A Ticking Bomb -- After several encounters with BGA component repair, a wiser-if not more humble-operator emerges.

One of the most overlooked, difficult and misunderstood steps in the process of ball grid array (BGA) rework is the handling of damaged mask at the rework site. Mask-defined BGA pads, where mask is used to contain the geometry of a BGA solder joint, are not often encountered. However, even when joints are not contained by solder mask, the vast majority of BGA pads are contained by mask on the connecting circuit, dog bone or via. Without this mask, the carefully calculated solder volume would likely be dispersed and create a short, an open or a starved joint.

Solder volume is critical. A complete break in the solder mask, on the connecting circuit or dog bone, may cause solder to drain down the attached via. Breakdown on closely spaced vias or circuits may cause bridging. Only one short or open can fail a BGA component.

Discovering and Assessing the Damage

Before a rework technician or engineer can even touch the board, the designers and bare board manufacturers should properly design and apply the solder mask. However, once the assembly is in the hands of the rework professionals, each must closely inspect the condition of the mask immediately after removing the component (or prior to placing the component if the board is received at that stage). If concern about the mask's condition exists, inspection, under magnification and prior to removing excess solder from the site (via wicking or vacuum), should be performed. The potential for failure exists if any fraying, lifting or bubbling of the solder mask, adjacent to the BGA pad, is found. During inspection, review the rework profile and board pre-bake cycle-just in case. Ensure the board is moisture-free prior to rework to prevent thermal shock at the site.

What if mask damage is discovered? Damage can range from light fraying, at the margin of the circuit and the BGA pad, to the complete removal of the protective mask covering the connecting circuit. Light fraying should be left as-is. However, missing solder mask must be restored. I normally allow 20 percent mask deterioration before repairing the mask, and the method of repair, for instances of greater than 20 percent, might surprise you. Most think a small amount of epoxy or touch-up mask will solve the problem, but the cause of mask lift is the capillary creep of solder along the circuit. Once that process occurs, merely placing mask on top of the solder is useless. Once the location is heated again, the solder will simply continue to creep along the circuit.

Repair Technique

To prevent further creep along the circuit, all solder must be removed from the circuit and replaced with fresh mask or epoxy (see inset). The solder is carefully removed from the circuit surface with a knife. The BGA pad is left untouched, but the circuit, up to and including the connecting via, may be scraped. Care must be taken not to scrape through the circuit or to damage the adjacent board surface. Once the area is prepared, a small amount of high strength, thermosetting epoxy may be placed over the copper. Care must be taken to keep the epoxy at or below the mask level and off the BGA pad surface. Observe the recommended cure cycle time of the replacement material-do not skimp on the time. To help ensure 100 percent reliability, a full cure is essential. No coloring agents should be mixed with the epoxy, as they could weaken the bond.

Conclusion

Obviously, this type of repair can quickly become a time-consuming procedure. If extensive damage exists at the BGA site, each and every pad affected will require this repair procedure. Unfortunately, experience indicates that efforts to shortcut the process lead only to failure.

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Jeff Ferry is CEO of Circuit Technology Center, Haverhill, MA; [phone]; www.circuit techctr.com.

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