What Is a BGA? Ball Grid Array Explained for Beginners

A ball grid array replaced the fine pitch lead frame for one practical reason: a processor with a thousand connections cannot be soldered reliably around its perimeter, but it can be soldered underneath. The BGA places a grid of small solder balls on the underside of the package, and those balls become the electrical and mechanical connections to the board once the assembly passes through reflow. That change solved a density problem and created a set of new ones, because the joints are invisible from above and the package is far less forgiving of a badly designed pad or a warped board.

What a BGA Actually Is

A ball grid array is a surface mount package in which the connection points are distributed as a two-dimensional array of solder balls on the package underside rather than as leads along the edges. The balls are attached to a substrate, which is a small multilayer board that redistributes the fine pitch of the silicon die to the coarser pitch of the array. The die sits on top of that substrate, connected by wire bonds or by flip chip bumps, and is covered by a lid or encapsulated in molding compound.

Because the balls are underneath the package, the whole footprint is available for connections. A package that would need a lead pitch well below the capability of any assembly process around its perimeter can be built with balls on a pitch the process handles comfortably in two dimensions. That is the entire idea, and every advantage and every difficulty of the package follows from it.

How the Structure Is Built

The chain starts at the wafer and ends with the finished package mounted on a printed circuit board. Understanding the intermediate steps explains the behaviour of the part, particularly its thermal and mechanical characteristics, and it also explains why the same silicon can appear in packages with very different electrical performance.

Die, substrate and interconnect

The die is the silicon itself. It is thinned, diced and attached to the package substrate, and the electrical connection is made either by fine wires bonded from the die pads to the substrate or by flip chip bumps that connect the active face of the die directly to the substrate. The substrate is a small multilayer board, and its layer count determines how easily power can be distributed to the centre of the array.

Solder balls and the second level

The solder balls are attached to the underside of the substrate in a grid. The pitch is fixed by the package standard, and the ball diameter follows. When the package is placed on the board and reflowed, each ball collapses and forms the joint, and that second level of interconnection, between package and board, is the one the assembly engineer controls.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/Materials-of-Green-Circuit-Board.png" alt="BGA package showing the solder ball array on its underside” />

Why an Array Beats Perimeter Leads

The first advantage is connection density, and it is the reason the package exists. The second is electrical performance: an array provides many short, low-inductance paths for power and ground, which reduces the simultaneous switching noise that limits supply integrity in a large digital device. Signal paths are also shorter on average than they would be in a perimeter package of the same pin count, because the outer rows and the inner rows are all roughly equidistant from the die.

Thermal behaviour is the third advantage. Heat flows from the die through the substrate into the array of balls and into the board, and because the array covers a large area rather than a few leads it does so with much lower thermal resistance. This is why high power processors can be packaged as a ball grid array and still be cooled adequately by the board itself, provided the board has enough copper and enough thermal vias beneath the footprint. Our component tolerance and reliability notes describe how the mechanical consequence of that arrangement is assessed.

The Costs of the Package

Nothing about the array is free. Two rows of balls must be routed on the surface layer, and the remaining rows require vias to reach the inner layers, which means the board beneath a large array is full of via holes and the routing channels between them are narrow. Fine line widths, small vias and often blind or buried vias become necessary, and that drives the board into high density interconnect territory. Our blind and buried via article describes the process steps that follow.

Inspection is the other cost. Because the joints are hidden, visual inspection cannot confirm that a ball has formed properly. X-ray inspection becomes mandatory for any serious product, and the only way to make the joints reliable is to control the process rather than to screen the result. That means a well designed pad, a stencil aperture that deposits the right volume of paste, a flat board and a reflow profile that keeps every ball in the array above the melting point for long enough without overheating the package.

Designing the Board for a BGA

The pad is usually a non-solder-mask-defined circle slightly smaller than the ball, which leaves room for the mask dam between pads. Solder mask defined pads are sometimes used on tight pitches, but they reduce the barrel of solder around the joint and should be adopted deliberately rather than by default. Escape routing should be planned layer by layer before the layout starts, because the number of rows that can be escaped on each layer determines the number of vias and therefore the layer count.

Power delivery deserves its own planning. The balls carrying the core supply are numerous and distributed across the array, and they should be connected to a plane through short vias rather than routed to a corner of the package. Decoupling capacitors belong on the opposite side of the board directly beneath the array, where the loop inductance is lowest, and their placement should be checked against the specific ball groups they serve. Our stencil fabrication notes explain how the paste volume for each aperture is derived.

Assembly, Reflow and Inspection

Assembly of a ball grid array is a process control exercise. The package must be kept dry, because absorbed moisture turns to steam during reflow and causes the internal delamination known as popcorning, so a bake is required if the floor life has been exceeded. Placement accuracy has to be sufficient for every ball to land on its pad, and the reflow profile must bring the whole package, including the centre of the array, above the liquidus temperature for long enough to form a proper joint.

Warpage is the recurring difficulty. The package and the board expand differently as they heat, and on a large thin package the two can bow away from each other at the centre, producing joints that look formed under X-ray but are mechanically weak. Controlling the board thickness, using a symmetric stack-up and keeping the package within its moisture specification are the standard mitigations. Inspection relies on X-ray for voiding and on cross section for samples, and it is supplemented by electrical test through boundary scan where the device supports it.

Rework and Underfill

Rework is possible but it is not a routine operation. The package is removed with a focused infrared or hot gas rework station after the board has been preheated from below, the site is dressed and repasted with a stencil designed for the purpose, and the replacement is placed and reflowed. Each cycle exposes the board and the neighbouring components to additional thermal stress, so a board that has been reworked several times is not equivalent to a new one.

Underfill is applied where the joint must survive thermal cycling or mechanical shock. A capillary underfill flows into the gap between package and board and cures into a rigid layer that distributes the strain away from the individual joints. It greatly improves fatigue life at the cost of making rework very difficult, so the decision is usually made deliberately and early, and it is common in automotive and portable products where the thermal and drop requirements justify it.

FAQ

Can a BGA be soldered by hand? Not reliably. The joints are underneath the package and cannot be reached with an iron, and heating the package from above without a controlled profile produces joints of unknown quality. Rework requires dedicated equipment with infrared or hot gas heating.

How is a BGA inspected if the joints cannot be seen? X-ray inspection shows the shape and voiding of each ball, and cross section on a sample confirms the metallurgy. Electrical test through boundary scan verifies connectivity on devices that support it.

Why do BGA parts need baking before assembly? The plastic package absorbs moisture from the air. If it is reflowed while wet, that moisture flashes to steam and delaminates the package internally, which is why a bake is required once the floor life has been exceeded.