SECTION IV

Engineered Wipe Solvents

The solubility character of today’s solder pastes require an engineered solvent designed to match up with the solder paste flux composition. When considering the functions the engineered solvent needs to do, it must match up (solubilize) the flux package, release the solder spheres, rapidly dry, leave behind no residue, and not interact with materials used within the process. The engineered solvent needs to be either nonflammable or have a very high flash point. Additionally, the engineered solvents typically have less adverse health effects.

Many printers have the option to add a solvent and/or an auxiliary removal step, typically involving a vacuum, to the wiping material to either prevent or reduce these two problems. The solvents are typically isopropyl alcohol (IPA) or an engineered solvent blend.

Figure 1: Stencil Printer Orifices designed to wet understencil paper

During the stencil printing process, solder paste and flux build-up in both the apertures and bottom side of the stencil. Figure 2 shows the flux vehicle and some trace solder balls following the first print.

Figure 2: Flux Build-Up on Highly Dense Apertures after 1 Print

Figure 3 shows the build-up on the bottom side of the stencil after additional prints. As expected, solder flux combined with solder balls increases with additional prints. The risk is that flux and stray solder balls can get deposited next to the stencil aperture on subsequent prints and get transferred to the PCB. Additionally, small apertures become clogged at a faster rate.

Figure 3: Build-up on Stencil Bottom Side after 3 Prints

For wider pitch features, dry wiping the bottom side of the stencil has become an accepted practice. On larger feature prints, a small level of solder paste on aperture walls does not materially affect the printing process. However, as feature size reduces, chemical assistance is often 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/Vac
  5. Engineered Solvent Wipe/Dry/Vac
  6. Engineered Solvent-Aqueous Wipe/Dry/Vac

In the dry wipe studies, there appeared to be streaking on the bottom side of the stencil. On closer examination, the flux vehicle tended to become wiped over the bottom side of the stencil. Increasing the number of prints increased the level of flux spread on the bottom side of the stencil (Figure 4). As the board being printed is sandwiched to the stencil, there is a risk that the errant flux can be deposited onto the surface of the board. There may or may not be a reliability risk.

Figure 4: Flux Streaks Following Dry Wipe

IPA (isopropyl alcohol) is the common solvent wipe used when a wet wipe is used. Historically, the choice of IPA made sense as most solder flux formulations were based on IPA. However, solder paste manufactures are moving away from IPA-based fluxes for several reasons, the most notable of which is higher soldering temperature alloys. IPA is a flammable solvent with a flash point (the minimum temperature required for a substance to produce flammable vapors) of 12°C (54°F), which can be a risk factor.

In addition to the flammability considerations,IPA is also becoming an inefficient solvent for modern solder pastes. When natural rosins were the primary constituents of solder paste flux, IPA was an excellent choice as a solvent material.Because rosin is highly soluble in IPA, the IPA readily evaporated, and the IPA was extremely affordable. Current fluxes, however, especially no-clean formulations, have materials in them that are not as soluble in IPA as rosin is, and require more specialized solvents.

The solder paste used to print Figure 4 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, there are flux streaks over the bottom side of the stencil.

Figure 4: IPA/Dry Wipe Example

There also appears to be flux streaks on the bottom side of the stencil in the third sequence of IPA/Dry/Vac (Figure 5).

Figure 5: IPA/Dry/Vac Wipe Example

A solvent-based stencil cleaning agent that is 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 has a tendency to stick to the aperture walls and bottom of the stencil. Figure 6 shows an example of the bottom side of the stencil following the Solvent Wipe/Dry/Vac sequence. 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 a solvent wipe with vac + vac wipe process. For slower evaporating solvents, a second dry wipe may be a good practice.

Figure 6: Engineered Solvent Wipe/Dry/Vac

The last solvent tested was a solvent-water azeotrope engineered composition. The benefits of engineering a solvent-water azeotrope is the uniform evaporate rate, non-flammability, and low-VOC content. The potential risk of using this type of wipe solvent is its effectiveness at removing no-clean flux resins and drying effectiveness following the wipe sequence.

The solvent-water Wipe/Dry/Vac 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 7). Additional dry and vac cycles should be evaluated when using a slower-to-dry wipe solvent.

Figure 7: Solvent-Water Azeotrope Wetness on Stencil Bottom

Using an engineered cleaning agent to wipe the bottom side of the stencil has some risk factors that need to be understood and anticipated. The risk of chemically-assisted wiping has the potential to contaminate the solder paste. To mitigate this risk, understencil wipe cleaning solvent must both clean and readily dry from subsequent dry wipe and vac processes.

The desirable properties of an understencil cleaning agent are: (1) the ability to rapidly dissolve the solder paste flux vehicle; (2) have material compatibility with the nanocoating and equipment; (3) be non-flammable; (4) have low odor; and (5) have sufficient volatility to rapidly evaporate and dry post cleaning. Deficiencies in any of these properties can reduce process repeatability and reproducibility.

One question that continuously arises is how often should an understencil wipe be done? As a rule, smaller apertures require a greater wiping frequency. As the density and miniaturization of the components placed onto the assembly being printed, a wiping frequency after 3-5 prints is common. The user can program a sequence of these features, as well as the frequency that the understencil wipe process occurs. The current recommendations of what solvent, the process sequence, and frequency of the process have not been based in publically available empirical data.

The following steps are commonly used to determine wipe frequency:

  1. Wipe the Stencil thoroughly
  2. Print 1 board
  3. Jog vision system out with stencil light on to inspect stencil apertures for paste residue (Squeeze Out)
  4. If there is no evidence of Squeeze Out, print another board
  5. Repeat this until you see Squeeze Out
  6. Subtract 1 - 2 prints from the determined number of prints for your “wipe frequency”

Excessive delays between prints may require more aggressive wipes.

  1. Start with default settings for a single wipe
  2. Dry Wipe
  3. Use vision system to verify cleanliness of stencil.
  4. If stencil is not clean, use of solvent or additional wipes may be necessary.
  5. Use vacuum wipe if unclogging of apertures is necessary.
  6. Use Idle-time wipe if available

Most wipe solvents, frequencies and sequences are set by engineers or technicians based on observations. Some are derived by DOE’s. Most selections are product-specific, based on the solder paste formulation, PCB configuration and yield history.