BGA Reballing

Estimated reading time: 5 minutes

This article details the prerequisites and options for BGA reballing.

Howard Rupprecht

When working with ball grid arrays (BGA), two of the most commonly asked questions are, "Can I reuse components?" and "How do I reball the components?" While these are obviously relevant concerns, there are very few companies that are currently reballing. The following areas should be considered before starting.

Component Reusability

The component supplier should answer this question, as they will know what amount of thermal cycling their components will stand. Consider a BGA assembled on a double-sided SMT printed circuit board (PCB) that was removed, reballed and replaced; potentially, it would have had to undergo six reflow cycles during this process, in addition to any ball reflow during the manufacturing of the component:

1.Reflow assembly - topside.

2.Reflow assembly - bottomside.

3.Removal of component.

4.Removal of excess solder from component.

5.Reflow of new solder balls.

6.Replacement of component.

Another consideration is the amount of handling and potential ESD damage that could occur during the reballing process.

In many cases, the reballing process is not commercially viable because of the extended labor times. In cases where component value is very high, or because of component availability, it may be necessary to reball.

Reballing Methods

There are two basic methods most commonly used for reballing BGAs: preforms and reballing fixtures. Micro-stencils can be used for reapplication of both fluxes and solder paste to components.

Method One

Preforms. One method for reballing involves using solder preforms (normally an array of solder balls to match the component either held in a paper sheet or joined together). These are available from some solder manufacturers. In the case of eutectic balls, these preforms need to be soldered to the device in a controlled environment (reflow oven or head) using flux. High-temperature, non-melt solder balls need to be attached to the device using screen printed or dispensed solder paste.

The typical process steps for attaching an array of solder spheres using the BGA preform are:

Removal of residual solder. The pads on the part need to be prepared for the replacement solder balls. Residual solder can be removed with solder braid, used in conjunction with a direct-power soldering iron equipped with a blade tip. Work row by row across the pads with the hot blade tip. The operator must be careful to keep the wick between the tip and the board. The old solder can be quickly removed without risking pad damage that is likely to result from direct contact between a hot tip and the substrate. Subsequently, the flux residue on the part should be removed with an approved solvent, leaving the land area clean.

There are also non-contact solder removal methods available that use hot air or nitrogen to reflow the solder, while a vacuum pipette simultaneously "sucks" the molten solder off the pad area. This method is available either in automated form or as a hand tool. While it involves less contact with the pads, it can be more time consuming and require specific equipment not normally available on the rework bench. Consequently, this method may be less cost-effective and is less widely used.

Flux application. Prior to placement, the correct flux should be applied to the pad areas using either dispensing, printing or, more commonly, brush application.

Preform placement. Placement of the new preform is generally a manual operation where alignment is controlled by either edge registration or a fixture to align the component pad with the preform.

Reflow. After the part is fluxed and the preform placed, the next step is to reflow the part. Reflow should be multi-zone controlled to match the profile of the solder paste flux being used. When the reflow cycle is complete, the new array of solder balls should have been attached successfully. At this point, the carrier material needs to be removed, which can be accomplished by a variety of methods. Let the carrier soak in some deionized water for a few seconds, peel away with a tweezer, or remove it with a spray rinse or batch cleaning system.

The part then needs to be cleaned of flux residue, which can be done by almost any standard aqueous method. If the paper is removed with a batch cleaner or spray rinser, this step may not be necessary. If a no-clean flux gel is used, the paper can be easily removed when hot. This stops the need to soak the paper off and is of great benefit, as no pre-drying will be needed. Only a full convection reflow from top and bottom will work well. If top heat alone is used, then the coupons will bow and attach the balls to the sides of the part. The center balls will solder correctly.

It should also be noted that some components may be sensitive to moisture and will therefore require a pre-baking operation prior to any reattachment to a PCB.

Method Two

Reballing fixtures. This method normally involves component-specific stencils and tooling to allow paste or flux to be printed and loose solder balls to be located in the correct position before being reflowed - again in a controlled environment. Solder balls of different sizes and compositions are available from solder paste manufacturers.

The diagrams show the process being used with printed solder paste and non-melt, high-temperature solder balls. These are used for ceramic BGA (CBGA) components and are usually 90/10 composition.

Stencil printing. Figure 1 shows solder paste being printed onto the component. The part is held in a fixture and the solder paste is manually applied using a chemically etched stencil.

Solder ball application. In the second stage (Figure 2), the paste printing stencil has been removed and replaced with a larger stencil that will dictate the position of each solder ball. Loose solder balls are then poured onto this stencil and fall into their intended positions above each printed pad. When all the apertures have been filled, any excess solder balls can be brushed off or poured away.

Reflow. After all of the balls have been placed, the component is ready for reflow. The method shown in Figure 3 completely removes the stencil and uses forced convection from above to reflow the solder paste and attach the non-melt solder balls.

Some methods advocate leaving the ball location stencil in position during the reflow operation; as the ball`s natural reaction is to self-center, this will make little or no difference. The stencil may also act as a heat sink that will increase the required reflow temperature.

The reballed part is removed from the fixture and is ready to be attached to the PCB as a normal component.

REFERENCES

1. Winslow Automation: www.winslowautomation. com/bga.htm.

2. P. Wood, OK International, Menlo Park, Calif.

HOWARD RUPPRECHT is product manager, Soldering and Rework Systems, for OK International (Europe), Eagle Close, Chandlers Ford, Hampshire SO53 4NE, England; E-mail: hrupprecht@okinternational.com; Web site: www.okinternational.com.

Figure 1. The squeegee prints solder paste onto the component being reworked.

Figure 2. Loose solder balls are poured onto the stencil.

Figure 3. Controlled forced convection is used to reflow the solder paste (a). This is an example of a reflow profile for BGA 225 component (b).