CISTELAIER S.P.A.

With this article and some others that will follow later we would like to share available knowledge in the PCB design and manufacturing field.

Following some articles related to HDI PCBs we would like to describe the three related more common build-up structures.

HDI PCB types
The design designation system of this standard recognizes industry approved design types used in the manufacture of HDI printed boards. The designations determine the HDI design type by defining the number and location of HDI layers that may or may not be combined with a substrate.

Type I, Standard build-up for HDI
This construction describes an HDI in which there are both blind vias and through vias used for interconnection. Type I constructions describe the fabrication of a single blind via layer on either one or both sides of an PCB substrate core.
The PCB substrate is typically manufactured using conventional PCB techniques and laser via hole technology. The substrate can have as few as one circuit layer or may be as complex as any number of inner layers.

Type II, Sequential build-up 1+N+1
Tipe II is characterized by buried vias in the core and may have through vias through the outer layers. Type II constructions describe an HDI board in which there are plated microvias, plated buried vias and may have PTHs used for surface-to-surface interconnection.
Microvias on layer 1 and layer n grant the connection to conductors on layer 2 and layer n-1, making the connection with the core. Type II constructions can also have PTH to connect layer 1 directly to layer n.

Struttura Tipo III, build-up sequenziale X+N+X
Type III constructions describe an HDI in which there are plated microvias, paste/resin filled holes and through vias used for interconnection. Type III constructions are distinguished by having at least two microvia layers on at least one side of a substrate core. The substrate core is usually manufactured using conventional PCB techniques, may be rigid or flexible, and have as few as one or as many as any number of layers with buried vias.
Where space is minimal and if expressly requested, stacked vias - stacked blind holes - must be used - while where conditions allow it, staggered vias - non-overlapping blind holes on several levels - can be used. Staggered vias are less critical, so preferable, for manufacturing than stacked vias.

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In PCB market, HDI technology required the adoption of new technical skills, new machineries and the implementation of the related production processes including that of f illing the through holes and laser holes, necessary for the realization of" filled and capped vias”, technique also known as "plating over filled vias” well described by the IPC 4761 standard as Type VII filling.

The capped vias technology with hole filling with resin aims to increase the density of the interconnections in printed circuit boards by using the space occupied by through holes as SMD assembly points. This technique consists in filling the holes after plating them, with copper thicknesses on the walls, normally >25µm, and, in any case, defined with the customer.

The resins adopted for "filled and capped vias” have specific insulation properties and dimensional variation with temperatures, they are treated with heat for the relative polymerization and consequent hardening, and, subsequently, are first planarized and then covered with a layer of copper with a thickness of at least 15µm.

This technique can be applied for the realization of different types of printed circuits and with multiple application variants which for this reason are in strong expansion:

- pads used for µBGA, "ball on via technology" and SMD assembly: "Via in Pad Assembly Technology"
- pads used as test points, "Via in Pad Test Technology”
- heat dissipation pads under the cases of the components for "heat management”
- SBU - sequential build up" construction with laser stacked holes on buried holes treated as capped vias

The fundamental operations for filling the holes with resin are carried out in two distinct phases: in the first, the holes are filled with variable pressure and vacuum to allow perfect filling of the holes without the risk of gaps in the resin, while, in the second phase, a surface cleaning of the panel is performed to eliminate the excess resin, facilitating its subsequent planarization.

Whatever the final technology chosen, after complete polymerization, the resin is removed by a mechanical brushing action, called planarization, which can be performed with specific machines that work with cup brushes or with horizontal brushing machines.

The target is to remove the excess resin and create a uniform surface: this operation is a prerequisite for the subsequent over-plating of the filled vias with copper to allow ideal soldering of electronic components. (inserisci disegno).

The knowledge of "capped vias" technology is fundamental today to create compliant pcbs to market and regulations standards required, especially relating to the growing HDI technology demand.