Ball Grid Array (BGA): Features, Soldering Technique, and X-Ray Inspection

As the electronic devices are shrinking in size, the pitch size is drastically decreasing. The components are getting packed on a PCB like the grains in an ear of corn. All these things have led to one of the most efficient component package, the Ball Grid Array (BGA).

The way in which the components are attached to the PCB will majorly influence the stability and design efficiency. The BGA came to existence since the pin counts on chips rose significantly.

What is a BGA?

A Ball Grid Array is a Surface Mount Device (SMD) component that possesses no leads. This SMD package employs an array of metal spheres that are made of solder called the solder balls for interconnections. These solder balls are affixed to a laminated substrate at the bottom of the package.

The chip/die of the BGA is connected to the substrate by wire bonding or flip-chip technology. The interior part of the BGA consists of internal conductive traces that connect the die-to substrate bonds and to the substrate-to-ball array bonds.

In a BGA, the pins are replaced with metal balls made of solder.

Of all the packages, the BGA is the most popular package used in high I/O devices in the industry. It has a high lead (solder ball) count which is greater than 208 leads.

Features of BGA

High lead count
High interconnection density
Occupies lesser space on the board
No leads to bend
Reduced coplanarity problems
Minimized handling issues
Self-centering during reflow process which reduces placement problems during surface mount
Thermal and electrical characteristics better than conventional QFPs and PQFPs
Improved design-to-production cycle time (can also be used in Few Chip Package(FCP) and Multichip Modules (MCM))
Learn how to breakout a .4mm BGA.

Types of BGAs

There is a wide variant of BGAs available in the market. Here are a few popularly used BGAs:

Plastic over-molded BGAs (PBGAs) (Ball pitch 1.0mm, 1.27mm) – Are an alternative form of the standard BGA. These BGA incorporate a plastic-coated body, a glass-mixture laminated substrate, and etched copper traces. PBGAs feature improved temperature stability and preformed solder balls.
Flex Tape BGAs (TBGAs)
Hight thermal metal top BGAs with low profile (HLPBGAs)
High thermal BGAs ( H-PBGAs)
How the BGA is soldered to a PCB
The BGA soldering technique
In the PCB assembly process, the BGA is soldered on to the circuit board by the solder reflow process, using a reflow oven. During this process, the solder balls melt in the reflow oven.

Sufficient heat must be applied to ensure that all the balls in the grid melt sufficiently for every BGA solder joint to establish a strong bond.

The surface tension of the molten balls assists in keeping the package aligned in the right location on the PCB until the solder cools and solidifies. An optimum temperature controlled BGA soldering process is necessary for solid solder joints and also to prevent solder balls from short-circuiting each other.

The composition of the solder alloy and soldering temperature are precisely chosen so that the solder doesn’t completely melt but stay in semi-liquid, allowing each ball to stay separate from its neighbors.

BGA Solder Joint inspection

The optical techniques cannot be used to inspect BGAs since the solder joints are hidden from sight beneath the BGA components.

The electrical tests aren’t much reliable since the tests reveal about the electrical conductivity of the BGA at that particular instant. This test doesn’t predict if the solder will last long enough. The solder joint may fail over a period of time.

X-rays for BGA inspection

The BGA solder joint bonds are inspected using X-rays. The X-ray inspection helps to look through the device solder joint underneath the components. Due to this ability, the Automated X-ray Inspection (AXI) technology is widely used for BGA inspection.

Removing Faulty BGAs (BGA Rework)

If the BGA components are found to be faulty, then they are carefully detached from the board by melting the solder joint. This is done by locally heating the BGA component until the solder joint melts underneath it.

In the rework process, the component is heated in a dedicated rework station. This consists of an infrared heater, a thermocouple that monitors temperature, and a vacuum device to raise the package.

With great precaution, it must be ensured that only the faulty BGA component is heated without damaging the neighboring components on the board.

The BGA components are well established in the electronics industry for their numerous advantages in both mass production and prototyping. As the components grow in number the routability and component placement gets intricate. All these can be managed with the BGA packages.

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