HDI stands for high-density interconnect. As the name suggests, an HDI PCB is characterized by its high density of components and routing interconnections that use the latest PCB technologies. An HDI design is by its very nature a high-performance design.
HDI design uses the latest advances in the PCB interconnection technology. Keeping in mind the latest state of the art PCB technology, we can define the HDI PCBs as those printed circuit boards that use some or all of these features: microvias, blind and buried via/microvia techniques, built-up PCB laminations and high-signal performance considerations.
The total number of layers of your final HDI PCB is determined by BGA (or the highest pin count device), signals break-out, the number of signal layers, as well as the number of power and ground layers. The number of power and ground layers can be determined by taking into account the number of grounds, the number of different major voltages in the circuit, as well as signal integrity/controlled impedance requirements.
The sequence in which the layers are put together (signal, power, ground, etc.) is another factor that affects signal performance and allows for a balanced PCB structure. It is always good practice to order the layers in a balanced manner. You should have the number of plane and signal layers both odd or both even (both even is the best for a balanced structure). They should also be symmetrically placed.
Manufacturability of HDI design primarily has to do with via structures. Microvia structures can have a big impact on the manufacturing process since they directly affect the number of lamination cycles. The more variations you have of layers where microvias start and stop at, the more number of sequential laminations are needed for the PCB manufacturing.
Take these facts into consideration when selecting your materials:
Proper material selection is important for the layout design since materials will affect the electrical performance of the signal traces. The physical thickness of the material is important when considering the aspect ratio of the microvia to be plated. The current standard aspect ratio for a microvia is 0.75:1. (The microvia diameter should be larger than the height of the material it is penetrating to the next adjacent layer.)
In this 1-N-1 type of stack-up, the ‘1’ represents one sequential lamination on either side of the core. One sequential lamination adds two copper layers for a total of N+2 layers. This stack-up does not feature stacked vias. There is one extra lamination and no stacking of the vias. The buried via has been mechanically-drilled. There is no need to use a conductive fill for the via. It will naturally fill with the dielectric material. The second lamination adds the top and bottom layers. Then, we finish up with a final mechanical drill. The pcb manufacturer plans the right amount of prepreg between layer one and two so the resin flows into the buried via.
Software design tools do not check for drill-to-copper. If you provide enough annular ring, it does not mean that you cannot have any drill-to-copper issues. PCB materials move: When you laminate, a core might shift one way and another core might shift another way. If this happens, it might hit the copper.
If you use mechanical drills along with your laser drills, the first thing your manufacturer should check for the registration of the mechanical drills to the laser drills, which means more spacing is needed for drill-to-copper. If you do not use mechanical drills, the yield will be higher. With laser drills, drill-to-copper is not a problem because the manufacturer can easily laser drilled vias more accurately than mechanically drilled vias.
Soft gold is used a lot for wire bonding – ENEPIG can also be a process for wire bonding. Make sure that you can get non-contaminated surface finishes. In a HDI board where the traces might be smaller, there is no reason to use HASL, which is very rough and not smooth enough to put on BGAs. HASL is a very aggressive process which weakens the strength of the copper. For immersion gold, the typical thickness is 0.05 - 0.23 µm (2 - 9 µ in) gold over 2.5 - 5.0 µm (100 – 200 µ in) electroless nickel. This is not good for gold wire bonding and black pad. For soft gold, the typical SMT thickness is 0.25 – 0.8 µm (10 - 30 µ in) gold over 2.5 – 8 µm (100 - 300 µ in) nickel. This is expensive but good for dual surface finish for fine pitch and wire bond surfaces, and overhang.