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HDI technology for high-performance printed circuit boards


The global electronics market has been pushing for years towards increasing miniaturization.

This trend is imposed by the market demand for increasingly portable devices, therefore of low weight, and capable of integrating many functions: the real challenge is to reduce the size of the devices, miniaturizing them, and, at the same time, to increase their performance.

The target can be reached basically by working on the structure of the printed circuit board by increasing the density of the components on the surface and the number of connections between the layers that make it up: this need has therefore led to the birth and diffusion of "HDI - High Density Interconnection" structures.

All circuits with pitches’ density per surface with values greater than 20 pitches/cm² greater than traditional printed circuit boards (PCBs) are defined as high interconnection density PCBs (HDI). This therefore results in smaller size for design parameters such as tracks less than 100 ?m/4mils wide, laser holes with a diameter less than 150 ?m/6mils and pitches with a diameter less than 400 ?m/16mils.

The above-mentioned structures have opened up rather suggestive scenarios for designers as the need for weight reduction and size has led to a more widespread development of real "System in Package" where the above-described Design Rules are combined with other characteristics quite interesting at the design level:

- Thick copper technology (from 70 to 210 ?m, up to 400 ?m): use of copper planes with dissipative function or transport of high currents in HDI structures where digital and power must be integrated;

- Aluminum or copper (IMS) supports with sequential build up multilayer structures where the high power needs to be managed digitally giving the possibility of integration of the two systems;

- Construction of flexible and rigid-flexible circuits where all the boards that were previously interconnected with connectors are now connected to each other with flexible material layers (Polyimmide) that thanks to HDI techniques allow maximum integration and reduction of weights and sizes of multiple board devices finally giving the designer the opportunity to draw printed circuits with a 3D vision instead of a 2D perspective as in the past.

The increase in the number of interconnections in the board has increasingly made the printed circuit board, once considered as a mechanical support of the components, an "active" and not "passive" element of the final devices.

On the basis of this new scenario, especially in the "HDI - High Density Interconnection" structures, it has become more and more important to verify the printed circuit board also as a "sophisticated" connection element where the impedance of the tracks, the electrical values (DK & DF) of the laminates used, the copper thickness of the tracks, the thickness of the copper in the holes are analyzed and evaluated with specific simulations to ensure the "signal & power integrity" of the project. For the same purpose, specific "thermal analysis" are conducted to ascertain a correct "heat management".

All these aspects are necessary, and now essential, to guarantee the correct and reliable operation of the new components.


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