HDI PCB: microvias, via-in-pad and sequential lamination

Each build-up level adds one more pass through press, drill, plate and inspect, so the cheapest HDI decision is often the one that removes a level. Most HDI cost is fixed in the design phase. The package choice sets the pitch. The pitch sets the via size and the number of build-up levels. The levels set the lamination cycles, and each cycle is a new chance for layer-to-layer registration to drift. When our engineers draw the schematic, they check parts availability and alternates first, and they ask whether a package with a coarser pitch does the same job. A second engineer then checks the schematic and the layout (power tree, footprints, connector pinouts, parts against the BOM) before delivery.

Via-in-pad needs the same early care. A via inside a BGA pad must be filled and capped, or solder drains down the barrel during reflow. Via-in-pad does not by itself need the advanced route: the standard route asks for the via plating method, and "Filled and plated over" goes to engineering review there. Our DFM review in the design phase sorts each finding by when it must be fixed: before the demo build, before EVT, or before production. Signal integrity is the other reason for HDI. Short microvias leave short stubs and low parasitic inductance, which helps fast digital links. Controlled impedance is declared with its targets and the critical nets named, held to ±5% impedance and checked by TDR testing. Low-loss and Rogers laminates are declared laminate families. Boards above the configurator's 30 layers, up to 40 layers, are quoted after engineering review.

Start from the densest package on the board. If its pitch leaves no room to route a signal between two vias at your minimum trace and space, you are at the HDI boundary. If the board is only small or only crowded, better placement and more layers usually solve it without HDI.

A second test is the layer count. If through-hole routing pushes the stack past what the board can carry mechanically, microvias may bring it back. Trace and space below the configurator's 0.10 mm minimum, down to 2 mil (0.05 mm), is quoted after engineering review, and so are boards above 30 layers. If you are not sure, send the BGA part numbers with the order; our engineers answer this question in the free engineering review.

Staggered microvias are offset from one level to the next, so no via sits on another. Stacked microvias sit on top of each other and route straight down, which uses less area. Stacked is denser and harder to build, because each level must be filled flat enough for the next one to land on it.

A defect at any level of a stacked column can fail the whole column, so a staggered structure is the more forgiving build when the routing allows it. Both are declared options and both go to engineering review. The choice follows from the escape routing.

A laser can only drill a microvia through the outer dielectric layer that is being formed. To reach deeper, the board is laminated, drilled, plated and filled, then laminated again with the next layer pair. Each cycle repeats press, drill, plate and inspect, and each one adds cost and registration risk.

This is the real cost of HDI, and the number of build-up levels is a design decision worth time. We run up to 4 sequential-lamination cycles. Sequential lamination, backdrill and press-fit holes are each declared per project, so the review plans the process around them early. A high-speed stack-up guide explains how layer order affects the same decision.