Electropolishing Laser-Cut Stencils

Hi all, We are currently having problems with a laser-cut stencil that is electropolished and nickel plated with 9-mil openings. The paste does not release well from the apertures. We are having to wipe after every print. The board is fixtured well and we have been using the same paste for the last two years. We had a different laser-cut stencil in production from a different manufacturer with the same 9-mil opening, but this stencil was not electropolised or nickel-plated. With this stencil, the paste release was excellent and we never had to worry about wiping. Does anyone have any input into whether or not laser-cut stencils should be electropolished and nickel plated? I have heard contradicting opinions on this subject. One source has said that you do not want to electropolish laser-cut stencils because the rough laser walls will give you a better paste release. Whereas, another source has said that the smooth laser walls will give you a better paste release. Also, what benefit does nickel plating give you? Does the paste tend to roll better with nickel plating? Do you tend to get more or less paste deposit on the boards with nickel plating? Does it increase the life of the stencil? Has anyone using laser stencils seen major differences between the quality of different stencil manufacturers? Any input into this subject would be greatly appreciated. Thanks, Todd

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To electropolish or not to nickel plate, that is the question. Whether it be nobler in the minds of the competition could be the answer. I think it will probably depend upon whom you ask the question. If the stencil mfgr. has nichel plating capibility, then you will probably find that nickel plating is the answer. The same for electropolishing. Something you should keep in mind is that nickel plating is an "additive" process and electropolishing is a "removal" process. So, if you are starting with a cut aperture of 9-mil, after nickel plating you will have something less than 9-mil and visa versa for electropolishing and visa-versa for nickel plating. When laser-cutting became popular, the chem-etch people said the inner side walls of a laser-etched stencil were not as smooth as chem-etch. So, the laser people had to come up with a way to compensate - electropolish. When the E-Fab nickel stencil was introduced the compensation was to nickel plate a laser-cut stencil in an attempt to stay competitive with the E-Fab. You may have confirmed this logic with your previous experience with the non-plated, non-polished stencil. Now, lets get to what I think the problem is - aperture contamination. How are you cleaning the stencil? A 20-mil pitch stencil is difficult enough to print through. With 9-mil, you have to take extra care in keeping the apertures clear (rough inner walls or not). Just wiping the stencil will just push paste back into the apertures. I recommend scoping the apertures under high power magnification. I'm willing to bet that you will find solder ball contamination. If solder balls are allowed to stand in the apertures overnight, they will dry like cement and be even more difficult to remove with successive cleanings. Dry paste attracts wet paste and obstructs the apertures resulting in poor paste transfer and poor paste release. I also recommend and article on our Web Site that was written by a stencil mfgr. "SMT Stencil Cleaning: A Decision That Could Impact Production." (www.smartsonic.com/article.html) You can also find it in the July,1996 issue of EP&P or the August, 1996 issue of Asian Electronics Engineer. Now, comes the sales pitch. Smart Sonic is the only stencil cleaning process that guarantees to clean any type of solder paste (wet or dry) from any fine-pitch stencil...or your money back! Enough said. If you have any questions, please call me. Tel.: 1(800) 906-440-R Regards, Bill Schreiber | Hi all, | We are currently having problems with a laser-cut stencil that is | electropolished and nickel plated with 9-mil openings. The paste does | not release well from the apertures. We are having to wipe after every | print. The board is fixtured well and we have been using the same | paste for the last two years. We had a different laser-cut stencil in | production from a different manufacturer with the same 9-mil opening, | but this stencil was not electropolised or nickel-plated. With this | stencil, the paste release was excellent and we never had to worry | about wiping. | Does anyone have any input into whether or not laser-cut stencils | should be electropolished and nickel plated? I have heard | contradicting opinions on this subject. One source has said that you | do not want to electropolish laser-cut stencils because the | rough laser walls will give you a better paste release. Whereas, | another source has said that the smooth laser walls will give you a | better paste release. | Also, what benefit does nickel plating give you? Does the paste tend | to roll better with nickel plating? Do you tend to get more or less | paste deposit on the boards with nickel plating? Does it increase the | life of the stencil? | Has anyone using laser stencils seen major differences between | the quality of different stencil manufacturers? | Any input into this subject would be greatly appreciated. | Thanks, | Todd

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Todd: You might want to speak with IRI Stencils about laser cut stencils and platting.

Wayne

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| Hi all, | We are currently having problems with a laser-cut stencil that is | electropolished and nickel plated with 9-mil openings. The paste does | not release well from the apertures. We are having to wipe after every | print. The board is fixtured well and we have been using the same | paste for the last two years. We had a different laser-cut stencil in | production from a different manufacturer with the same 9-mil opening, | but this stencil was not electropolised or nickel-plated. With this | stencil, the paste release was excellent and we never had to worry | about wiping. | Does anyone have any input into whether or not laser-cut stencils | should be electropolished and nickel plated? I have heard | contradicting opinions on this subject. One source has said that you | do not want to electropolish laser-cut stencils because the | rough laser walls will give you a better paste release. Whereas, | another source has said that the smooth laser walls will give you a | better paste release. | Also, what benefit does nickel plating give you? Does the paste tend | to roll better with nickel plating? Do you tend to get more or less | paste deposit on the boards with nickel plating? Does it increase the | life of the stencil? | Has anyone using laser stencils seen major differences between | the quality of different stencil manufacturers? | Any input into this subject would be greatly appreciated. | Thanks, | Todd

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| Hi all, | We are currently having problems with a laser-cut stencil that is | electropolished and nickel plated with 9-mil openings. The paste does | not release well from the apertures. We are having to wipe after every | print. The board is fixtured well and we have been using the same | paste for the last two years. We had a different laser-cut stencil in | production from a different manufacturer with the same 9-mil opening, | but this stencil was not electropolised or nickel-plated. With this | stencil, the paste release was excellent and we never had to worry | about wiping. | Does anyone have any input into whether or not laser-cut stencils | should be electropolished and nickel plated? I have heard | contradicting opinions on this subject. One source has said that you | do not want to electropolish laser-cut stencils because the | rough laser walls will give you a better paste release. Whereas, | another source has said that the smooth laser walls will give you a | better paste release. | Also, what benefit does nickel plating give you? Does the paste tend | to roll better with nickel plating? Do you tend to get more or less | paste deposit on the boards with nickel plating? Does it increase the | life of the stencil? | Has anyone using laser stencils seen major differences between | the quality of different stencil manufacturers? | Any input into this subject would be greatly appreciated. | Thanks, | Todd

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| Hi all, | We are currently having problems with a laser-cut stencil that is | electropolished and nickel plated with 9-mil openings. The paste does | not release well from the apertures. We are having to wipe after every | print. The board is fixtured well and we have been using the same | paste for the last two years. We had a different laser-cut stencil in | production from a different manufacturer with the same 9-mil opening, | but this stencil was not electropolised or nickel-plated. With this | stencil, the paste release was excellent and we never had to worry | about wiping. | Does anyone have any input into whether or not laser-cut stencils | should be electropolished and nickel plated? I have heard | contradicting opinions on this subject. One source has said that you | do not want to electropolish laser-cut stencils because the | rough laser walls will give you a better paste release. Whereas, | another source has said that the smooth laser walls will give you a | better paste release. | Also, what benefit does nickel plating give you? Does the paste tend | to roll better with nickel plating? Do you tend to get more or less | paste deposit on the boards with nickel plating? Does it increase the | life of the stencil? | Has anyone using laser stencils seen major differences between | the quality of different stencil manufacturers? | Any input into this subject would be greatly appreciated. | Thanks, | Todd Todd, Straight up answer time. Watch your apertures. In all honesty, you're on the hairy edge with your aspect ratio. I've done a ton of work in the screen printing arena and have a good understanding of what's happening to you. Nine is tough with a 6 mil foil. Bill was right about the clogging. It doesn't matter what stencil you use with nine mil apertures. You're going to have a process that's less than optimal. It doesn't matter if you were semi-successful before, you weren't optimal. I would guess that you chose nine mil apertures because of bridging issues. The cure didn't lie in the aperture geometry alone. It was in the pad too. I'm going to give you the formula for .020" pitch success. .010" apertures on .012" to .013" pads. Bingo. Works every time. I have used more stencils from the big guys and bucket shops than most process guys will ever see in their lives. EFAB is the best. Hands down. You're gonna pay through the nose for it though. There is a certain amount of surface roughness required for good paste release. There was a great paper in the NEPCON West '97 proceedings written by Richard Cloutier of AMTX. It talks about roughness in relation to release. E-polished will release better than not electropolished but EFAB is superior to that. It has to due with the inherent surface finish caused in process. An EFAB stencil is "grown". This process is good for release and accuracy. (.012" pitch, no brainer) If you're doing .016" pitch, you should laser cut. It's more stable than chem etch. A prerequisite for .0075" wide apertures. Electropolish it too. .020" is chem etch, hands down. If you've got tons of money to blow, buy a laser cut stencil but you don't really need it for this pitch. I use the aforementioned formula on a variety of assemblies with great success. (I'm in a contract prototype shop that offers everything from circuit layout, DFx, through assembly, test, and box build) I don't know if you are in an OEM and have the freedom to "grow" pads on your assemblies but look into it. At least convince the boss to run a test vehicle to prove it. You'll save a ton of $$$$ in the long run. Give a jingle 978 932 3218 direct Regards, Justin Medernach Flextronics Int'l

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Electropolishing definitely improves the print characteristics of both Laser-cut and Chem-etch stencils. The caveat here is avoid over polishing which results in rounding of the edges and/or enlargement of the openings. Over polishing is typically the result of over compensation to improve the looks or enlarge the opening of a Laser-cut or Chem-etched stencil but, it can also be the result of just plain negligence on the part of the operator. Your Laser-cut 9 mil opening should not be a problem on a 6mil or less thick stencil. The pad to be printed must be larger than the stencil aperture as measured on the bottom the stencil. In this case,12 mils is ideal but, no smaller than 10. Measure both sides of the stencil! Laser-cut stencil apertures taper from top to bottom and the tapers vary with manufactures. If you want 9 mils on the bottom, check for it. Also, use a 60x plus microscope to view the radius of the opening. A radius of 3 tenth is too much. I've seen the print results of Laser-cut 4.5 mil electropolished stencils and can say that: "you will be there soon." I recommend a visit to your stencil manufacture to review their procedures. Unfortunately, Chem-etching is an art and a science. There are only a few companies that employ the talent to maintain "day in and day out" quality. I recommend that you use either Laser-cut or Electro-form to create your 9 mil and below opening. You can achieve the benefit of the tapered walls associated with Laser-cut stencils with chemical etching by enlarging the bottom tool used to create the stencil. Again, measure the opening and check for consistent tapers throughout the stencil. Inaccurate or poorly maintained photoplotters will produce parallelogram shaped aperture where you least expect them. Even Lasers can produce the infamous parallelogram. Check for them on every stencil.

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| To electropolish or not to nickel plate, that is the question. Whether it be nobler in the minds of the competition could be the answer. | I think it will probably depend upon whom you ask the question. If the stencil mfgr. has nichel plating capibility, then you will probably find that nickel plating is the answer. The same for electropolishing. | Something you should keep in mind is that nickel plating is an "additive" process and electropolishing is a "removal" process. So, if you are starting with a cut aperture of 9-mil, after nickel plating you will have something less than 9-mil and visa versa for electropolishing and visa-versa for nickel plating. | When laser-cutting became popular, the chem-etch people said the inner side walls of a laser-etched stencil were not as smooth as chem-etch. So, the laser people had to come up with a way to compensate - electropolish. When the E-Fab nickel stencil was introduced the compensation was to nickel plate a laser-cut stencil in an attempt to stay competitive with the E-Fab. You may have confirmed this logic with your previous experience with the non-plated, non-polished stencil. | Now, lets get to what I think the problem is - aperture contamination. How are you cleaning the stencil? A 20-mil pitch stencil is difficult enough to print through. With 9-mil, you have to take extra care in keeping the apertures clear (rough inner walls or not). Just wiping the stencil will just push paste back into the apertures. I recommend scoping the apertures under high power magnification. I'm willing to bet that you will find solder ball contamination. If solder balls are allowed to stand in the apertures overnight, they will dry like cement and be even more difficult to remove with successive cleanings. Dry paste attracts wet paste and obstructs the apertures resulting in poor paste transfer and poor paste release. | I also recommend and article on our Web Site that was written by a stencil mfgr. "SMT Stencil Cleaning: A Decision That Could Impact Production." (www.smartsonic.com/article.html) You can also find it in the July,1996 issue of EP&P or the August, 1996 issue of Asian Electronics Engineer. | Now, comes the sales pitch. Smart Sonic is the only stencil cleaning process that guarantees to clean any type of solder paste (wet or dry) from any fine-pitch stencil...or your money back! Enough said. | If you have any questions, please call me. | Tel.: 1(800) 906-440-R | Regards, | Bill Schreiber | | Hi all, | | We are currently having problems with a laser-cut stencil that is | | electropolished and nickel plated with 9-mil openings. The paste does | | not release well from the apertures. We are having to wipe after every | | print. The board is fixtured well and we have been using the same | | paste for the last two years. We had a different laser-cut stencil in | | production from a different manufacturer with the same 9-mil opening, | | but this stencil was not electropolised or nickel-plated. With this | | stencil, the paste release was excellent and we never had to worry | | about wiping. | | Does anyone have any input into whether or not laser-cut stencils | | should be electropolished and nickel plated? I have heard | | contradicting opinions on this subject. One source has said that you | | do not want to electropolish laser-cut stencils because the | | rough laser walls will give you a better paste release. Whereas, | | another source has said that the smooth laser walls will give you a | | better paste release. | | Also, what benefit does nickel plating give you? Does the paste tend | | to roll better with nickel plating? Do you tend to get more or less | | paste deposit on the boards with nickel plating? Does it increase the | | life of the stencil? | | Has anyone using laser stencils seen major differences between | | the quality of different stencil manufacturers? | | Any input into this subject would be greatly appreciated. | | Thanks, | | Todd

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