Screening & Drying Conductive Inks: FAQs
One of the biggest problem areas when screening conductive inks is that they are not dried completely after screening. For silver ink, a dried trace thickness of .0004 to no more than .0008” (approximately 10 to 20 microns) is recommended. One easy way to check for completeness of drying is to run a circuit through the drier and then measure the point-to-point resistance on one of the longer ink traces. Run the circuit through the drier again and check the resistance. If the drop is less than 10% (5% in critical applications), then the ink can be considered dried after the first pass.
Because inks harden by drying, or evaporating the solvent out of them, there are three factors that contribute to how fast they will dry; temperature, amount of air flow and air humidity. Too often an operator will rely only on the readouts of the oven to determine temperature. In order to optimize drying efficiency it is essential to profile the oven by running a small thermocouple connected to a spool of wire and a hand held temperature readout. Place the thermocouple on a circuit and record the temperature at regular intervals as the circuit runs through the drier. Sometimes you will find an amazing difference between the actual temperatures and the temperatures indicated on the oven readouts – especially on a hot air oven with no IR heat sources. Once an oven is profiled, you can begin to increase the temperature settings until you reach an optimal actual temperature on the substrate that does not damage the substrate.
We frequently get calls from customers who say that all of a sudden the ink is not drying on a process that they have been using for a long time. Sometimes the culprit is the fact that they have been drawing dry air from the outside of their facility for the ovens, and all of a sudden it is the middle of August and the air has high humidity. If the air is saturated with water, it cannot efficiently carry the solvent away from the printed ink.
This is not recommended because the ink will have a tendency to “skin over” when drying, and the solvent will not come out easily. If a thick trace is needed, it is recommended that the pattern be printed in two passes.
The short answer is that inks can be made to dry very quickly by using a faster evaporating solvent system. The downside to this is that if it dries faster in the oven, it will also dry in on the screen more quickly, causing clogging. Good ink systems will provide an optimal balance between long screen time and short drying time.
Silver ink can be reused with a few precautions. When collecting used silver ink it should be stored in a separate container. After being on a press for long periods it will eventually lose enough solvent that it will be too thick to print. When this happens, small amounts of solvent should be added to restore the viscosity. It is essential that the right solvent be used. Adding an incompatible solvent to any thermoplastic ink can cause the binder to “crash” and you will be left with a thick, lumpy material.
Many times an operator will add an incompatible solvent (or too much of a solvent) to a reclaimed ink and find out after a production run that the printed traces have a higher resistance and are no longer in spec.
Because different thermoplastic binders are soluble in different solvents, it is important to find out from the ink supplier what solvents are recommended for thinning silver inks and also screen cleaning.
When adding used silver ink to the press, it is recommended that small amounts be added to fresh material.
The easiest way to check for adhesion during a production process is to utilize a tape adhesion test. Because the ink is printed in long, thin traces it is not recommended to do a “cross hatch” cutting before tape testing. If the ink comes off of the substrate after tape testing, there may be a compatibility issue between the ink and substrate, or the ink may not be dried enough. Run the ink through the drier once more and check tape adhesion. One thing to keep in mind is that it is normal to see a small “shadow” pattern of silver ink on the tape after testing.
Another quick test to evaluate adhesion of the ink is to do a hard “crease” test. Measure the electrical resistance along a trace, then bend and crease the circuit across the trace. Measure the resistance again, to assure that the ink is still conducting, and look at the crease area under magnification to see if it is breaking away from the substrate completely. There is a complete library of test procedures for evaluating membrane switch materials available from ASTM (www.astm.org).
The difficulty in making a UV curable conductive ink is that both silver and carbon reflect UV energy. Some solid fillers are transparent to UV energy and will allow it to pass through them so that the UV ink can be cured completely. Conductive fillers do not allow UV energy to pass through them to reach all of the polymer binder in a conductive ink.
When you add silver or carbon, the UV energy is reflected immediately as it hits the surface of the ink, and continues to diffuse rapidly as it passes further into the ink.
Some claims for UV conductive ink are that special silver particle configurations are used to redirect the UV energy like a series of small mirrors so that it can reach entirely through the cross section of ink.
The simple reality is that it is not possible to obtain a complete crosslinking by UV energy in a conductive ink. The only way to do this is by using a dual cure chemistry, where the UV energy starts a reaction to cure the surface of the ink and give partial curing to the top part of the ink layer. Once this is done, another chemical reaction starts that keeps the polymer crosslinking through the entire cross section.
Unfortunately, this secondary reaction is held to the same laws that govern all other chemical reactions, and it takes a long time (usually 24 to 96 hours) to complete unless the substrate is heated.
The UV component of the curing mechanism can be enhanced by longer exposure to more UV lamps, but even under the best UV curing conditions a “post curing” will be required.
Even under the best of conditions, UV conductive inks still are not as mechanically strong, solvent and heat resistant, or electrically conductive as traditional solvent, water or epoxy based conductive inks.