The demand for dense and compact integrated circuits is rising, and there is a need to shrink electronic devices. Because of this, ball grid array technology has become the go-to option for building surface-mount packages.
Therefore, if you want to create a compact printed circuit board, you need an in-depth understanding of ball grid array packages. We have looked at the technology in detail below, so take a look!
What is the Ball Grid Array?
The system descended from PGA (Pin Grid Array) and improved the design by having an array of solder balls instead of pins to conduct the electrical signals.
How? The ball arrays allow the bottom section of the surface mount device to be usable, not just the perimeter.
Additionally, the structure creates a higher pin density, resulting in more connectivity. The balls also make the die-to-package distance shorter, resulting in better performance at high speeds.
BGA Internal Construction
Two types of methods exist to link the silicon die to the substrate.
- Wirebond: Wirebond BGAs use wires to connect the two parts.
- Flip Chip: Flip Chip BGAs use solder bumps instead of cables to connect the die to the substrate.
In this case, the silicon die is a small semi-conducting block that hosts a fabricated circuit.
On the other hand, the substrate is a small PCB with traces specifically suited for the package. It usually consists of several layers (depending on the design complexity) and connects the bumps to the solder balls.
The substrate material differs depending on the signal transmission requirements of the circuit board.
Features of Ball Grid Array
- High lead count
- No leads to bend
- High-density connections
- Low inductance
- Small footprint (takes up little space on the board)
- Low thermal resistance between the package and PCB
- Self-centering during reflow
Advantages of BGA Technologies
- Higher pin density
- Lower inductance
- Better heat conduction
- Improved electrical conductivity
- Better performance at high speeds
- Robust and highly reliable
- High potential for self-alignment
Disadvantages of BGA Technologies
- Difficult inspection
- Wide inductance distribution
- Presents routing difficulties
- No solder joint visibility
- Prone to flexural stress from the PCB
- Somewhat expensive
Types of Ball Grid Arrays
There are several types of BGAs, but the most popular ones are:
Plastic Ball Grid Array packages get this name from the plastic coating covering the glass-mixture laminated substrate with etched copper traces.
It is the most popular type in double-sided PCB designs and delivers excellent thermal and electrical performance. PBGAs usually carry 200 - 500 ball arrays, each with a composition of 64% tin and 37% lead.
The C in CBGA stands for ceramic, and this type features a co-fired ceramic substrate.
Compared to PBGAs, the ball composition in CBGAs is also different, consisting of 10% tin and 90% lead. The design offers better electrical and thermal conductivity but is more expensive to build.
A Tape Ball Grid Array uses a flexible interconnection to create fine lines between the solder balls, resulting in slim ball grid assemblies with excellent thermal performance.
Although more expensive than PBGA, TGBA is versatile enough to face the chip or integrated circuit up or down. The wire-bonded construction is better if facing up, but the flip-chip design is better when facing down.
Enhanced BGAs are PBGAs with additional heat sink options. They feature dams built around the chips and electronic components on the substrate.
EBGAs usually have a wire bond construction with chips facing down and liquid compounds to seal the electronic components on the dam.
Flip Chip BGAs are similar to the ceramic type but have a BT resin substrate instead of ceramic. This design saves on cost but, most importantly, creates shorter electrical pathways to improve electrical properties and performance.
Metal BGAs feature a metal-ceramic substrate and a wire bonding as the internal construction with down-facing chips and a sputter coating on the circuits. MBGAs offer superb electrical and thermal properties.
A micro BGA is a CSP chip with µBGA architecture. It places fine pitches between the solder balls for higher-density connections.
The BGA also features unique elastomer layers that better withstand temperature changes between them and the circuit board.
Assembly of BGAs
Solder dispensing involves depositing the solder paste to the substrate to attach the array of metal spheres. There are two dispensation methods:
As the name suggests, this method involves using a syringe to deposit controlled amounts of the paste as dots, lines, or circles.
An application head with X and Y-axis movement, and sometimes the Z-axis (height sensing), controls the syringe movement while air pressure pushes the material through the needle bore.
The process is slow and not ideal for high-volume production, but it is perfect for applying the paste during rework.
A stencil is a thin metal foil with slots cut into it to allow the paste to pass through at specific spots.
After the material gets deposited on the upper side of the stencil, a squeegee traverses through the print area, resulting in rapid deposition in one operation. As such, this method is ideal for high-volume production.
The Solder printing machine
Solder jetting is an alternative method to the solder dispensing techniques above and works similarly to laserjet printing using ink. The process can apply solder paste precisely to the substrate or electronic component.
Flux Only Attachment
Flux-only attachment is possible when using a tin-lead eutectic or a close alloy. There are two ways to do this attachment. Before joining, you can deposit the flux on the substrate or dip the BGA in the change.
The purpose of the flux is to remove all the surface oxides and hold the BGA in place before the reflow process. However, the process requires an inert gas to be successful.
Reflow soldering might not form a BGA joint if there is insufficient solder paste or deformed/missing solder balls. Therefore, it is necessary to inspect the solder paste deposit using offline measurement systems or line printers with measurement systems.
Measure the height, size, and position of the solder paste deposits. The system should detect a missing ball and determine if the size and shape of the array of solder balls are consistent.
Solder paste inspection machine
The next step is to place the component and align it using the fiducial marks or package edges as placement guides for the machine's vision systems. BGA solder joints have self-aligning properties that accommodate up to 50% placement tolerances.
Solder reflow is the last step in the assembly process, and it involves using infrared or convection furnaces for ball grid array soldering.
An infrared rework soldering station
Reliability Concerns and Failure Modes of BGA Packages
Even though BGA packages have numerous advantages over PGAs, mounting these electronic components can have potential problems. These include:
The substrate design has a significant impact on the reliability of the BGA assembly. Since it holds the copper pad, it can affect its geometry. The solder-resist mask can create a high-stress point on the ball, resulting in a cracked joint. If this crack propagates around the ball, it might cause an electrical failure.
Movement During/After Reflow
Surface-mount components can move after reflow if there are sudden vibrations on the conveyor because each molten solder ball has not yet solidified. To minimize this problem, manufacturers use the 62Sn36Pb2Ag alloy, which has a melting range, instead of the eutectic 63Sn37Pb.
Popcorning can occur in PBGAs if moisture gets absorbed between the package over-molding and the substrate. Since plastic has a low moisture absorption rate, the damp air expands rapidly on reflow and can cause a sharp crack.
Manufacturers can avoid this by heating the components in nitrogen/dry air atmospheres or designing packages with thermal vias below the die.
Voids in the Solder Joint
Moisture, shrinkage during solidification, and flux volatilization can create voids in the solder joint. However, these voids are not all that bad. Those with small gaps up to 16% of the pad area are more reliable than the void-less. You should only avoid large ones that exceed 40% of the pad size.
Differences in thermal expansion between the board and package can result in thermal fatigue, which can cause the joints to crack. The issue is common in CBGAs because such packages have the most considerable CTE material mismatch.
Is Your Contract Manufacturer Experienced With BGAs
Outsourcing your BGA manufacturing can help you save time and money, but you need to pick a manufacturer with the following qualities:
- High production efficiencies
- Controlled and consistent quality
- Engineering expertise
- Low costs
We are experienced BGA manufacturers and assemble these components using the best materials and designs to avoid failure modes and reliability concerns.
As you can see, BGAs are highly efficient and dense surface-mount packages that are perfect for carrying chips and integrated circuits. Contact us for more details if you need to partner with an experienced manufacturer to make them for your project.