News & Events
Solving problems in the real world – continued
In this fourth extract from the ‘How to be a great Screen printer’ e-book produced by MacDermid Autotype, Professor Steven Abbott takes another look at some of the more complex screen faults that can occur and their root causes.
Over the years we’ve built up a large number of case studies of problems found in the real world for screen printers. Here we share some of our experience of the issues and the solutions we have found and we’re sure you may just recognise some of them! The faults are not listed in any particular order.
Stencil thickness V ink thickness
We all know that small increases in the stencil thickness (EOM) can have a big impact on the amount of ink that is printed. Therefore, it stands to reason that, ‘you would need to use a thick stencil to get a thick print’. Unfortunately that is not always true. Yes, a high EOM will dramatically increase the ink deposit for small or narrow images/tracks. However, if for example you want to reduce the electrical resistance of a Silver conductive track, the assumption is to simply use a high EOM stencil which will give you a higher ink deposit (and therefore improve conductivity). Unfortunately, the downside of this approach is that it can dramatically reduce your process window for an in-spec print.
The reason for this is simple, you need to print with as low a squeegee pressure as possible to minimise image distortion for this printed circuit, but if you combine a low squeegee pressure and a thick stencil, the ink simply doesn’t reach the substrate evenly, so you get negative sawtoothing (see image). As a result you increase the squeegee pressure and the ink now fills the stencil, but you get image distortion and a thick edge to the print. The result is now a narrow process window between negative sawtoothing and image distortion.
Using a high EOM stencil to produce a block image is not a solution either. In reality, a thick stencil merely gives you a thick edge. This is simply because the stencil has no influence on the amount of ink that’s printed just a few mesh holes away from that edge. You can see this for yourself if you print a 2 cm square with a high EOM stencil, the thick edge typically extends no more than 200 microns in from the edge. This is because the ink deposit in the centre of the square is determined by the mesh only and the stencil thickness has no influence, as the mesh is pushed down into contact with the substrate (the only exception to this is if you are using an ultra-high tension screen).
Thick edges to the print are a problem in many ways:
- First, if you are printing a conductive track they give you non-uniform electrical properties over the printed circuit as the conductivity is directly proportion to the cross sectional area of the track.
- Second, they will cause increased ink slump which can causing shorting with a circuit.
- Third, if you are not into printed electronics, thick edges can really mess-up anything you print on top of them as their uneven topography guarantees to give uneven printing.
- Fourth, we’ve even seen examples where the thick edges destroy the inter-layer ink adhesion! Not a good thing for Membrane Touch Switch manufacture.
- Fifth, and finally, they can look really ugly, especially when printing a back-lit transparent ink
So what is the answer? The simple answer is to use the mesh as the primary controller of the ink deposit and the stencil is there purely to control just the print acuity (resolution and definition). This can be achieved by using a thin and flat stencil.
It is straightforward to measure the EOM of your stencils by using a hand held measuring device and this will take all the guesswork out of controlling just how thick it is.
Positive sawtoothing is the classic defect that is present in so many screen prints. The ink spreads outside the line in a regular wavy pattern. We’ve all seen it a million times and it always comes from stencils that have an uneven print surface (high Rz). Fortunately it is also easy to measure roughness as there are several hand held Rz measuring devices available, although these are a bit more expensive than the thickness measuring devices. Rz is easy to reduce by either using a stencil film or applying face-coats to an emulsion stencil.
The main symptom of a high Rz stencil is that it automatically causes the ink to spread out under the stencil surface. Consequently the amount the ink spreads depends strongly on the amount of squeegee pressure used and ink viscosity, so it is almost impossible to control. For line printing you see this as a saw-toothed image, whilst in halftone printing it manifests itself as dot-gain. To minimise dot-gain you have little choice other than to create a low Rz stencil to eliminate the problem at source.
However, don’t be tempted to create a lower Rz stencil by building EOM with additional emulsion coats. This will lead to a bigger ink deposit which also spreads (slumps) more to give you dot-gain.
Skipping is typically a problem that occurs when printing four colour process with UV inks (you have to be completely out of control to get this in a single colour!). We know that a high EOM = high ink build, and as UV inks are 100% solids they do not shrink when they are cured (wet height = dry height). Typically you will see skipping when you print the 2nd, 3rd or 4th colour in the set. Basically your stencil is sitting on top of a big mound of dots and the ink has even further to travel before it can reach the substrate. The squeegee pressure isn’t high enough to force the ink down the extra distance caused by the previous dots. Therefore, when the stencil rises, it only leaves little dots where the ink touches the surface of the substrate, these can look like ‘puppy paws’ but is classic skipping.
As the skipping fades in and out depending on where your new dot is with respect to previous dots, the skipping isn’t uniform. To the uneducated eye it can look like moiré and the printer then wastes a lot of time trying to fix the non-existent moiré!
In fact, after classic mesh moiré, skipping is the single largest source of “moiré” that we’ve seen. The industry solution is to increase the squeegee pressure, but this leads to ink spreading, loss of quality and resolution. The only way to resolve skipping at source is to ensure that your previously printed dots are as small as possible. You do this by using a fine mesh and a very thin, flat stencil which gives the minimum deposit.
All too often in screen printing, the solutions to print defects focus in on the symptoms not the root causes. Positive and negative sawtoothing, thick edges, dot-gain, process drift and skipping are all manifestations of the same problem; a mismatch between stencil thickness (EOM) and stencil roughness (Rz). Time spent in producing a thin flat stencil in production really can pay dividends.
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