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Printed Electronics World
Posted on November 8, 2012 by  & 

Capability, limits and considerations for screen printing

Guest article by Fernando Zicarelli, Asada Mesh
 
Screen Printing is a technology that advances with the help of mesh, paste and machine technology; we all share a common goal, to push screen printing technology to a higher and more valuable level. In this article we will share our own experiences and the ones from our customers to try to clarify the limits and capabilities of screen printing in general.
 
Currently, there are a few R&D Labs in the world who have achieved line widths down to 20µm (please see images below) but the truth is that line widths of 75-100µm is what most electronic companies are still printing today.
 

1. How do we choose the right Screen Mesh?

Before selecting the screen tool, you should know two important pieces of information from the desired imaged pattern - line width and thickness. From these two pieces of information, we can begin to work on selecting the proper mesh.
 
To understand better how Mesh Technology works, I have to explain what the specifications are:
  • Mesh Count (Number of Wires per Inch or per Centimeter)
  • Wire Diameter (in microns)
  • % Open Area (% number)
  • Thickness of the Mesh (in microns) **
  • Index Strength (strength of the wires) ***
 
 

2. How does the Emulsion affect the end result?

In every screen there is a photosensitive layer which covers the mesh called "emulsion". This emulsion is normally applied with a direct wet coating onto the mesh or it could be applied to the mesh with a dry emulsion film ormat. This emulsion is the material used to create the image pattern on top of the mesh. Years ago, one of the limiting factors was that emulsion resolutions were not capable of developing narrow line widths; however, with advances in technology today most emulsions have resolutions down to 25-50 microns.

3. Theoretical Print Thickness ****

It can be calculated by multiplying the mesh thickness by the percentage of open area. Once you have performed this calculation, we will obtain the Theoretical Print Wet Thickness. The example below will show how to calculate the Theoretical Wet Print of 640 mesh with 15 micron wires:
 
Mesh Thickness of 640-15 (35µm) X % Open Area (39%) = 13.65µm
 
  • The printed thickness varies depending on the characteristic of the paste bleeding.
  • Emulsion thickness affects the printed thickness depending on line width.

4. Theoretical Dry Print

It is calculated by multiplying the Theoretical Wet Print Thickness by the % of solids in the paste/ink, in this case we will use a Paste with 50% Solid content:
 
 
Theoretical Wet Print Thickness (13.65 µm) X % of Solids in the Paste (50%) = 6.825 µm

5. How do we know what an individual Screen Mesh Tool is capable of printing?

Well.., this is the question that I get asked the most in Exhibitions and Shows; the answer is simple..., "it really depends... "
.
One must ask these two questions:
 
  • Will the paste/ink go through the openings and will it define the feature properly?
In my initial paragraph I said "Screen Printing is a technology that advances with the help of mesh, paste and machine technology"; we do not know what the smallest feature size our screens can print until we test the pastes that our customers choose.
 
As a reference, Asada Mesh is currently working with most paste manufacturing companies to be able to provide an accurate answer to all of our customers; to help choose the proper mesh we tested each of our new meshes with appropriate pastes and determined their line width capability; below I show as an example the latest generation of Asada Mesh products called HSD:
 
We encourage everyone interested in producing Screen Printed Components to contact us; our Center of Screen Printing Excellence in Kagoshima, Japan is equipped with everything necessary to figure out the best tool for your job.
 
**Thickness of the Mesh can be varied by Calendering down to 30% thinner if necessary.
***Index Strength of the Wires; the higher the number the stronger the mesh is.
****Theoretical Wet Print is a mathematical calculation with an accuracy of +/-2 microns; it can vary depending on Printing Parameters (speed of stoke, snap-off, squeegee type, squeegee hardness, etc.).
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