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The Optional TheoryOur 50µm line becomes a 75µm lineAt the other extreme, slumping can causea different problem. Some specialist applicationsrequire ink deposits which haveto achieve a large thickness specification.If, for example, you are trying to print a100µm thick line, 500µm wide with a reasonablyviscous ink (10Pa.s) then within 2seconds the line is 700µm wide and only82µm thick. If the specification were 90µmthickness then you’d be in trouble. Thecomputer model shows that you wouldhave to have cured the ink within 0.4 secondsto avoid it slumping below 90µm.The Drop Spread software models the 100µm high, 500µm wide line32
Controlling slumpSo what can you do about slump? Thetable gives you good indications, but thereare lots of complications.Increasing surfactant levels should reducethe surface tension and therefore reduceslump, though this isn’t always the case incomplex cases. But surfactants can interferewith other aspects of the ink formulationso this isn’t always possible. Unfortunatelyin the example above even if youhalve the surface tension you only gain afew extra µm thickness.The initial contact angle is largely a functionof your ink deposit. A thin ink deposit(fine mesh, low-EOM stencil) will give youless slump. Remember that slump speed isproportional to the cube of the initial contactangle, so even modest reductions inink thickness can give large reductions inslump. In the thick ink example, reducingthe starting thickness to 90µm reduces thewidth of the slumped line by 20µm.The equilibrium contact angle is oftenignored but it can be a vital part of yoursolution. The ink will stop slumping whenthe contact angle reaches the equilibriumvalue. So if you tuned your substrate sothe equilibrium angle were equivalent tothe initial angle you would get no slumpat all. This trick has been used in the worldof fine-line inkjet printing where they havevery low viscosities and therefore verylarge slumps.As we’ve discovered in the previous section,“viscosity” is not a simple concept soit’s important to know which aspect ofviscosity is important for slump. It’s nowobvious that a perfect ink has a low viscosityduring the shearing action of the meshcoming out of the ink, followed by a rapidrecovery to a high viscosity to avoid slump.In the ceramic conductor industry theycan often come close to this ideal as theirformulations don’t contain polymers. Polymericinks tend to have less of a reductionin viscosity with shear and strong thixotropictendencies so they are slow to recover.Hence the battle with slump is much moredifficult. Specialist additives exist whichcan improve the situation and if the inkdesigner knows what has to be done thenthere is hope that the slump can be reduced.For example, the computer modelsays that to attain the 90µm thickness targetwith a 2 second delay before cure, thelow-shear viscosity should be increasedfrom 10 to 50Pa.s.Ceramic conductive inks are printed ontoceramic substrates. These substrates areoften microporous and they rapidly suckthe solvent away at the leading edge ofthe slumping ink. This sends the viscosityskyrocketing and the slump comes to ahalt. Crude porous substrates (such as paper)are obviously not a good idea as theydestroy edge quality. But micro-porousmaterials (holes in the µm range) do nothave a big effect on edge quality. Thereare some debates about whether micro-33
Page 1 and 2: HowHow to be agreat ScreenprinterEd
Page 3: About this eBookI’m passionate ab
Page 6: 06Table ofContents07 The Theory Bit
Page 11 and 12: The wrong way: a strong-deep flood.
Page 13 and 14: why a light flood is recommended so
Page 15: This is the single most important f
Page 18 and 19: When I first pointed this out to my
Page 20 and 21: The Theory Bit20Why is the 1-2-3 so
Page 22 and 23: The mesh does the meteringThe 1-2-3
Page 24 and 25: This gives us the ‘real’ TIV fo
Page 26 and 27: simple, in-control process.Years of
Page 28 and 29: The Optional Theory28At the shear r
Page 30 and 31: I’ve come across examples in all
Page 34 and 35: oughness can slow slumping; it prob
Page 36 and 37: The Optional TheoryThe exact part o
Page 38 and 39: atches of the ‘same’ mesh. My f
Page 40 and 41: The Optional Theory40The industry r
Page 42 and 43: the screen frame itself to compensa
Page 44 and 45: Print Faults - Making and fixing th
Page 46 and 47: Print FaultsThey include tape marks
Page 48 and 49: Wrong ink - Too much viscous drag (
Page 50 and 51: DustPrint FaultsWe threw some dust
Page 52 and 53: What’s the best order to print yo
Page 54 and 55: Colour shifts54high-frequency (non-
Page 56 and 57: We formed our own (subjective) opin
Page 58 and 59: And the winner is...Colour shifts58
Page 60 and 61: Getting the right exposureAnna Harr
Page 62 and 63: Getting the right exposureThe light
Page 64 and 65: 3/4 full exposureEFFECTS: Emulsion
Page 66 and 67: Getting the right exposureThere is
Page 68 and 69: Undercutting from a rough stencilSo
Page 70 and 71: Getting the right exposure(or numbe
Page 72 and 73: Solving problems in the real worldA
Page 74 and 75: Solving problemsThe simple act of r
Page 76 and 77: Emulsion coatingThe advice about cl
Page 78 and 79: Solving problemsA quick tip to help
Page 80 and 81: stencil causes a thick edge needed
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Solving problemsThis screen-shot fr
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A modest Rz gives amodest absolute
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Solving problemsThe root cause of n
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Now the ink fully fills the extra s
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Solving problems90Stencil X gives b
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Moiré, causes and curesSteven Abbo
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Moire, causes and cures94fluential
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Figure ii. The overlayedpattern fro
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Moire, causes and cures98Causes and
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2 15° moiréIt’s an unfortunate
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Moire, causes and cureson top of an
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Ideal. Find a RIP with an option fo
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Moire, causes and curescan’t pick
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frequency tend to have a lower ampl
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Moire, causes and curesis a half-tr
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Incidentally, note that the ratio o
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Moire, causes and cures114So, surpr
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grid) in a conventional set that wa
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Moire, causes and cures118what this
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Arguably the skipping takes place p
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Moire, causes and curesSummaryAs Ma
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MoiréDescriptionAn unexpected patt
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Problem solving GuideCause1. The mo
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Dark spots / HickiesDescriptionDark
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Problem solving GuideTechnical refe
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6. Substrate unstable.7. Changes in
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Glossary of terms used in this eBoo
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Glossary136Moiré amplitudeThe ampl
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A Macdermid Autotype HowTo eBookEdi
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