SECTION III

Understencil Wiping

The primary goal of the stencil printing process is to deposit an exact amount of material in a precise location on a PCB. To achieve this goal, the stencil and PCB must be well aligned and in very tight contact. Poor contact, or gasketing, between the stencil and the board can cause poor quality prints that result in soldering defects. A common root cause of gasketing issues is solder paste on the stencil’s contact side. Understencil wiping removes the undesired solder paste and flux from the contract side of the stencil. It can also remove paste buildup from the inside of the aperture.

Figure 15: Image of the Stencil Printing Process

Paste deposit volumes can be measured by Solder Paste Inspection (SPI) equipment. SPI machines precisely measure solder paste deposits to monitor solder paste deposit volumes and positions. They compare all the measurements with programmed tolerances, and determine if prints pass or fail the inspection criteria.

Deposit volumes are often reported as transfer efficiencies (TE). TE is the percentage of paste that is actually transferred to the PCB, as opposed to that left inside the stencil aperture. It is calculated as the paste deposit volume divided by the stencil aperture volume, and is expressed as a percent. A common benchmark is 80% TE. Paste that remains in the apertures has a tendency to dry out and build up on the aperture wall, effectively reducing the size of the aperture.

Deposit volume consistency is often measured using the Coefficient of Variation, CV. The CV is a simple metric that is calculated as the standard deviation of a set of similar deposits divided by the average volume of the deposits. It is also typically expressed as a percent. CVs of 10% or less are desirable; they can rise as high as 15% for very challenging deposit sizes and still be considered acceptable.

Aperture size is important, because as apertures get smaller they become more challenging to print correctly. A stencil aperture’s Area Ratio (AR) is calculated as the area of the aperture’s PCB-side opening divided by the area of the aperture walls. Because solder paste sticks to the surfaces it contacts, AR is an indicator of the relative adhesive forces on the solder paste deposit during separation from the stencil. As ARs decrease, so do TEs. The minimum acceptable area ratio is often considered to be 0.66 for typical SMT purposes. Solder paste buildup inside the apertures can reduce ARs to below the recommended 0.66 threshold. Production yields are the ultimate indicator of process capability and fitness for use. Print test yields are measured at the PCB level, not the per- deposit level. Most boards have thousands of paste deposits per print. All prints must fall within their control limits. An output of 1 bad paste deposit and 9,999 good paste deposits on a PCB would not be considered a 100 ppm process; it would be considered a zero yield process. It is important to consider yield because fallout impacts production. PCBs with failed prints must be taken out of production, cleaned, dried and reintroduced into the production run. Failed prints are labor intensive and slow down production processes.

TE, CV, AR and yield are all metrics that can be applied to the stencil printing process to characterize the relationship between process inputs and outputs.

The paste transfer process has two distinct steps: aperture fill and aperture release. In a properly adjusted process, the main factors that influence the fill of the aperture include the solder paste formulation, squeegee or print head

speed and pressure, and the orientation of the aperture in relationship with the print direction. Factors that affect aperture release include the solder paste formulation, the aperture’s AR, the smoothness and cleanliness of the aperture walls, and the three major print parameters: squeegee speed and pressure which influence paste viscosity, and separation speed between the PCB and the stencil. Occasionally, air pockets, clumps of solder spheres, dried solder paste (flux) or foreign objects in the solder paste will appear on a production line and affect the volume and repeatability of the solder paste.

Understencil wiping addresses many of the situations that adversely affect print quality. In addition to removing unwanted paste from the bottom of the stencil to improve gasketing, wiping removes dried paste from apertures that choke down the ARs, and can also remove the occasional anomalous detriments like solder powder clumps resin crystals or other foreign material. Most stencil printers have built-in wiper with options for dry wipes, wet wipes with solvents and vacuum wipes. They can be programmed in numerous sequences and intervals to optimize the production stencil printing process.

Figure 16: Smoothness and cleanliness of aperture walls are critical factors in solder paste volume repeatability of high density interconnects

2.1 Understencil Wipe Process Understencil wipe process is designed with a roll of fibrous wiping material for wiping across the underside of the stencil. The stencil printing machine software provides the operator a receipe of options for programming the wipe sequence. A common wipe sequence is a dry, wet, vac sequence. Each sequence goes the opposite direction of the last print.

Figure 17: Image of an Understencil Wipe design within the stencil printing machine

Most stencil printing machines offer the ability to program a wet wiping cycle within the understencil programmed wipe sequence. Isopropyl alcohol (IPA) is the common solvent used when a wet wipe is programmed into the wiping receipe. IPA has been the “traditional” solvent used in essentially all applications involving unreflowed

solder paste. Historically the choice of IPA made sense, as most solder flux packages were based on IPA. The problems with IPA are flammability and poor solubility match for many lead free no-clean solder pastes.

The solvent is programmed to wet the wipe material through small orifices. The wipe roll wetted with the solvent is passed across the print area. The goal is to remove the flux on the bottom side of the stencil and to release solder spheres from inside apertures and solder paste on the bottom side of the stencil. Figure 18 is an image of the wiping mechanism that has been wetted with the wipe solvent.

Figure 18: During the wet wipe, solvent is wicked through small orifices to wet and area of the wiper roll.

For each wipe programmed into the software, a vacuum sequence is passed over the bottom side of the stencil to clean out solder stray solder paste out of the apertures and to remove and evaporate the wipe solvent. Following the wipe programmed sequence; the goal is to remove both flux and stray solder spheres from the underside of the stencil while leaving the stencil clean and dry for the next round of board prints. The number of prints before wiping is a function of aperture size and board density.

Figure 19: Vacuum system designed to extract trace solder balls and chemical residues from the bottom side of the stencil.

2.1.1 Dry Wipe

Dry wiping is one option for removing solder balls on the underside of the stencil. The wiping paper is designed with a highly porous fiber that allows solder balls to be pushed into the fiber. For boards with larger aperatures, dry wiping is an effective method for cleaning the bottom side of the stencil. On highly dense prints, dry wiping can smear flux residue over the bottom side of the stencil.

Figure 20: Flux wiped over the bottom side of the stencil after a dry wiping.

As the film of flux builds on the bottom side of the stencil, flux residue transfers onto subsequent printed boards. The flux transfers to the printed area and starts to weep away from the solder print. Figure 21 illustrates how the flux transferred to the board starts to bridge between solder paste deposits. During reflow, this thin layer of flux can burn, oxidize and leave a white residue. All of the above are problematic.

Figure 21: Flux bleed away from solder prints