Advanced PCB Technologies — High Density Interconnect

Engineers often face a peculiar dilemma. On one hand, they need to enhance the functionalities of electronic gadgets they design so that customers have more value for their money, while they are constrained to use a sleek form factor. Not only does this impose a tremendous challenge to cram many components within a highly restricted space, but the challenge extends to maintaining the quality and integrity of the design as well.

Designers meet the challenge in different ways. They use subminiature passive SMD components, often as small as 0402 (0.4×0.2 mm), special fine pitch ICs in packages such as CSP, TQFP, and BGA, and advanced printed circuit technologies that offer thin flexible, multilayer boards, especially the high density interconnect (HDI) types.

Designers use several advanced technologies in producing HDI boards. For instance, rather than using glass fibers for producing the base substrate, HDI boards use Polyimide and similar materials, as these are flexible, more durable, and can withstand very high temperatures without degenerating.

Designers use special plated through vias to interconnect different layers in a multilayer HDI board. Rather than drill holes in the PCB layers using metal drills, fabricators of HDI PCBs use lasers to drill extremely small microvia holes in the layer, which they later electroplate with copper. Since these microvias can be as small as 15-30 µm, they take up very little space on the PCB, leaving a large area for routing the traces.

Designers use traces with width as small as 20 µm to route the circuits on HDI PCBs. In combination with microvias, these thin traces allow them to achieve extremely high routing densities impossible to achieve on regular boards. This is especially helpful when designing with fine pitch ICs and high pin count BGA IC packages that have a pitch as small as 0.5 mm.

BGAs are surface mounting packages with solder ball arrays on their bottom surface. Large BGAs may have as many as 560 solder balls. With pitch size as small as 0.5 mm, it is nearly impossible for designers to run traces from each pad under the BGA. However, engineers have solved this problem in a rather unique way.

In regular PCB design, using vias within pads is taboo, as this causes dry solders. The plated through via wicks away molten solder, leaving very little solder between the pad and the IC pin. However, designers regularly use via-in-pads in HDI PCBs, as this allows them to save a lot of space that they can use for routing. Molten solder does not travel down the microvia in HDI PCBs, as fabricators fill them up and plate them over. This has another advantage, as filled vias become better conductors of heat.

Another trick a designer often uses for gaining higher routing density in HDI PCBs is placing different types of vias such as blind and buried types. Vias connecting inner layers in a multilayer PCB are buried vias, while those originating on one of the outermost layers and connecting to one of more inner layers are blind vias. Unlike a through via that passes straight through the board, designers can stagger blind and buried vias in different layers to achieve higher routing density.