HDI, short for High Density Interconnection, is a type of PCB
(printed circuit board) technology starting to develop at the end of 20th century. For traditional PCB boards, mechanical drilling is utilized, with some disadvantages including high cost with aperture being 0.15mm and difficulty in improvement as a result of the influence of drilling tool. However, for HDI PCBs, laser drilling is utilized and as soon as it was introduced, it received massive popularity. HDI boards are also called laser boards whose aperture is ordinarily in the range of 3.0-6.0mil (0.076-0.152mm) and line width 3.0-4.0mil (0.076-0.10mm), which results in the fact that the pad size can be drastically decreased so that more layout can be arranged in each unit area. HDI technology adapts and pushes the development of PCB industry and now HDI circuit boards have been massively applied in all kinds of devices.