Ball Grid Array, BGA
- SMD ball grid array, BGA packages enable high density connections to be made more easily to integrated circuits by allowing the under-side of a chip package to be used for the connectivity.
A Ball Grid Array or BGA package is a form of surface mount technology, or SMT package that is being used increasingly for integrated circuits.
The BGA offers many advantages and as a result it is being used increasingly in the manufacture of electronic circuits.
The Ball Grid Array, BGA package was developed out of the need to have a more robust and convenient package for integrated circuits with large numbers of pins. With the levels of integration rising, some integrated circuits had in excess of 100 pins.
The conventional quad flat pack style packages had very thin and close spaced pins, and these were very easy to damage, even in a controlled environment. Additionally they required very close control of the soldering process otherwise the level of solder bridges and poor joints rose. From a design viewpoint, the pin density was such that taking the tracks away from the IC also proved to be problematic as there could be congestion in some areas. The BGA package was developed to overcome these problems, and improve reliability from the soldered joints.
Ball Grid Array BGA aims
The Ball Grid Array was developed to provide a number of benefits to IC and equipment manufacturers as well as providing benefits to the eventual users of equipment. Some of the BGA benefits over other technologies include:
- Efficient use of printed circuit board space, allowing connections to be made under the SMD package and not just around its periphery
- Improvements in both thermal and electrical performance. BGA packages can offer power and ground planes for low inductances and controlled impedance traces for signals as well as being able to route heat away via the pads, etc.
- Improvements in manufacturing yields as a result of the improved soldering. BGAs allow wide spacing between connections as well as a better level of solderability.
- Reduced package thickness which is a great advantage when many assemblies need to be made much thinner, e.g. mobile phones, etc.
- Improved re-workability resulting from larger pad sizes, etc.
These advantages have meant that despite initial scepticism about the package, it provides some useful improvements in many circumstances..
What is a BGA package?
The Ball Grid Array, BGA, uses a different approach to the connections to that used for more conventional surface mount connections. Other packages such as the quad flat pack, QFP, used the sides of the package for the connections. This meant that there was limited space for the pins which had to be spaced very closely and made much smaller to provide the required level of connectivity. The Ball Grid Array, BGA, uses the underside of the package, where there is a considerable area for the connections.
SMD BGA Ball Grid Array package diagram
The pins are placed in a grid pattern (hence the name Ball Grid Array) on the under-surface of the chip carrier. Also rather than having pins to provide the connectivity, pads with balls of solder are used as the method of connection. On the printed circuit board, PCB, onto which the BGA device is to be fitted there is a matching set of copper pads to provide the required connectivity.
Apart from the improvement in connectivity, BGAs have other advantages. They offer a lower thermal resistance between the silicon chip itself than quad flat pack devices. This allows heat generated by the integrated circuit inside the package to be conducted out of the device onto the PCB faster and more effectively. In this way it is possible for BGA devices to generate more heat without the need for special cooling measures.
In addition to this the fact that the conductors are on the underside of the chip carrier means that the leads within the chip are shorter. Accordingly unwanted lead inductance levels are lower, and in this way, Ball Grid Array devices are able to offer a higher level of performance than their QFP counterparts.
BGA package types
In order to meet the variety of requirements for different types of assembly and equipment, a number of BGA variants have been developed.
- MAPBGA - Moulded Array Process Ball Grid Array: This BGA package is aimed at low-performance to mid-performance devices that require packaging with low inductance, ease of surface mounting. It provides a low cost option with a small footprint and high level of reliability.
- PBGA - Plastic Ball Grid Array: This BGA package is intended for mid- to high-performance devices that require low inductance, ease of surface mounting, relatively low cost, while also retaining high levels of reliability. It has some additional copper layers in the substrate that enable increased power dissipation levels to be handled.
- TEPBGA - Thermally Enhanced Plastic Ball Grid Array: This package provides for much higher heat dissipation levels. It uses thick copper planes in the substrate to draw heat from the die to the customer board.
- TBGA - Tape Ball Grid Array: This BGA package is a mid- to high-end solution for applications needing high thermal performance without an external heatsink.
- PoP - Package on Package: This package may be used in applications where space is at a real premium. It allows for stacking a memory package on top of a base device.
- MicroBGA: As the name indicates this type of BGA package is smaller than the standard BGA package. There are three pitches that are prevalent in the industry: 0.65, 0.75 and 0.8mm.
When BGAs were first introduced, BGA assembly was one of the key concerns. With the pads not accessible in the normal manner would BGA assembly reach the standards that could be achieved by more traditional SMT packages. In fact, although soldering may have appeared to be a problem for a Ball Grid Array, BGA, device, it was found that standard reflow methods were very suitable for these devices and joint reliability was very good. Since then BGA assembly methods have improved, and it is generally found that BGA soldering is particularly reliable.
In the soldering process, the overall assembly is then heated. The solder balls have a very carefully controlled amount of solder, and when heated in the soldering process, the solder melts. Surface tension causes the molten solder to hold the package in the correct alignment with the circuit board, while the solder cools and solidifies. The composition of the solder alloy and the soldering temperature are carefully chosen so that the solder does not completely melt, but stays semi-liquid, allowing each ball to stay separate from its neighbours.
As many products now utilise BGA packages as standard, BGA assembly methods are now well established and can be accommodated by most manufacturers with ease. Accordingly there should be no concerns about using BGA devices in a design.
Ball Grid Array, BGA, inspection
One of the problems with BGA devices is that it is not possible to view the soldered connections using optical methods. As a result there was some suspicion about the technology when it was first introduced and many manufacturers undertook tests to ensure that they were able to solder the devices satisfactorily. The main problem with soldering Ball Grid Array devices, is that sufficient heat must be applied to ensure that all the balls in the grid melt sufficiently for every joint to be satisfactorily made.
The joints cannot be tested fully by checking the electrical performance. It is possible that the joint may not be adequately made and that over time it will fail. The only satisfactory means of inspection is to use X-ray inspection as this means of inspection is able to look through the device at the soldered joint beneath.It is found that once the heat profile for the solder machine is set up correctly, the BGA devices solder very well and few problems are encountered, thereby making BGA assembly possible for most applications.
Ball Grid Array, BGA rework
As might be anticipated, it is not easy to rework boards containing BGAs unless the correct equipment is available. If a BGA is suspected as being faulty, then it is possible to remove the device. This is achieved by locally heating the device to melt the solder underneath it.
In the BGA rework process, the heating is often achieved removed in a specialised rework station. This comprises a jig fitted with infrared heater, a thermocouple to monitor the temperature and a vacuum device for lifting the package. Great care is needed to ensure that only the BGA is heated and removed. Other devices nearby need to be affected as little as possible otherwise they may be damaged.
BGA repair / BGA reballing
Once removed, the BGA can be replaced with a new one. Occasionally it may be possible to refurbish or repair a BGA that has been removed. This BGA repair may be an attractive proposition if the chip is expensive and it is known to be a working device once removed. Undertake a BGA repair it needs to have the solder balls replaced in a process known as reballing. This BGA repair can be undertaken using some of the small ready-made solder balls that are manufactured and sold for this purpose.
There are many organisations that have been set up with specialist equipment to undertake this BGA reballing.
By Ian Poole
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