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.
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.”