How Surface-Mount Technology Increased PCB Component Density

The transition from through-hole technology (THT) to surface-mount technology (SMT) revolutionized electronics manufacturing by drastically increasing component density on printed circuit boards (PCBs). This article provides an overview of how SMT eliminated the need for drilled mounting holes, reduced overall component sizes, and enabled manufacturers to fully populate both sides of a circuit board, ultimately leading to the compact and powerful electronic devices used today.

Elimination of Drilled Holes

In traditional through-hole technology, components have long wire leads that must be inserted into holes drilled completely through the PCB. These holes take up a significant amount of physical space on every layer of the board, blocking routing channels and preventing other components from being placed directly on the opposite side.

Surface-mount technology solved this problem by soldering components directly onto flat copper pads on the surface of the board. Because SMT components do not require leads to pass through the board, the space directly beneath the component on the opposite side remains completely free. Holes on an SMT board—known as vias—are only used to pass electrical signals between layers and are significantly smaller than component lead holes.

True Double-Sided Board Population

The most direct impact of removing through-hole leads is the ability to populate both the top and bottom of a PCB simultaneously. With THT, placing components on both sides was difficult and inefficient because the soldered leads on the bottom would collide with the bodies of components placed in the same area on the reverse side.

With SMT, components sit entirely flat on the surface. Manufacturers can apply solder paste and mount components on the top side, flip the board over, and repeat the exact same process on the bottom side. Two complex integrated circuits can sit directly back-to-back on opposite sides of the same board without interfering with one another, instantly doubling the usable surface area of the PCB.

Radical Miniaturization of Components

Because SMT components do not need heavy wire leads strong enough to be pushed through a board, the components themselves can be manufactured at a fraction of the size of their through-hole counterparts.

Passive components like resistors and capacitors were shrunk to microscopic proportions, often smaller than a grain of sand. Furthermore, integrated circuits transitioned from large dual in-line packages (DIP) with wide pin spacing to compact surface-mount packages with extremely fine pitch (the distance between pins). The invention of Ball Grid Array (BGA) packages took this even further by placing the solder connections in a grid directly underneath the chip, maximizing connectivity while minimizing the component's footprint.

Tighter Component Spacing and Routing

Through-hole components require considerable clearance around their leads on the underside of the board to prevent accidental solder bridges during wave soldering. This limitation forced designers to leave wide gaps between parts.

SMT utilizes reflow soldering, a precise process where a pre-measured amount of solder paste is melted in an oven. Combined with high-precision automated pick-and-place machines, SMT allows components to be placed fractions of a millimeter apart on both sides of the board. Additionally, because huge drilled holes are no longer blocking the internal layers of a multilayer PCB, engineers can route copper traces much closer together, supporting the high density of components sitting on the exterior surfaces.