What are Footprints and Land Patterns

3. Surface Mount Basics

Understand Surface Mount Terminology

On the side/bottom of every part are a series of metalized part pins/pads that need to be connected to metalized land pads on a printed circuit board with solder.  This chapter provides a brief overview of package types and terminology.

Hard Way Hughes"In an attempt to avoid confusion in the following text, the metalized pads on a PCB will be referred to as lands or land pads while the metalized pads/pins attached to a specific component will be referred to as part pins/pads.  In real life, the distinction isn't quite as clear since both metalized parts are referred to simply as pads."

The Evolution of Parts

Integrated Circuits are available in a variety of packages.  The packages all have the same goal -- to provide an easily manufacturable way to electrically and thermally bond to a PCB.  As technology advances, the packages increase in size and pin density.

Image of Semiconductor History

A few package designs along with their timeline of introduction from anysilicon

In the early days of PCB manufacture, Dual In-Line (DIP) packaging was quite common.  Inside DIP packages, there is a silicon die that contains the integrated circuit, a metal frame, and bonding wires.  This packaging method was often standardized on 0.1" (2.54 mm) spacing.  (This is one reason why solderless breadboards evolved to have 0.1" pitch.)  As integrated circuits grew more complicated, the number of pins grew as well, which meant the packages could become quite long and wide.  Another disadvantage of DIP packages is the fact that through-hole mounting consumes a great deal of real-estate on a printed circuit board since it requires board space on every layer of a board and prevents components from being mounted on the opposite side.

One solution to the problem of limited PCB area was to use gull-wing pins on surface-mount devices.  These abandon through-hole attachments in favor of metalized lands.  Surface-mount technology allows multi-layer PCBs that don't have pesky through-holes that interfere with routing.

DIP Package

This artistic interpretation of a Small Outline Package shows the lead frame, die and bonding wires of a fictional device.

Quad Flat Packages offered a simple modification of the lead frame to add additional pins.  This technique can increase the number of pins without dramatically increasing the size of the package.

QFP28 Part

This artistic impression of a QFP28 simply demonstrates that pins extend from all sides of the package.

And as another example, Ball Grid Arrays offer even more connection points by using an XY grid of connections.  Tiny solder balls melt during reflow to connect the part to the PCB.

Ball Grid Array

This artistic impression of a Ball Grid Array offers a glimpse into the regularly spaced pattern of balls used in the device.

Our design uses a combination of package designs to allow you to learn how to work with a variety of land patterns.