers to core layers, the HDI PCB structures can be categorized into various types.
HDI PCB (1-N-1):
‘1’ represents 1 build-up HDI layers with laser-drilled microvias. It is the simplest form of HDI PCBs.

Fig 1. HDI PCB 1-N-1

HDI PCB (i-N-i):

      ‘i’ stands for 2 or more build-up HDI layers. There are staggered or stacked microvias on HDI layers.

Fig 2. HDI PCB i-N-i

Any Layer HDI (or Every Layer Interconnection):

      Any Layer HDI is the most complicated form of HDI PCBs with all layers on the Any Layer HDI PCBs being high density interconnection layers. Microvias on these boards can freely interconnect any conductors on each layer.

Fig 3. Any Layer HDI

4.Requirements for HDI manufacturing

      HDI PCBs are manufactured by sequential build-ups or laminations on core layers. Its feature of miniaturization puts forward new requirements for HDI manufacturing. Usually, the high density interconnection is embodied in three aspects:
Miniaturization of through holes – microvias with no more than 150 um in hole diameter require laser drilling and improved hole position control accuracy.
Refined trace width / space – requirements for wire defects and surface roughness are increasingly stringent.
Decrease in dielectric thickness – the dielectric thickness between copper layers tends to be 80um or below. Meanwhile, the requirements for thickness uniformity have become stricter.