What is HDI PCBs?
High density interconnect (HDI) PCBs represent one of the fastest-growing segments of the printed circuit board market. Because of its higher circuitry density, the HDI PCB design can incorporate finer lines and spaces, smaller vias and capture pads, and higher connection pad densities. A high-density PCB features blind and buried vias and often contains microvias that are .006 in diameter or even less.
1.Multi-step HDI enables the connection between any layers;
2.Cross-layer laser processing can enhance the quality level of multi-step HDI;
3.The combination of HDI and high-frequency materials, metal-based laminates, FPC and other special laminates and processes enable the needs of high density and high frequency, high heat conducting, or 3D assembly.
HDI PCB Builds: 1+N+1, 2+N+2, 3+N+3, 4+N+4, 5+N+5
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.
Note
Microvias are drilled by laser (CO2 or UV) and penetrate only one PCB layer.
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 laser drilling, the higher the 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, the iPhone 6 uses the 5+N+5 HDI PCB as its mainboard.