Solutions in solder paste: new stencil developments make the BGA rework process more reliable.

No matter how much we attempt to streamline and automate the process, ball grid array (BGA) rework has been, and will likely always be, a mixture of art and science. The need for a predictable, repeatable process continually drives us to attempt to remove as much "art" as possible from the BGA rework process, as new tools and knowledge become available. Such machinery is preferable to having the success of a rework operation dependent upon the uniquely developed skills of an individual operator. Rework processes typically begin with logical, sometimes obvious, solutions and then slowly evolve--driven by the continuous need to improve quality and cost efficiency.

Traditional Methods

The process of preparing a BGA component, or BGA location, for rework is a perfect example of how such improvements may evolve. A key step in the BGA rework process is the application of solder paste at a specific, individual location at either the component footprint on the circuit board or on the component itself. For some time now, to achieve such precise application, the stenciling process is mimicked through the use of a miniature metal stencil. This, method, however, is time consuming and awkward, utilizing a taped-to-the-board flat metal mini-stencil or a formed version held in place with a positioning arm. This stop-gap method simply cannot consistently produce the same result as a stencil printing machine that features controlled, automated squeegee functions, stencil lift-off and optical alignment mechanisms.

With my organization's six BGA rework systems, I suspect that we do as much BGA rework volume as anyone, and then some. The metal mini-stencils that we use to apply solder paste for BGA rework continue to challenge us with inconsistent print volume, stencil flatness issues, site access and stencil cleaning. We work hard to make these mini-stencils work properly, and the results often depend on the artistic skills of a dexterous operator. Nevertheless, we often must accept less than perfect print quality in the name of efficiency.

Emerging Methods

The current proliferation of metal mini-stencils ensures they will be around for some time to come, despite their shortcomings. But two recent developments in solder paste stenciling for BGA rework are viable alternatives that may offer significant yield improvements for those willing to apply and master them.

The first of these alternative processes involves stenciling solder paste directly onto the balls of the BGA component. The solder paste application is accomplished by attaching the component to a stencil plate--essentially a thick stencil with a mechanism for keeping the component balls pressed into the apertures of the plate. Following attachment of the component to the plate, a squeegee is used to apply solder paste to the plate's opposite surface. The plate is then inverted, component side up, and placed in a nest on the rework machine that has been tooled to accept the stencil plate. The component is unclamped from the plate, allowing the pick-up nozzle to access the component and extract it from the stencil plate with the solder paste attached to the component balls. The print quality is inspected through the split image optics of the rework machine prior to placement on the circuit board surface. Heating and reflow follows.

Another recant development replaces the rigid metal mini-stencil with a flexible, polymer film stencil featuring an adhesive backing. Unlike the mini-metal stencils, these polymer stencils are flexible and will conform to irregularities in the circuit board surface. The most interesting feature is the adhesive backing, which seals around BGA pads to ensure solder paste will not bleed under the stencil when the paste is applied. Fold-up side tabs, used to locate and place the stencil, also serve as solder dams by preventing solder paste spillage onto the circuit board surface.

Conclusion

Rework may not be getting easier, but the constant evolution of new tools, materials and methods for performing rework tasks helps the rework technician make the process more reliable and repeatable. At times, these improvements are fairly simple, as with the above-mentioned polymer stencils. Stay abreast of new developments and constantly look for innovative ways to improve the rework process.

Jeff Ferry is president of Circuit Technology Center, Haverhill, MA; [phone]; www.circuitnet.com.