Damp will provide a higher cleaning consistency. Make sure you chose a product that is stable and able to clean the solder paste in the first place. Print delineation is a common problem with unsuitable cleaning products. Also ask for the products that are approved for your printer.

Micro Care has observed numerous instances where the solvent used in understencil wiping has caused inconsistent results in the stencil printing process. There are two possible causes for the problems:

(a) tiny amounts of solvent are being released from the wipe into the solder paste itself, changing the viscosity of the paste on the next printing cycle. Slumping, bridging and solder balls are the likely results.

(b) The solvent is causing tiny amounts of solder paste to be released from the walls inside the aperture of the stencil. When the vacuum cycle moves past the aperture it sucks out the loosened paste, removing too much -- not just removing the paste from the bottom of the stencil, but from inside the aperture as well. The result of this is insufficient paste deposition, tombstoning, and missing components (looks like a P&P error but is really a printing error).

In short, the solvent should be considered contamination and avoided whenever possible. The stronger the solvent and/or the slower-drying the solvent, the more prominent these errors become. The net result is slower cycle times, more misprinted boards, more defects during test and inspection, and more rework -- all expensive processes. However, most stencil papers are of insufficient strength to allow high-quality stencil wiping without the use of solvents, so most companies are stuck between the proverbial rock and a hard place.

My recommendation would be to find the highest quality stencil wiping roll you can find, such as DuPont Sontara FP paper, and wipe dry. The additional expense of the stencil roll is more than offset by the savings in solvent, the extended roll life, reduced rework, and improved throughput.

I prefer damp paper with a solvent that is compatible with the solder paste being used. Dry paper does not always remove dry paste and can allow the paste to build up over time.

The exact combination of solvent, vacuum, and dry wipes for optimum underside stencil cleaning is at least in part determined by the solder paste, and the application. The limitation of a simple dry wipe is that it tends to push material back into the apertures while much of the mediums solvent is absorbed in the paper. The net result of this type of process is that you often have dry paste residing in the apertures, which typically reduces the volume of past that is deposited on the following print.

A vacuum wipe minimizes the amount dry paste that gets pushed back into the aperture by drawing the metal into the paper. The use of a solvent in combination with a vacuum wipe can actually start to remove material from in the apertures, however care needs to be taken to insure that all the solvent is removed from the aperture.

These more complex cleaning regimens obviously take longer and can impact throughput and a solvent wipe is typically not necessary every time the stencil is cleaned. A good compromise if your process can accommodate it is to run vacuum wipes with intermittent solvent/vacuum wipes.

There is no yes or no answer to this often asked question. The effectiveness of a stencil wipe is dependent on a number of factors including the type of paste, stencil design, stencil fabrication, etc.

We suggest that you conduct an experiment to determine the effectiveness of stencil wipe with and without a solvent. Certainly if you can achieve an effective wipe without using a solvent then why use it. A solvent adds another expense and variable to the process.

If solvent does provide an improved stencil wipe then by all means you should use it.

Another experiment will be to determine which of the numerous solvents available will provide the most effective stencil wipe.

As I said, since every process has a different combination of factors, only experimentation with your particular process can determine what is best for you.