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.

Edward Zamborsky Regional Sales Manager, OK International Inc. Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

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.

Leo Lambert Vice President, Technical Director, EPTAC Corporation At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

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.

Peter Vigneau General Manager, Circuit Technology Center, Inc. Mr. Vigneau is the General Manager of Circuit Technology Center, a world-leader in the repair and rework of populated and non-populated circuit boards. Peter has over 30 years of experience in the electronics industry.