of conductive ink for screen printing (case 06)
How to improve a conductive ink?
A customer wanted analytical support for the development of a conductive ink for screen printing of electronics. The ink, once applied on a substrate as a very thin line, started to spread tremendously, which resulted in undefined broad conductive lines causing unwanted short-circuits. We were asked to provide a failure analysis and propose chemical solutions.
Screen printing process analysis
The screen printing process was analyzed in terms of relevant shear conditions to which the ink is subjected and the wetting of the substrate was examined as well.
Shear rates varying by several orders of magnitude were identified in the process (‘squeeze’ vs. ‘spread’) and simulated with a rheometer. Contact angle relaxation of ink droplets on the substrate was monitored with the sessile drop technique. Requirements for a good ink were specified in terms of viscosity at rest, elastic recovery potential after squeeze treatment and size and relaxation of the contact angle. The components in the recipe were correlated to their spreading function (deformulation).
New conductive inks for screen printing
The tremendous spreading could be attributed to a mismatch of surface energy and rheological parameters in the current formulation. A number of specific rheological additives were proposed to steer the visco-elastic properties.
Furthermore, the amount and purity of surfactants as well as the type of ink carrier were named as possible root causes. In addition, selection of specific surface energy or use of a pre-coating with corresponding property were specified. Based on this analysis, new variants of conductive inks for screen printing could be formulated that had superior line quality without introducing short-circuits.
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