Reprinted with permission from the July 2010 issue of ...

Repair, Rework, and Reball Solder Ball

By Thorsten Teutsch, Pac Tech - Packaging Technologies, Santa Clara, CA

There is a growing demand for equipment and processesthat will restore defective solder balls on surface mount(SMT) devices. The three most common reasons for

restoration are: to repair missing or bridged solder bumpscreated during the bumping process; rework parts that weredamaged during assembly; or to replace lead-free sphereswith lead-based spheres for specialty applications (reballing).Repair. There is always some yield loss associated with thecommon solder bumping technologies — paste printing, elec-troplating, or sphere transfer. For some applica-tions this yield loss is manageable, but for otherapplications, bump yield losses have a profoundeffect on the die yields and therefore also affect theoverall profit margin. This is especially relevantwhen the device is of very high value, or the I/Ocount per die is high. By implementing an inlinerepair station into the bumping process, yields canbe dramatically increased.Rework. During the assembly process, defectiveSMT parts (BGA, CSP, and packaged flip chipparts) are often scrapped due to defects in the sol-der interconnects. Recovery of these devicesrequires disassembly and then rework of the indi-vidual components. This rework includes removal ofthe damaged solder balls from both the device and substrateand then placing new solder balls onto the part. A flexiblerework tool, which can remove and replace defective solderballs, will lead to a significant increase in overall systemyields.Reballing. The electronics industry has eliminated metalliclead (Pb) from nearly all components and processes (RoHScompliance). Most backend packaging companies which pro-

duce BGA packages have switched all of their productioncapacity to lead-free alloys (SnAgCu), leaving specialty appli-cations like defense, aerospace, and medical without the"qualified" lead-based BGAs required for their systems.Because of this need, it is important to have a flexible processto remove and then replace the lead-free spheres with thehigher reliability and qualified lead-based alloys. A singletechnique that can accomplish all three restoration processes— repair, rework, and reballing — involves the use of the SB2laser-based bumping system. The tool removes and replaces

the solder bump utilizing a localized heating source to selec-tively melt a single solder sphere, then uses a vacuum toremove that molten solder material from the device (wafer,chip, or package) through a capillary tube. This same toolthen drops a new preformed solder sphere onto that bond padwhile simultaneously reflowing the sphere using a laser (sol-der jetting).

The basic principle behind the operation of the SB2-M

PRODUCTION

a. SB2-M tool; b. Reballing tool bondhead; c. Schematic diagram of reballing tool.

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tool is to: align the capillary tube to the solder sphere thatneeds to be removed. The heated capillary head is lowereddown toward the solder ball and vacuum is applied to the cap-illary to suck the molten solder up and out through the capil-lary. To replace the balls, the tool is realigned to the bondpads, and a preformed solder sphere is singulated within thebondhead and dropped down through the capillary. As thesphere approaches the bond pad, a laser beam is pulsed with-in the capillary, which reflows the solder sphere just as itreaches the pad (solder jetting). The bondhead is then reposi-tioned over the next pad and the process is repeated.Reballing speeds of up to 6-8 balls per second have been real-ized using this technology.

Analysis of the solder joints which are created using thislaser-based method has shown comparable ball shear forcesto that observed for the original part. This method has theadvantage over traditional rework methods in that the veryshort laser pulse (1-20 millisec) creates only a minimal

amount of additional intermetallics. This results in restoredparts that have the same reliability observed for the originalpart. In addition, localized laser heating significantly reducesthe amount of stress that can build up in a system.

The versatility of this tool is realized by its wide flexibil-ity in solder alloy composition and the ability to acceptdevices with different sizes and form factors. Solder balls ofnearly any solder alloy composition and size can be bothremoved and replaced using this technology. Solder balls assmall as 150µm and as large as 760µm are currently beingdeposited using the repair and rework tool in volume applica-tions. The low cost structure of this tool is realized in its ver-satility used to repair, rework, and reball solder intercon-nects.

Contact: Pac Tech Packaging Technologies, 328 Martin Avenue, Santa Clara, CA 95050 408-588-1925 E-mail: sales@pactech-usa.com Web: www.pactech.com