Once the pads have had their solder removed and cleaned off properly in the so-called “site prep” process, the BGA can be placed onto the correct pads on the PCB using the proper temperature profile using a BGA rework machine.
When the BGA replacement occurs using a modern area array rework system, split prism optics are used to assist in the precise placement. Using a prism, optical images of both the solder balls on the device as well as the PCB pads corresponding to the BGA device pattern are superimposed onto one another on an operator screen. The two images can then be aligned with X, Y and rotational adjustments. These controls either can be manual, semi- or fully automatic in nature. For the ultra fine-pitched devices the BGA placement process may need to be completed using a stepper motor to precisely move the different alignment stages in a precise enough fashion. The device can then be placed onto the PCB after either paste flux or patterned solder paste is deposited onto the PCB or selectively on the bottom of the solder spheres of the replacement device. One needs to insure that the BGA placement is lined up such that the pin 1 designator is lined up with the marking on the board.
BGA replacement accuracy criteria is the same as the BGA initial placement at the time of initial assembly. The ball grid array should be placed with a force per the manufacturers' specifications such that the solder paste contacts 50% of the PCB pad and not be in contact with the vias. For test boards, the placement accuracy can be confirmed using double-sided tape adhered to the PCB BGA site. The BGA can then be placed onto this site, firmly held in place and examined at an angle under a stereomicroscope for accuracy. This is the basic process for ball grid array placement. In practice this placement is usually less than 2 mils. BGAs tend to be self-centering so their placement accuracy is not very critical on older 1.0 and 1.5mm pitch replacement devices. This means that they will have a tendency to move back to the center of the pad assuming that thermal characteristics of all of the pads is pretty consistent.
Alternatively, the ball grid array can be hand-placed when using the StencilQuik™ process for larger pitch sizes (1.00 and 1.27mm). Care must be taken in handling the parts at the edges while making sure the operator is properly grounded. This stay-in-place stencil is first placed on to the board where the stencil apertures are aligned over the correct pads. After peeling back the release liner and placing the device, pressure or temperature can be used to begin curing of the underlying adhesive on the stencil. The ball grid array placement process is completed when the device is tactily “fitted” into the holes which have been filled with either paste flux or solder paste for the BGA placement process.
There are several potential problems that a BGA rework technician needs to watch out for during BGA placement. Controlling the downward placement force during the BGA placement process is critical. If the site has been paste printed too much downward force can cause the paste to move outwards and short out neighboring devices if on the outside row or short internal BGAs underneath the package. A different phenomenon can occur if paste flux is used for rework. If too much downward pressure is applied during the BGA placement process then the underside of the BGA can “float” and end up a whole row off. Too much pressure during the BGA placement process can also lead to potentially uncleanable flux areas between the balls which made lead to electrical performance problems or even long term reliable concerns.
In either case, the BGA placement process should be confirmed by optical, automtic optical inspection, endoscopic and/or x-ray inspection.