Origin of the Name
HDI is the abbreviation of High Density Interconnector. Printed circuit board is a constructive element made of isolating material and conductive tracks. Integrated circuit, transistor, diode, and active devices (like resistors, capacitors, and connectors) are generally soldered on the PCB. It mechanically supports and electrically connects electronic components using conductive tracks. Hence Printed Circuit Board is the platform to get the elements connected and foundation to link the components.
The final definition of PCB name is kind of confusing since it is not a common terminal product. For example, it is called mother board in the personal computer other than PCB even if there is PCB in the mother board. Here is another example. Since there is integrated circuit part loaded on a printed circuit boards, PCB is called IC board by some press. However, they are not the same thing.
Current electronic products tend to be multifunctional and more complicated. The distance between joints of IC elements becomes closer and the speed of signal transmission gets faster. Accordingly the quantity of connections gets increased and the length of wiring gets partially shortened. Thus it needs high density circuit board configuration and micro-aperture technique to get the goal accomplished. Single or double layer PCB cannot fulfill the requirement of wiring and crossover. That’s why multilayer boards will definitely become the mainstream. Now it’s necessary to have more power plane and ground plane due to increased signal lines. All the above push multilayer printed circuit board into general use.
As a requirement of high speed signal, PCB should be able to ensure impedance control featuring AC, high frequency transmission, and decreased unnecessary EMI. Multi layer becomes a necessary design thanks to Stripline and Microstrip structure. Low-k insulation materials with low damping decrement are applied to reduce quality problems in signal transmission. It keeps leveraging the density of PCBs to work in with miniaturization and array form of electronic components framework. The introduction of BGA (Ball Grid Array), CSP (Chip Scale Package), and DCA (Direct Chip Attachment) helps boost the development of HDI.
Any hole with diameter of less than 150um is called Microvia in the business. The geometry of Microvia is very helpful in space utilization and compatibility of electronic products.
There used to be many terms for this type of PCB products in the business. It was called SBU (Sequence Build Up Process) in US and Europe since the sequence composing method is used in manufacturing. In Japan, there was a different name, MVP (Micro Via Process) since the hole is much smaller than it used to be. Besides, someone also called it BUM (Build Up Multilayer Board) due to MLB (Multilayer Board).
To prevent from confusion, US IPC proposed to set a general term for this type of products, namely HDI (High Density Intrerconnection Technology). Thus people start to use High Density PCB or HDI board.
The application of HDI
In electronic design, people make every best to improve the functionality of the product while keep trying to minimize the size. HDI technology just comes out in the right time to help the terminal product design achieve the two goals. Currently HDI is widely applied in cell phones, digital cameras and camcorders, MP3 and MP4 players, laptops, automotive electronics and other digital gadgets. Normally HDI board is made in Build-up method. So more layers mean more advanced technology. The common HDI board is single layer basically. High level HDI board use 2 and more layers together with advanced technology including hole laying, electrolytic plating hole filling, and laser drilling. High level HDI boards are mainly used on 4G cell phones, top grade digital camcorders, and IC support plates.
The vision: Thanks to the application in 3G/4G boards and IC support plates, it is clear that high level HDI will grow very fast in the future.
Advantages of HDI
1. Lowered PCB cost: if using HDI in manufacturing PCB with more than 8 layers, its cost will be less than that using traditional complicated pressing manufacturing.
2. Increased circuit density: interconnection between traditional circuit board and parts
3. Helpful for the application of advanced construction technology
4. More accurate signal
5. High reliability
6. Improved thermal stability
7. Improved RFI/EMI/ESD
8. Improved design efficiency