What Do HDI1, HDI2, HDI3, HDI… and ELIC Mean in PCB Boards?
If you move from making standard 4-layer or 6-layer through-hole PCBs (like traditional TV boards) to mobile phone products, you will run into a new concept: HDI high-end boards. So what is an HDI board?
HDI stands for High Density Interconnect. It uses laser microvias and buildup lamination to achieve fine-pitch VFBGA fan-out. The "order" number (HDI1, HDI2, HDI3…) represents the number of stacked laser blind-via layers on each outer side.
There is also a special type of HDI board: ELIC, short for Every Layer Interconnect (also called Anylayer). It is the highest-end HDI process.
Let's start with a question to show why HDI boards are needed.
Extra tip: From the numbers above, if the BGA uses 0.65mm ball pitch, you can fit a 0.3/0.5mm mechanical via between the balls. In that case, an HDI board is not required.
If you use a 0.1mm laser microvia, the total diameter with annular ring is 0.22mm. When the laser via is placed directly on the 0.25mm chip pad, the via-to-via spacing is still 0.4mm (same as the chip ball pitch), so no interference happens.
In summary: VFBGA chips need laser microvias, and laser microvias are a unique HDI process. That is why VFBGA chips require HDI high-end boards.
Before diving into the HDI types, here are two key concepts:
1. Stacked Via vs Staggered Via
Stacked Via: The L1-L2 and L2-L3 microvias share the exact same center point, aligned vertically to form a straight copper pillar, allowing a direct L1-to-L3 connection. The lower L2-L3 microvia must be fully copper-filled and planarized before the upper L1-L2 microvia can be made. This requires two separate lamination cycles, two laser drilling cycles, and two copper-filling plating cycles.
Staggered Via: The L1-L2 and L2-L3 microvias are offset horizontally and do not overlap. A short trace on L2 connects L1 to L3. The lower microvia does not need full copper fill — just barrel plating is enough, making it easier to manufacture.
In ELIC any-layer boards, every layer pair can use either stacked or staggered vias. In my experience, HDI board laser vias are almost always stacked. My take: stacked vias save space and shorten routing, giving better signal quality and power integrity.
2. Blind Via, Buried Via, and Through Via
(1) Blind Via: Connects the surface layer to an adjacent inner layer. One end is exposed on the PCB outer surface, the other end stops inside the board. It does not go through the entire board.
(2) Buried Via: Completely hidden inside the PCB, invisible from both top and bottom. Encased by dielectric layers. It only connects internal layers and does not reach the surface.
(3) Through Via: A mechanical hole drilled through all layers of the board.
Example for an 8-layer HDI2 board: Via 1-3 is a blind via, Via 3-6 is a buried via, and Via 1-8 is a through via.
With these basics covered, here is a breakdown of each HDI type:
1. HDI1 (1st-Order HDI, standard notation: 1+N+1)
One buildup cycle and one laser microvia cycle on each side of the core.
N is the core layer count. Inner layers are connected by traditional mechanical through vias or buried vias.
Blind vias can only connect adjacent layers: L1 to L2, and the last layer to the second-to-last layer (symmetric). Stacked vias are not supported. Cross-layer blind vias (L1 to L3) are not possible.
2. HDI2 (2nd-Order HDI, standard notation: 2+N+2)
1.Two buildup cycles and two laser drilling cycles on each side of the core. Typical stackup: 2+N+2.
2.HDI2 supports L1-L2 and L2-L3 blind vias, and supports both stacked vias and staggered vias.
3.The surface layer can jump directly to Layer 3 without needing a long through-via detour.
3. HDI3 (3rd-Order HDI, 3+N+3)
Three independent buildup cycles and three laser blind-via cycles on each side. Stackup: 3+N+3 (8-layer: 3+2+3, 10-layer: 3+4+3).
Blind vias can stack across three consecutive layers: L1-L2, L2-L3, L3-L4, forming a multi-step blind-via plus buried-via structure. With stacked vias, the surface layer can jump directly to Layer 4.
4. ELIC (Every Layer Interconnect, also called Anylayer)
Traditional 2nd-order or 3rd-order HDI boards have a thick solid core in the middle. Inside the core, only mechanical buried vias can be used — laser blind vias cannot cross the core.
ELIC has no thick core (note: not "no core" — it has no thick core). The entire board is built from thin laminated layers. Laser microvias can be placed between any two adjacent layers, without being limited by outer buildup count. This makes PCB routing extremely flexible.
Compared to HDI3, ELIC has no fixed 3+N+3 structure. Laser microvias can be stacked between every layer pair, giving the shortest possible cross-layer paths.
If the board is too thick (e.g. 1.6mm) and has a thick core, ELIC is not possible. For a 10-layer board, if the core is too thick for laser drilling, the maximum you can do is HDI4, and mechanical drilling must be used through the core.
HDI Board Cost and Lead Time
As a general rule, HDI3 boards cost 30%+ more than HDI2. ELIC boards cost more than double compared to HDI2.
For lead times, there is no absolute number, but here are some ballpark figures: HDI2 ~15 days, HDI3 ~25 days, ELIC ~35 days.
On a side note: AI is not only driving up memory prices lately, but also PCB prices significantly. So it pays to choose your PCB stackup wisely to balance cost and performance.
Two More Questions
That wraps up this overview of HDI boards. I have tried to be thorough, but my knowledge is limited — corrections and discussion are always welcome.
