A rapidly-developing area of screen printing, in which our experience and expertise is being requested more and more, is the printing of conductive elements. Here are a few things that need to be carefully considered to achieve a successful outcome...
As with any ink solution we need to be clear what the customer requires. Questions specific to conductive ink applications may be:
- Resistance required (normally specified in ohms per square at 25um dry ink film)
- End use
- Mechanical requirements (flexibility, humidity resistance, abrasion resistance)
Once there is an indication regarding application, to narrow down suitable ink, it is possible to recommend the suitable mesh types and drying.
The most critical part of the process is the drying as most performance issues (target track resistance) are related to drying.
- Conductive inks are not difficult to process, but the method must be consistent, repeatable and measured.
- Pre press is critical and must be controlled and consistent. (Mesh, tension, angle, eom, Rz)
- The resistance of the conductive in Carbon, Silver, Silver/Silver Chloride etc, can be varied if the solvent is not fully removed from the ink. Residual solvent in the dry ink film will result in a non-conformance, ie Higher than expected track resistance.
- The drying temperature should be ramped gradually. This will ensure that the conductive ink does not 'skin' causing solvent to be trapped in the ink film. This is normally achieved using a multi-stage drying conveyor.
- Gradually ramping the temperature will also avoid rapid removal of the solvent which can result in solvent boiling and causing voids in the dry ink film. This will also result in increased track resistance.
- The specified temperature to dry the ink should be used. The solvent in the conductive ink will have a boiling point which is required to remove the solvent from the ink film. If the tech data specifies 110C for ten minutes, this should be the starting point. Using 90C for 40 minutes does not mean that the solvent will be removed.
- Test - When the initial tests are made with the ink, the resistance should be measured against the target. If the resistance is higher than expected with the initial tests, re-run the drying. If there is a difference in the resistance by re-drying the drying schedule needs to be adjusted to either increase the temperature or dwell in the oven.
- Measure - During screen print production there are many variables which might be difficult to control accurately. As a result, the final printed conductive needs to be measured after correct drying off-line, normally with 2 test tracks or worms of known and standardiased dimensions printed on either side of the main print. Measurements should include elecetrical resistance, thickness and registration as a minimum.