Sunday, September 25, 2016
VOLUME -24 NUMBER 6
Publication Date: 06/1/2009
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Archive >  June 2009 Issue >  Partnering > 

Automated Reballing Tool Fulfills MIL Needs
Schematic diagram of automated reballing tool.

There has been a growing demand to automate the rework and repair processes associated with BGA packages (reballing). Historically, there have been three main reasons for reballing these packages: deformed balls, damaged balls, or missing balls.

A fourth reason that has emerged, is based on the need for certifiable reliability for the military, and this only comes from lead-based products. Because of the RoHS-mandated elimination of metallic lead from the electronics industry, the lead-free balls that are on nearly all of the recently manufactured BGA packages, need to be replaced with the old-fashioned lead-based alloys (typically PbSn) to meet MIL specs.. Most backend packaging companies which produce BGA packages have switched all of their production capacity from lead-based solder balls over to lead-free alloys, leaving specialty applications like military and medical without the "qualified" lead based BGAs required for their systems. These customers have found it nearly impossible to procure BGA packages with lead-based solder balls on them.
Image of automated reballing tool bondhead.


To date, the industry has used what amounts o manual techniques to remove the lead-free solder balls from the existing BGA packages and then replace them with lead based balls. The ball removal methods include: heated solder wicks, hot air knives with vacuum, and heated mechanical blades with vacuum assistance. Replacing the balls is traditionally accomplished by mating solder-preforms to the package, printing solder paste onto the package, or dropping solder balls onto the prefluxed package and then reflowing. All of these techniques for removing and replacing are highly manual, require a number of fluxing and cleaning operations in the process flow, and use a furnace to reflow the entire device, which often leads to increased intermetallic (IMC) formation.

Automatic Ball Replacement
A technique that both removes and replaces the ball automatically utilizes a localized heating source to selectively melt a single ball, then uses a vacuum to remove that molten solder material. This same tool then drops a new preformed solder sphere onto that bond pad while simultaneously reflowing the ball in-place using a laser. This tool has been used for many years in the wafer level packaging industry for both Flip Chip and Chip Scale Packaging applications and has now been modified by PacTech Packaging Technologies for the reballing of BGA packages. It is commonly know in the industry as a Solder Ball Bumping tool (SB2).

By using this technology to selectively remove and then replace a solder ball, one imparts heat only to the localized area of the solder ball itself, thus greatly reducing the chances of damage to the rest of the package or to the integrated circuits and interconnects that are contained within the package. The potential damage eliminated include: "popcorning" effects in the package, solder fatigue of internal solder interconnects due to multiple reflows, thermal stresses, excess intermetallic formation, residues from fluxes, damage from mechanical cleaning cycles, and solvent attack.

The basic principles behind the operation of the automated reballing tool is to first align the tool bondhead (capillary) to those solder balls on the package that need to be removed. This alignment is accomplished by aligning the head visually to two known positions on the package (e.g. fiducials or corner pads) and then programming in the coordinates for each ball position into the X-Y stage software. Alternatively, one can manually control the stage servos to position the head directly over a user-selected ball position.

Heated Capillary Head
The heated capillary head is lowered down toward the solder ball and a vacuum is applied to the capillary to suck the molten solder up and out through the tube. Analysis of BGA packages after solder removal has shown that a finite amount of solder remains on the reworked pad which averages around 30-40µm and is in the form of a solder meniscus on the pad. Variables which define the efficiency and speed of ball removal include: capillary temperature, capillary head height, vacuum pressure, and mechanical speed of the stages. To replace the balls, the tool is realigned to the bond pads of the package, and a preformed solder sphere is singulated within the bondhead and dropped down through the capillary. The downward motion of the sphere is assisted by a back-pressure flow of nitrogen within the capillary. As the sphere approaches the bond pad, a laser beam is pulsed within the capillary, which reflows the solder sphere just as it reaches the pad. The bondhead is then repositioned over the next pad and the sphere dropping process is repeated. Reballing speeds of up to 10 balls per second have been realized using this technology.
Close-up image of solder ball being placed and reflowed.
Analysis of the solder joints which were created using this method to both remove and replace solder balls, has shown higher ball shear forces than those reballed using traditional methods, no solder voids, no observed anomalies in the solder grain structure, and a consistent intermetallic layer within the solder joint.

The versatility of this tool is realized by its wide flexibility in solder alloy composition, the ability to accept devices with different sizes and form factors, and a software interface which allows a high level of automation for higher volume applications. Solder balls of nearly any solder alloy composition and size can be both removed and replaced using this technology. Solder balls as small as 80µm and as large as 760µm are currently being deposited using the automated reballing tool in volume applications. The limitations in solder alloy composition are only dictated by the composition of preformed solder spheres that can be purchased. There are a large number of vendors who now supply spheres for most of the common solder alloys like: eutectic lead/tin, high lead, lead-free SAC alloys, indium/tin, and gold/tin eutectic. Other specialty alloys are available from most vendors upon request.

The cost of ownership for this tool is based on three main factors: initial tool cost, speed of ball removal/replacement, and sphere cost. The intrinsic cost of the system, however, is realized by the cost of recovering packaged BGA parts where the balls are damaged, missing, or deformed; or by enabling the use of "off-the-shelf" BGA packages by replacing the lead-free balls with lead-based balls.

Contact: Pac Tech — Packaging Technologies, 328 Martin Avenue, Santa Clara, CA 95050 408-588-1925 fax: 408-588-1927 E-mail: sales@pactech-usa.com Web:
http://www.pactech.com

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