cleaning
Understencil Wipe Cleaning Yield Improvements
A study of the behavior of flux-stencil interactions.
by MIKE BIXENMAN, DBA
Understencil wiping has gained increased interest over the past several years. Changes in circuit design due to miniaturized components and highly dense interconnects have increased the importance of stencil cleanliness, both inside the aperture wall and on the seating surface of the stencil. A technology that wets the understencil wipe with a solvent-based cleaning agent is being studied to improve print performance and better understand the behavior of flux-stencil interactions. The cleaning agent dissolves the flux component of the solder paste to improve solder ball release from the stencil’s bottom side and aperture walls.

Kyzen and Indium performed a study to characterize the relationship between wipe processes and bottom-side stencil flux/paste flow. A highly dense circuit board and a stencil with nanocoating was used to study the effects of the understencil wiping process. After each print, the stencil was removed from the stencil printer. The apertures were examined to inspect buildup in both the apertures and bottom side of the stencil. FIGURE 1 shows the flux vehicle and some trace solder balls following the first print.

As expected, solder flux combined with solder balls increases with additional prints. Additionally, small apertures clog quickly. For wider pitch features, dry wiping the bottom side of the stencil is an accepted practice. On larger feature prints, a small level of solder paste on aperture walls does not materially affect the printing process. As feature size reduces, however, chemical assistance often is needed to dissolve the flux vehicle within the solder paste. Solder balls are released and collected within the wiping fabric. To better understand stencil wiping, the following wipe sequences were studied:

  1. Dry/vac
  2. Dry/vac/dry
  3. IPA wipe/dry
  4. IPA wipe/dry/vacuum
  5. Engineered solvent wipe/dry/vacuum
  6. Engineered solvent-aqueous wipe/dry/vacuum.

In the dry wipe studies, there appeared to be streaking on the bottom side of the stencil. Upon closer examination, the flux vehicle tended to become wiped over the bottom side of the stencil. Increasing the numbers of prints increased the level of flux spread on the bottom side of the stencil (FIGURE 2).

close view of flux buildup on highly dense apertures after one print

Figure 1. Flux buildup on highly dense apertures after one print.

close view of flux streaks following dry wipe
Figure 2. Flux streaks following dry wipe.

IPA (isopropyl alcohol) is a common solvent wipe used when a wet wipe is used. However, paste manufacturers are moving away from IPA-based fluxes because they are a flammable solvent with a flash point and are becoming inefficient for modern pastes. Current fluxes, especially no-clean formulations, require more specialized solvents. The solder paste used for this research was a lead-free, no-clean formulation. Following the IPA/dry wipe, the bottom side of the stencil was dry and mostly clean. Similar to the dry wipe, flux streaks were observed over the bottom side of the stencil. Flux streaks were also observed on the bottom side of the stencil in the third sequence of IPA/dry/vacuum.

solvent-water azeotrope wetness on stencil bottom
Figure 3. Solvent-water azeotrope wetness on stencil bottom.
A solvent-based stencil cleaning agent known to be effective at cleaning no-clean, rosin-based, wet solder pastes was evaluated. The engineered cleaning agent solvates the flux resin components within the wet solder pastes. The engineered solvent composition cleans and removes solder paste that tends to stick to the aperture walls and stencil bottom. The bottom side of the stencil appeared to be free of the flux stains. Unlike IPA, the engineered solvent is formulated within the combustible range. Due to its lower vapor pressure, the solvent dries somewhat slower than IPA. The bottom side of the stencil was dry following the dry + vac wipe process. The final solvent tested was a solvent-water azeotrope-engineered composition. The benefits of engineering a solvent-water azeotrope is the uniform evaporate rate, nonflammability, and low-VOC content. Potential risks of using this wipe solvent include ineffectiveness at removing no-clean flux resins, drying following the wipe sequence, and potential solder paste contamination. The solvent-water wipe/dry/vacuum process did not appear to clean the no-clean flux vehicle as well as the engineered solvent wipe. Of greater concern was a light film of the solvent-water azeotrope on the bottom side of the stencil after the wipe sequence (FIGURE 3). Additional dry and vac cycles should be evaluated when using a slower-to-dry wipe solvent.
Conclusion
The research performed here found that understencil wipe solvents match with the flux compositions used in lead-free solder pastes and remove fluxes more effectively than IPA. Engineered understencil wipe solvents removed flux stains and left a dry surface following the wipe sequence. The solvent-aqueous understencil wipe solvents appeared to clean well but were slower to dry, and may require additional dry wipe and vacuum steps.

Mike Bixenman, DBA, is chief technology officer at Kyzen (kyzen.com); mikeb@kyzen.com.