HDI Structure And Cost: Type I, II And III

Two boards can have the same layer count and the same outline and still differ by a factor of three in price, because the cost of a high density board is driven by how the layers are connected rather than by how many there are. The HDI structure determines how many times the panel has to be drilled, plated and laminated, and each of those cycles adds cost, yield risk and lead time.

This article explains the three common HDI structures, what each one adds to the manufacturing sequence, and how to decide which is needed.

The comparison is worth making early, because the choice is usually made during layout, when it is still possible to reduce the structure without disturbing the design.

Why The Structure Drives The Cost

A conventional multilayer board is drilled once, plated once and laminated once, regardless of how many layers it has. A build up structure is made by pressing additional layers onto a finished core, and each of those additional steps is a separate lamination cycle with its own drilling and plating operations. The cost of the board therefore scales with the number of cycles rather than with the number of layers.

The other driver is the drilling method. A microvia is small enough that it is formed by laser, which requires different equipment and different process control from mechanical drilling, and the holes then have to be filled and plated over so that the next layer can be pressed on top. Both operations add steps, and both reduce yield compared with a simple through hole. The filling and plating of the microvia is where much of the process difficulty is concentrated.

Type I: Microvias On The Outside

The first level of HDI adds laser drilled microvias to the outer layers only, connecting the surface to the layer below. The core of the board is a conventional multilayer construction, so the inner layers are made and laminated in the ordinary way, and the build up is applied afterwards on each face.

This is the least expensive form of HDI because it adds only one extra lamination cycle and the through vias that pass through the whole board are still mechanically drilled. It buys a significant amount of routing freedom, since the microvias free the area directly beneath a dense component, and it is often enough to solve a routing problem that a thicker through via cannot. Compared with an equivalent standard construction, the cost increase is in the range of forty to seventy percent.

Type II: Adding Buried Vias

The second level introduces buried vias, connections between inner layers that are made before the outer layers are pressed on. That means the inner core has to be drilled and plated as a separate sub assembly, then laminated inside the board, which adds a drilling operation, a plating operation and a lamination cycle.

The benefit is a further increase in wiring density in the inner layers, which matters when the ball pitch of the largest device leaves too little room to escape between the planes. The cost of the additional steps is substantial, and the increase over a standard construction is typically in the range of eighty to one hundred and twenty percent. Whether that is worth paying for depends on whether the routing genuinely needs the extra inner layer freedom. The selection of blind and buried via structures is the decision that sets this in motion.

Type III: Multiple Build Up Cycles

The third level uses more than one layer of buried vias, so the board is built up in several stages, each with its own drilling, plating and lamination. A ten layer board of this type may involve three separate lamination cycles, four drilling operations and three plating steps before it is finished.

Each additional cycle multiplies the opportunity for something to go wrong, and the yield falls accordingly. The cost increase over a standard construction is commonly quoted at one hundred and eighty percent or more, and at the extreme end the structure becomes the dominant term in the price of the board. This level of HDI is used where the routing density cannot be achieved any other way, typically on very dense products with fine pitch devices and a hard limit on board area.

The Percentages Are Only A Starting Point

The figures quoted for each type are useful for orientation and misleading if taken literally. They are calculated for a particular layer count and board size, and both of those change when an HDI structure is introduced. Adding build up layers can allow the through via count to fall, which changes the layer count, and the panel utilisation changes with the design rules.

The only reliable number is a quotation for the specific stackup, and it should be obtained while the layout can still be changed. A fabricator who is asked early can also point out where the structure is more elaborate than the design requires, which is worth more than a discount negotiated after the artwork is finished. Bringing the stackup into the review before the panel is released is the usual moment for that conversation.

Choosing The Least Structure That Works

The sensible approach is to start with the simplest construction that can be routed and then add structure only where the routing fails. A standard multilayer board with ordinary through vias should always be the first attempt, because it is the cheapest to build and the easiest to yield. Where the escape routing from a fine pitch device does not fit, the outer microvias of a Type I structure are usually the next step.

Buried vias should be added when the inner layers are genuinely congested rather than because they seem more capable. gopcb reviews proposed HDI structures against the routing that actually has to be achieved, and will say when a simpler stackup will do the job, because a board that is harder to build than it needs to be costs the customer in both money and yield.

Process Control and Verification

On a design of this kind, HDI structure is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

FAQ

Is HDI only about fine pitch components? No. Microvias free routing area under any dense device and also help where a thin board has to carry many nets, even if the components are not the finest available.

Do more layers always mean more cost? Not in the same way. The number of lamination cycles and drilling steps drives cost more strongly than the layer count alone.

Can a Type III board be converted to a cheaper structure later? Sometimes, by increasing the layer count of a simpler build. That is worth exploring with the fabricator before committing to a complex stackup.