Using preforms, balls can be removed and reattached in an hour.

Removing and reworking the BGA is often an economically attractive alternative to replacement with a new part. If the intent is to reuse the BGA, the solder balls must be replaced before it can be resoldered to the circuit board. The BGA removal process renders existing solder balls unusable.

Starting with a BGA removed from the board, the first step is to bake the moisture out of the component to prevent a defect known as popcorning. This defect appears as craters in the BGA package, caused by the explosive expansion of absorbed water vapor during reflow. It is easy to see on the top and sides of components, but after the solder balls have been reflowed, it can be more difficult to see on the underside of the component.

Next, remove the existing solder balls and clean the surface. Solder balls can be removed using a soldering iron with a blade tip attached. Uniform heating of the part with auxiliary heating is necessary to avoid localized thermal shock to the BGA from the hot blade. This method is adequate for bulk solder removal. Care must be taken not to scratch the BGA surface. Generous use of flux increases the effectiveness of the heat transfer and also removes any oxides. Solder braid is used next to remove smaller flecks of solder, also with generous flux.

As a final cleaning and preparation, the BGA is carefully washed with isopropyl alcohol (IPA) to remove flux. The part is then inspected. Cleanliness can be confirmed using standard resistivity of solvent extract (ROSE) or SEC (solvent extract conductivity) testing.

Specify the BGA type and part number to BGA reballing vendors in order to obtain the correct preform parts.

Continuing with a clean, solder-free BGA, the next step is to perfectly position the balls under the BGA. With hundreds of solder balls to be exactly positioned, a mechanical device is required to maintain the alignment and position the solder balls under the chip. The two most popular methods to actually attach the solder balls are the “preform” method and the “specialized fixture” method. The preform method holds solder balls in the exact pattern using a water-soluble paper carrier (Figure 1). This precise paper form is the key to this process and ensures the balls are aligned on the carrier within 0.0004".

The perimeter of the preform is precision-cut to perfectly match the BGA dimensions. Mating the preform to the BGA is accomplished by applying paste flux to the BGA (Figure 2). The paste flux is sticky, and holds the BGA to the preform. A fixture is often used to speed assembly and ensure accuracy (Figure 3).

The next step is to reflow the two mating parts using a profile based on whether the solder balls are SnPb or Pb-free. The BGA assembly is processed through the reflow oven the same way a board is processed. The solder balls are now attached to the BGA.

Then remove the paper carrier (which is designed for easy removal). After part inspection, the BGA is ready to be attached to the board.

With some experience, good equipment and a mature process, the entire rework and BGA reballing process can be accomplished within an hour.

ACI Technologies Inc. (aciusa.org) is the National Center of Excellence in Electronics Manufacturing, specializing in manufacturing services, IPC standards and manufacturing training, failure analysis and other analytical services. This column appears monthly.

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