Any circuits added due to repairs has to address what circuitry it is being removed or replaced. If the defective circuitry is removed, the replacement should be the same thickness and should not impact the installation of a BGA component.

In answer to the first part of your question, yes, you can add circuits under existing BGA’s. A similar procedure is shown in IPC-7721 procedure number 6.2.1. Essentially you remove the component from the board. Route a jumper or circuit replacement copper strip as needed.

Machine a shallow channel to route the wire/copper strip as needed. Bond the replacement trace into position with your choice of manufacturer’s epoxy or dry film replacement trace. Solder the circuit at both ends. Test as needed and then coat with solder resist to ensure isolation from adjacent circuits or components.

The procedure in IPC-7721 is specifically for changing the connection of a BGA pad, but the procedure can be modified as needed to suit your application.

To answer your second question, it depends on how good you are with the first part. If the new trace/circuit is low and flat, you should be able to re-install the component as needed. You will need to reball the BGA before placing it back onto the pad.

Check out http://www.solder.net/ for EZReball, the BEST option for reballing an area array device.

It would depend upon the type of BGA and the component you wanted to place underneath it. For example; if you had a plastic BGA and the center grids were vacant, you could place components under this BGA. You would have to ensure that they would not interfere with the post reflow height or clearance of the chip (which is in the .016”-.020” range).

In a ceramic BGA, you would have additional clearance as the solder balls are not melt-able and the post reflow height is usually .030” or higher.

Either way this is not a very common practice and most designers leave the space under a BGA for trace routing and thermal control.

Absolutely, it is possible to rework and add circuits under leadless devices like BGA's while maintaining the ever critical flatness characteristics required for successful BGA placement.

This is a modification we perform successfully on a regular basis. However I must caution, this is one of the more challenging rework and repair procedures we encounter. This rework can be difficult because it requires demonstrated skill in multiple disciplines.

Some of the more important capabilities required include BGA removal & replacement, X-Ray inspection, proficiency in milling & cutting of PCB's, the ability to add new circuit components like BGA pads, copper tracks, and jumper wires. The technician's focus on precision and quality of workmanship is paramount to successful results. The complexity of this type of rework and repair is directly proportional to BGA site array density.

If you would like to investigate the procedures in more detail, see BGA ECOs - Nifty Solution or Rework Nightmare?", an article found on the Circuit Technology Center web site. This article details step by step procedures required for a rework/repair of this nature.

Performing repairs/rework under surface mount components to resolve ECO issues is far more common than you might think. Although it is a challenging procedure, it can be done reliably and safely by following established proven procedures. Our experience has demonstrated that skill, attention to detail and patience yields robust, reliable results.

If a program's time to market and ultimate success has stalled because prototypes or production modules need rework or repair under a surface mount component go for the rework or repair. There is no better way to salvage your costly investment and keep your program on schedule.