Many PCB manufacturers have mistakenly delayed the incorporation of Ball Grid Array (BGA) packages into their product designs because of concerns over increased cost and complexity. Incorporating BGAs will not only require additional expenditures in equipment, but will also require additional skills training for employees. The increasing I/O capacity of BGAs and the need to provide more sophistication in less space, has forced many manufacturers to convert to BGA practices or risk losing their competitive advantage. As with any other component package, BGA rework capability is a necessary reality, and must be considered. Some important considerations for successfully reworking BGAs will be examined.

Removal Equipment
In selecting BGA rework equipment, some key elements are essential in obtaining versatility, reliability, and repeatability. Equipment available for BGA rework should incorporate the use of a split-vision alignment system to ensure the exact alignment of the component to the substrate. There should also be a programmable localizer reflow system, preferably one that uses hot-air convection while providing underside heating to ensure the correct reflow of the BGA. Some typical examples are the standard "hot gas" method, the sealed oven method, the horizontal flow method, and the conduction method. Also required will be an assortment of tools including a solder wick, a soldering iron with a wide chisel tip, and a micro-stencil for the application of solder paste if necessary. Finally, one very important factor in successful rework of any BGA begins in the design stage of the assembly. Ensuring enough free area (“keep-out distance”) around the periphery of the BGA is important to provide enough clearance for rework tooling and fixturing. In most cases, a "keep out distance" of 0.100in. is sufficient.

Removal
Before beginning BGA removal, it is important to understand the materials used. Is the solder used to attach the component eutectic or non-eutectic? What is the liquid temperature of the solder? Are the BGA balls eutectic or high temperature alloys? These questions must be answered in order to develop a thermal profile used to remove the BGA from the assembly. Practically all rework equipment available today incorporates the ability to develop thermal profiles specific to the requirements of a particular assembly. It is absolutely necessary to ensure the complete reflow of the solder attaching the BGA to the substrate before attempting to lift it. This will help eliminate damage to the substrate lands and traces. In most applications, the same profile can be used for re-attachment of the replacement BGA as long as enough time is provided above the liquid temperature to ensure complete reflow of the alloys used.

Reballing
In many cases, if may be cost effective to reball removed BGAs. The easiest and most cost effective method used to accomplish this task is through the use of BGA pre-forms. A pre-form is actually an array of alloy spheres placed in a cardboard backing that is impregnated with a water-soluble flux. By applying a compatible water-soluble, tacky flux to the underside of the BGA, and overlaying the pre-form onto the BGA, it is possible to re-apply new balls using a standard reflow cycle. BGA pre-forms are available in numerous pattern arrays, varying sphere sizes, and various alloys. It is important to select a pre-form that will match the original BGA specifications for alloy material and sphere size. This method is feasible for plastic, ceramic, and micro-BGA applications.

Site Preparation
Once the BGA has been removed from the substrate, carefully remove any residual solder from the land area. This can be accomplished by using solder wick and a soldering iron with a wide chisel tip. In most applications, solder wick that is 0.100 in. wide and impregnated with flux works well. When removing residual solder, apply the wick over the land area and place the iron tip in contact with the solder wick. Never move the solder wick over the land areas in a scrubbing motion. This will result in damaged lands and traces. Another important step in the preparation of the substrate is the complete removal of any remaining flux residue from the area where the new BGA will be attached.

Paste Application
In order to ensure successful BGA reattachment, a consistent application of solder paste to the substrate land area is required. The use of a dedicated, micro-stencil tooling makes this process step achievable. The two most important parameters involved with the application of solder paste is the registration of the micro-stencil to the substrate lands, and applying consistent squeegee pressure across the length of the micro-stencil. It is recommended, prior to application of the solder paste, that any adjacent components be masked with tape to prevent excessive solder paste build-up on them. Also, it is a good practice to thoroughly clean the micro-stencil tooling after each application to prevent clogging of the apertures.

Placement
Two critical factors to consider when placing a new BGA onto a substrate are BGA-to-land registration and "Z" placement force. The surface tension of the solder paste during reflow will usually be sufficient to cause self-alignment of most BGAs. However, heavier BGAs and BGAs with high temperature interface connections may not self-align. Alignment to the land pattern on the substrate is important in achieving stress free BGA interconnections.

To control "Z" placement force, most rework stations rely on manual movement of the Z-axis to place the component into position. It is important to limit this movement in order to bring the BGA balls in contact with the solder paste deposits without forcing the BGA too far into the paste. This can cause solder bridging under the BGA.

Reflow
In order to achieve complete reflow of all BGA interconnections, the proper thermal profile must be developed. The profile for rework should be similar to any profile used in the assembly process. It should adhere to the requirements of the solder paste during the initial preheat, soak, and reflow portions of the profile. It is important to ensure that the center of the BGA component reaches the liquid temperature of the alloy used, and is above this temperature for the required length of time. This will ensure the complete reflow of all the BGA interconnections.

Inspection
The use of X-ray inspection techniques (fluoroscopy) can be used to determine the success of the BGA rework process. Access to the BGA interconnections is not possible after reflow. The use of X-ray inspection, when performed properly, will detect any process anomalies such as solder bridging, misalignment, solder joint voids, and open solder connections. The ability to view the BGA interconnections at a compound viewing angle will aid in the detection of open solder connections between the BGA and the substrate. A viewing angle of 90o will work well in detecting solder bridging and solder voiding.