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48 DESIGN007 MAGAZINE I JULY 2025 As HDI technology becomes mainstream in high- speed and miniaturized electronics, understand- ing the PCB manufacturing process can help PCB design engineers create successful, cost-effective designs using advanced technologies. Designs that incorporate blind and buried vias, boards with space constraints, sensitive signal integrity require- ments, or internal heat dissipation concerns are often candidates for HDI technology and usually require sequential lamination to satisfy the require- ments. This complex multilayer construction tech- nique enables finer features, dense interconnects, and multiple layers of microvias and buried vias within a single design. However, it's also a source of significant cost and potential yield loss if not thoughtfully designed. Sequential Lamination and the Importance for HDI Designs In sequential lamination, a stepwise build-up pro- cess enables connections on specific layers. Fig- ure 1 shows a simple representation of the via Sequential Lamination in HDI PCB Manufacturing by Mat t Stevenson, ASC Sunstone C ircuits structures where sequential lamination proves effective. This design uses buried vias from layers 2–5, microvias on layers 1 and 6, and through-hole vias from layer 1 to layer 6. To build this design, we will need to use a sequential lamination process. The first step is to create the image on copper layers 3 and 4. We will use our standard single-sided inner layer process. Photoresist is applied to both sides of the cores, representing layers 2 and 3, and layers 4 and 5. We then image the copper layers on 3 and 4 using our LDI (laser direct imaging) process. We flood and expose layers 2 and 5 so that all the copper will remain after etching. We then etch the cop- per circuitry into layers 3 and 4. We laminate these cores together with the appropriate prepreg lay- ers, creating a multilayer sub-assembly (sub). To create the buried vias, we will drill through- holes on our newly created sub-assembly from lay- ers 2 to 5. We then process the sub through elec- troless copper deposition, which adds a thin con- ductive layer of copper inside the drilled holes, C O N N ECT T H E D OTS Buried laser Through vias micro via Hole pre-preg Core pre-preg Core pre-preg L1 L2 L3 L4 L5 L6 Figure 1: A design using burie d vias from layers 2–5, microvias on layers 1 and 6, and through-hole vias from layer 1 to layer 6. ▼