Build Sequence
Sequential build-up processes for complex circuit boards involve multiple cycles of lamination and laser drilling to increase routing density. Using multi-stage hdi allows for the placement of components with very high pin counts by utilizing stacked or staggered microvias across several layers. This method differs from standard production because the board enters the lamination press several times during its manufacture.
Microvia Stacking
Each stage of the process adds a new layer of dielectric and copper which is then drilled to connect to the layers below. A multi-stage hdi design might use a 2+N+2 or 3+N+3 construction where the numbers represent the number of sequential lamination cycles on each side of the core. These connections allow signals to travel from the outer surface to the inner layers through paths that take up very little space.
Fabrication Complexity
Producing these boards requires very high precision in layer registration to ensure that the small microvias land exactly on the pads of the previous layer. The cost of multi-stage hdi is higher than standard boards because of the extra processing time and the increased risk of yield loss. Each lamination cycle subjects the board to heat and pressure which can cause the material to shift or shrink.
Despite these challenges, the technique is necessary for the latest generations of high-performance processors and memory modules that require thousands of connections in a tiny area.
