Ball Grid Arrays (BGAs) are a common component in modern high value and often expensive circuit boards. When a failure is isolated to the BGA, the root cause is often the solder ball interconnection, not the BGA itself. Removing and reworking the BGA is often an economically attractive alternative to replacement with a new part. There are several manufacturers that offer equipment to remove and replace the BGA on circuit boards. However, if the intent is to reuse the same BGA, the solder balls must be replaced before it can be resoldered to the circuit board. The BGA removal process will render the existing solder balls unusable.

Reballing is also performed when the only available parts on the market have lead-free solder balls, and the application requires eutectic solder. The salvage market also reballs BGAs for resale. This article describes the required BGA reballing process, specifically the preform method.

BGAs can develop problems due to unreliable solder joints which have become more common with the advent of lead free solder balls. Lead free components require a more stringent process control, and without tight process control, the BGA solder joint can be unreliable.

Starting with a BGA removed from the circuit board, the first step is to bake the moisture out of the component to prevent a defect called “popcorning.” This defect appears as craters in the BGA package which are caused by the explosive expansion of absorbed water vapor during the reflow process. 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. This baking step eliminates “popcorning” risks and must be performed.

The reballing process continues by removing the existing solder balls and cleaning the surface. The 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 surface of the BGA. The 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 used with generous flux.

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

By specifying the BGA type and part number to the BGA reballing vendors, the correct preform parts will be obtained. Companies such as Winslow Automation offer a huge range of preforms and can deliver most in less than a week.

Continuing with a clean and 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 the solder balls in the exact pattern using a water soluble paper carrier. (Figure 4-1). This precise paper form is the key to this process and assures the balls are aligned on the carrier within 0.0004 inch.

The perimeter of the preform is precision-cut to perfectly match the dimensions of the BGA. Mating the preform to the BGA is accomplished by applying paste flux to the BGA (as shown in Figure 4-2). The paste flux is sticky, and holds the BGA to the preform. A fixture is often used to speed the assembly and assure accuracy (Figure 4-3).

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

The last step in the preform reballing method is to remove the paper carrier (which is designed for easy removal). After part inspection, the BGA is ready to be assembled onto the circuit board.

With some experience, good equipment, and a mature process, the entire rework and BGA reballing process can be accomplished within an hour. Reballing is a mature process with no special skills required. This makes the reballing option economically justifiable to rework circuit boards.

For more information on reballing contact the EMPF Helpline via email at helpline@empf.org or call 610.362.1320.