The late Alden Johnson demystified the printing process.

This column is dedicated to Alden Johnson, who recently passed away from a heart attack at the age of 64. Alden was not only a longtime member of the Speedline Technologies family, but also as iconic a figure as we see in the stencil/screen printing world. Alden made many significant contributions to the stencil printing process, including his attempt at demystifying the printing process. Below are some guidelines he wrote in an attempt to put a scientific bent on the printing process, which some likened to “black magic.”

Alden will be greatly missed by his friends and colleagues. Our sympathies are with his family.

Key Variables for the Stencil Printing Process

The paste printing process has four major variables: printer, stencil, substrate and solder paste. Solder paste is printed on the board to serve as an electromechanical and (sometimes) thermal connection for the component and pad on the board after reflow. Paste is pushed over the stencil by a squeegee and rolls into the apertures onto the component pad. Once the apertures are filled, the squeegee levels the paste to the top of the stencil and moves onto the next area of the stencil to be filled. The forces that adhere paste to pad must overcome the same forces that hold the paste to the stencil sidewalls. If the forces are not overcome, there will be a partial release and an insufficient amount of solder paste on the pad, which will result in a poor solder joint (at best) or open solder (worst case). The amount of solder paste deposited must contain enough metal and flux to result in a fillet between the component body or lead and the pad on the substrate.

The following is a list of attributes or key variables that must be under control to maximize solder paste transfer during the stencil printing process. With these attributes under control, you will take 90% of the black magic out of the SMT stencil printing process.

PCB variables.

• Board coplanarity – Impacts stencil gasketing, paste volume transfer, component placement. Legend, solder level and solder mask can impact coplanarity.
• Plug-in vias – Impact stencil gasketing by raising stencil.
• Solder mask thickness in relation to pads – Impacts stencil gasketing and solder paste volume transfer.
• Board warpage – Impacts stencil gasketing, paste volume transfer and component placement.
• Tooling holes, if used for alignment – Impact board and stencil alignment.
• Board stretch/step-and-repeat – Impacts printing accuracy and overall print quality.
• Pad width – Impacts stencil gasketing, print quality and solder paste transfer accuracy.

Solder paste variables.

• Powder size and distribution – Impact print quality, solder paste transfer, bridging and insufficients.
• Rheology – Impacts slumping, which affects deposition height, causing bridging and insufficients.
• Environmental factors – Temperature and humidity affect rheology.

Stencil variables.

• Aperture width relative to pad width – Impacts print quality, paste transfer deposition volume.
• Wall smoothness – Impacts solder paste release, solder paste transfer efficiency, deposition volume.
• Aperture and stencil thickness tolerance – Impacts deposition thickness and volume.
• Trapezoidal apertures – Impact deposition volume and paste transfer efficiency.
• Stretch during stencil assembly – Impacts deposition accuracy.
• Foil distance outside image area – Impacts print quality.
• Stencil thickness – Determines deposition height, print quality, bridging and insufficients.
• Fiducal location and fill – Impact board and stencil alignment, deposit accuracy.
• Gasketing – Impacts print quality and bridging.

Printer variables.

• Squeegee types – Metal, urethane and edge design affect print quality, solder paste transfer, deposition morphology; cause bridging and insufficients.
• Squeegee speed – Impacts print quality, deposition volume, paste rheology.
• Squeegee pressure – Impacts print quality, deposition volume, paste rheology.
• Pad orientation – Parallel apertures to squeegee stroke generally have 5-10% less paste than those perpendicular to stroke.
• Alignment accuracy – Impacts print quality.
• Board orientation – 0°, 45° or 90° can impact print quality, especially if coplanarity is poor.
• Quick or slow release – Affects print quality, deposition morphology.
• Environmental issues – Affect paste roll, release, paste rheology.
• Cleaning and wiping stencil – Impacts paste deposition quality and cycle time.
• Board stretch – Impacts board and stencil alignment and print quality.
• Consistency and accuracy of setup – Impact print quality and repeatability.
• Post-print QC and SPC – Affect overall quality.

Rita Mohanty, Ph.D., is director advanced development at Speedline Technologies (speedlinetech.com); rmohanty@speedlinetech.com.