At first sight soldering ball grid arrays, BGAs may seem to be difficult as the solder balls that solder onto the PCB are sandwiched between the BGA body itself and the circuit board.
However PCB assembly using BGAs has been proved to work, and work well. The soldering process and other areas of the PCB assembly may require to be slightly modified, but the benefits from using BGAs have been found to be quite significant, both in terms of reliability and performance.
The Ball Grid Array, BGA was introduced as a result of the pin count on many chips rising significantly. The pins on carriers like the Quad Flat Pack became very delicate and easy to damage. Also PCB routing was difficult as a result of the close proximity of many leads. Using the whole of the underside of the chip solved the issues of density on fragile chip leads in one go.
The BGA components provide a far better solution for many boards, but care is required in the PCB assembly process when soldering BGA components to ensure that the BGA is correctly soldered so that all the joints are correctly made.
BGA solder process
One of the initial fears over the use of BGA components was their solderability and whether soldering BGA components could be made as reliable as soldering devises using more traditional forms of connection. As the pads are under the device and not visible it is necessary to ensure the correct process is used and it is fully optimised. Inspection and rework were also concerns.
Fortunately BGA solder techniques have proved to be very reliable, and once the process is set up correctly BGA solder reliability is normally higher than that for quad flat packs. This means that any BGA assembly tends to be more reliable. Its use is therefore now widespread in both mass production PCB assembly and also prototype PCB assembly where circuits are being developed.
For the BGA solder process, reflow techniques are used. The reason for this is that the whole assembly needs to be brought up to a temperature whereby the solder will melt underneath the BGA components themselves. This can only be achieved using reflow techniques.
For BGA soldering, the solder balls on the package 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.
