Ink flow question

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John Kent
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Ink flow question

Post by John Kent »

I want to try and screen print tracks and pads onto copper. My question concerns what happens to the ink when it goes through the mesh. For example , I would imagine that the shape of the ink going through the mesh will be the same shape as the mesh holes, ie we are effectively extruding the ink. I can image that that would form an image made up of ink dots ?. I am wondering that if one made two passes or more with the squeegee would that deposit enough ink to flow under the mesh threads and give a complete ink coverage ?. If one is stuck with the dots, then it would not make electrically conductive tracks on the copper. At the other extreme, if the ink does tend to flow onto the surface would it close up the pad holes ?.

Thank you ....

John Kent
geneh
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Re: Ink flow question

Post by geneh »

My first question is what ink are you using is it a conductive thick film ink or is it a paste? If it is a thick film ink then you will have no issue with ink lay down depending on mesh count and type. If it is a paste I would recommend using a photo stencil. Yes 2 passes will help your ink flow out if it is a thick film ink. Conductive inks are user friendly if you get the right mesh and emulsion. Check with your ink supplier on what mesh and emulsion to use this will make your job easier. Let us know what happens I'm interested.
John Kent
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Re: Ink flow question

Post by John Kent »

Thank you very much for your reply. First I have no practical experience of screen printing and am basically an engineer. I have today made a hinged frame, with base board and 'snap' adjustment and am now waiting on some mesh. I want to print acid proof resist directly onto copper clad PCB board, prior to a 12 minute etch in ferric acid. I then need to chemically or mechanically remove the ink from the remaining copper so that it can be soldered. I have no idea if a water based in or acrylic is the ink I need, to last 12 minutes in the acid without re-dissolving. The PCB board is 1.6mm thick with copper on one side only.

I was planning on using light sensitive emulsion (when I can find some). The image positives are not a problem and I have already done those (see attached image below). I have made up boards using the normal photographic method and the artwork below works fine. If there was danger of ink spread I could reduce the widths of the tracks and pads to avoid shorts.

I have only been able to find one person (by email) who screen prints, but he has never tried to print tracks and pads on PCB boards. What he did say was that if he can print bar codes then I should have no problems.

If I could have found conductive inks then I think I would have made up a flat bed ink jet printer and printed the image directly onto the board. If the ink was UV cured then they usually cure in about 100 milli-seconds and that would have prevented having to handle boards with wet ink on them. I feel sure that there is a solution somewhere.

Thank you again for your reply.

John.
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geneh
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Re: Ink flow question

Post by geneh »

Hi John,

Your in luck I do some aicd etching of stainlees steel and this is the resist we use Ethone PR 4011 Solvent strippable plating resist you can find it on the net just google it. Also hopefully I'm not to late and you have not ordered your mesh yet I use a 230 stainless steel Mesh .0014 wire diameter. The reason for the 230 SS mesh is to get a good ink deposit down and still keep line definition. With this being a solvent based ink any good quality emulsion will work. We coat our screens 2 on the print side and 3 on the squeegee side. Keep us up to date on your progress.

Good Luck And May The Screen Printing Force Be With You,

Gene
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Re: Ink flow question

Post by John Kent »

Thank you very much, I will look them both up on the Internet. I feel like a dimbo because I didn't even know that mesh could be made of metal !.

Thank you again ....

John.
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d fleming
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Re: Ink flow question

Post by d fleming »

It's been years ago but I got a tour once of a facility that screenprinted circuit boards to be dipped and hot tanked so the solder attached only where needed on the board. I do remember they used blue poly 3 from ulano for stencil. That was a long time ago, there may be a better mousetrap by now.
John Kent
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Re: Ink flow question

Post by John Kent »

Yes that is the solder resist method. As large scale integration is introduced, tracks and pads get closer and closer together and there is a real danger of accidental solder bridges forming between tracks and pads, solder resist goes a long way to preventing that. I am grateful to your reference to a suitable ink. The learning curve is steep but every little bit of information adds to ones understanding. Thank you ...

John.
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Re: Ink flow question

Post by geneh »

For an emulsion I preferr Kiwo Poly Plus SRX direct emulsion. It is both solvent resistant and water resistant with excellent image definition. It is the closest thing I have found to a capillary film. The only issue is not to use heat to dry your screen after coating just air dry.
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