String in glue dispensing proces

Hi, good friends I found this site is so wonderful to learn SMT techniques meaningfully. We now employ 3 Fuji GL-541 dispensors and raw material with Soma IR-120. but we found glue string casually. From my experience from trouble-shooting the cabinet temperature and nozzle dia. are significant. but, I believe that there are still some factors affect it. Pls advise. Best Regards, Stoney Tsai VeriFone Taiwan ltd.

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| Hi, good friends | I found this site is so wonderful to learn SMT techniques meaningfully. | We now employ 3 Fuji GL-541 dispensors and raw material with Soma IR-120. but we found glue string casually. From my experience from trouble-shooting the cabinet temperature and nozzle dia. are significant. but, I believe that there are still some factors affect it. | Pls advise. | Best Regards, | Stoney Tsai | VeriFone Taiwan ltd. Stoney, Time for Design of Experiments. You have a decent handle on the process. Add factors of dispense amount and speed to environment and nozzle diameter and you have the makings of a nice little L9 array for DoE. Four factors, three levels per factor. Average analysis will suffice for quick testing. Signal to noise analysis will tell you how centered the process is. I don't recommend this as you are looking for attribute data. (String or no-string) Give it a try. Regards, Justin Medernach Flextronics Int'l

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Hi Stoney, Justin is giving you some good advice, unfortunately these kinda' problems sometimes call for the ol' D.O.E...otherwise, you could be chasing your tail for a while until you figure out exactly what's causing your problems. Has the epoxy you're using ever give you any stringing problems before? If it hasn't and you just started seeing the stringing recently, there's one thing you can do a quick check on if you haven't already, and that's the little stand-off pin at the tip of the syringe needles. The glue I used to use was really good at NOT stringing (Ciba Giegy EpiBond), but occasionalIy, I would get those pesky strings too. Most of the time I found that after using the needles for a while, the stand-off pin would get pushed back up into the needle just a bit, but it would be enough so that as the needle decended down to the board to dispense the dot, it ended up too close to the surface of the board and wound up getting epoxy all over the outside of the needle, then pull a string along with it heading over to the next dot. I can't remember off the top of my head what distance the pin should be from the tip of the needle...but you can check it against the stand off of a known good needle and see if it's been pushed back up. Which reminds me of question, are you getting the stringing from all three of your dispense heads in the machines? That'll give you a clue of whether you really need to do a D.O.E. In my opinion, if you're not stringing from all three heads, then it's pretty obvious you need to focus on what it is about those certain head(s) that's giving you the strings. One other thing that can cause stringing is to have too high of number in for "G" (Glue Time) data. If you dispense too long, the same thing can happen as having the stand off pushed back up into the needle, you'll get glue all over the outside of the needle which pulls strings along with it everywhere it goes. Each digit that you use represents 0.01 seconds...that number isn't really a fixed number either, the number you use will depend on glue viscosity, air pressure to the machine, ambient temperature, epoxy temperature, etc. One other thing that can cause problems, is the glue check function...it's a good concept ( I guess), but I've seen more problems with it than it actually solves. It's supposed to automatically adjust the amount of glue dispensed based on a binary image of a few trial glue dots that it spits out on the board at the beginning of a program... so what happens if there's a bubble in the syringe when the test dots get dispensed? You see my point? I never used it... Hope this helps a little... -Steve Gregory-

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| Hi Stoney, | Justin is giving you some good advice, unfortunately these kinda' problems sometimes call for the ol' D.O.E...otherwise, you could be chasing your tail for a while until you figure out exactly what's causing your problems. | Has the epoxy you're using ever give you any stringing problems before? If it hasn't and you just started seeing the stringing recently, there's one thing you can do a quick check on if you haven't already, and that's the little stand-off pin at the tip of the syringe needles. | The glue I used to use was really good at NOT stringing (Ciba Giegy EpiBond), but occasionalIy, I would get those pesky strings too. Most of the time I found that after using the needles for a while, the stand-off pin would get pushed back up into the needle just a bit, but it would be enough so that as the needle decended down to the board to dispense the dot, it ended up too close to the surface of the board and wound up getting epoxy all over the outside of the needle, then pull a string along with it heading over to the next dot. | I can't remember off the top of my head what distance the pin should be from the tip of the needle...but you can check it against the stand off of a known good needle and see if it's been pushed back up. Which reminds me of question, are you getting the stringing from all three of your dispense heads in the machines? That'll give you a clue of whether you really need to do a D.O.E. | In my opinion, if you're not stringing from all three heads, then it's pretty obvious you need to focus on what it is about those certain head(s) that's giving you the strings. | One other thing that can cause stringing is to have too high of number in for "G" (Glue Time) data. If you dispense too long, the same thing can happen as having the stand off pushed back up into the needle, you'll get glue all over the outside of the needle which pulls strings along with it everywhere it goes. | Each digit that you use represents 0.01 seconds...that number isn't really a fixed number either, the number you use will depend on glue viscosity, air pressure to the machine, ambient temperature, epoxy temperature, etc. | One other thing that can cause problems, is the glue check function...it's a good concept ( I guess), but I've seen more problems with it than it actually solves. It's supposed to automatically adjust the amount of glue dispensed based on a binary image of a few trial glue dots that it spits out on the board at the beginning of a program... | so what happens if there's a bubble in the syringe when the test dots get dispensed? You see my point? I never used it... | Hope this helps a little... | -Steve Gregory- :Steve, Nice call. I forgot about the most important stuff. The mechanical. That's what was cool about having Fuji. 9 out of 10 times, when something wasn't running right. It wasn't material related, thus hidden. It was a slight mechanical problem that would be eliminated with some light trouble shooting. No need for the time consuming experiments. Thanks, Justin |

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| | Hi Stoney, | | Justin is giving you some good advice, unfortunately these kinda' problems sometimes call for the ol' D.O.E...otherwise, you could be chasing your tail for a while until you figure out exactly what's causing your problems. | | Has the epoxy you're using ever give you any stringing problems before? If it hasn't and you just started seeing the stringing recently, there's one thing you can do a quick check on if you haven't already, and that's the little stand-off pin at the tip of the syringe needles. | | The glue I used to use was really good at NOT stringing (Ciba Giegy EpiBond), but occasionalIy, I would get those pesky strings too. Most of the time I found that after using the needles for a while, the stand-off pin would get pushed back up into the needle just a bit, but it would be enough so that as the needle decended down to the board to dispense the dot, it ended up too close to the surface of the board and wound up getting epoxy all over the outside of the needle, then pull a string along with it heading over to the next dot. | | I can't remember off the top of my head what distance the pin should be from the tip of the needle...but you can check it against the stand off of a known good needle and see if it's been pushed back up. Which reminds me of question, are you getting the stringing from all three of your dispense heads in the machines? That'll give you a clue of whether you really need to do a D.O.E. | | In my opinion, if you're not stringing from all three heads, then it's pretty obvious you need to focus on what it is about those certain head(s) that's giving you the strings. | | One other thing that can cause stringing is to have too high of number in for "G" (Glue Time) data. If you dispense too long, the same thing can happen as having the stand off pushed back up into the needle, you'll get glue all over the outside of the needle which pulls strings along with it everywhere it goes. | | Each digit that you use represents 0.01 seconds...that number isn't really a fixed number either, the number you use will depend on glue viscosity, air pressure to the machine, ambient temperature, epoxy temperature, etc. | | One other thing that can cause problems, is the glue check function...it's a good concept ( I guess), but I've seen more problems with it than it actually solves. It's supposed to automatically adjust the amount of glue dispensed based on a binary image of a few trial glue dots that it spits out on the board at the beginning of a program... | | so what happens if there's a bubble in the syringe when the test dots get dispensed? You see my point? I never used it... | | Hope this helps a little... | | -Steve Gregory- | :Steve, | Nice call. I forgot about the most important stuff. The mechanical. That's what was cool about having Fuji. 9 out of 10 times, when something wasn't running right. It wasn't material related, thus hidden. It was a slight mechanical problem that would be eliminated with some light trouble shooting. No need for the time consuming experiments. | Thanks, | Justin | | ******************* Thx for all of your input, I would get a try on it, and would communicate with you guys for further investigation. Thx again!

Stoney Tsai Verifone Taiwan Ltd.

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| Hi, good friends | I found this site is so wonderful to learn SMT techniques meaningfully. | We now employ 3 Fuji GL-541 dispensors and raw material with Soma IR-120. but we found glue string casually. From my experience from trouble-shooting the cabinet temperature and nozzle dia. are significant. but, I believe that there are still some factors affect it. | Pls advise. | Best Regards, | Stoney Tsai | VeriFone Taiwan ltd. Stoney, The key to a good adhesive process is proper dot shape. It all comes down to the surface tension of the adhesive. There is a constant battle of surface tension between the the adhesive itself, the adhesive to the PCB and the adhesive to the nozzle. The nozzles made by fuji are specially constructed for typical SMT adhesive in which the dot diameter should be twice the inside diameter of the needle. This ratio can be stretched from 1:1.5 up to 1:2.5, but using an "air over" system (volume regulated by pressure and time), you would want to stick near the 1:2 ratio. When using this ratio, the nozzle should be designed such that your dot is the shape of a "Hershey Kiss". With the stable shape of a "hershey Kiss" in which almost 80% of the adhesive is on the bottom 50% of the dots height, the 20% of adhesive on the top 50% of the dot is pulled from inside of the nozzle due to the surface tension of the material in its shape being greater than that tension to the inside of the nozzle. If the standoff wears down, the balance is upset. Stringing occurs when surface tensions are not properly balanced. If the ratio is below 1:1.5, the dot will be tall and skinny. The material from the tip of the needle will stick to the needle, and cause stringing. Many times the 1:2 ratio is comprimised when the standoff contacts a higher or lower area than with which the adhesive is being placed (via, solder mask or most commonly when the needle doed not contact the PCB- or the PCB starts to bounce). Sometimes the surface tension can be upset when perhaps the dot is placed on a via, and the surface contact area between the adhesive and PCB is no longer great enough to offset the surface tension between the needle tip and the adhesive. Both Fuji and Heraeus among others have some good info. Best of luck. Steve A

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| Hi Stoney, | Justin is giving you some good advice, unfortunately these kinda' problems sometimes call for the ol' D.O.E...otherwise, you could be chasing your tail for a while until you figure out exactly what's causing your problems. | Has the epoxy you're using ever give you any stringing problems before? If it hasn't and you just started seeing the stringing recently, there's one thing you can do a quick check on if you haven't already, and that's the little stand-off pin at the tip of the syringe needles. | The glue I used to use was really good at NOT stringing (Ciba Giegy EpiBond), but occasionalIy, I would get those pesky strings too. Most of the time I found that after using the needles for a while, the stand-off pin would get pushed back up into the needle just a bit, but it would be enough so that as the needle decended down to the board to dispense the dot, it ended up too close to the surface of the board and wound up getting epoxy all over the outside of the needle, then pull a string along with it heading over to the next dot. | I can't remember off the top of my head what distance the pin should be from the tip of the needle...but you can check it against the stand off of a known good needle and see if it's been pushed back up. Which reminds me of question, are you getting the stringing from all three of your dispense heads in the machines? That'll give you a clue of whether you really need to do a D.O.E. | In my opinion, if you're not stringing from all three heads, then it's pretty obvious you need to focus on what it is about those certain head(s) that's giving you the strings. | One other thing that can cause stringing is to have too high of number in for "G" (Glue Time) data. If you dispense too long, the same thing can happen as having the stand off pushed back up into the needle, you'll get glue all over the outside of the needle which pulls strings along with it everywhere it goes. | Each digit that you use represents 0.01 seconds...that number isn't really a fixed number either, the number you use will depend on glue viscosity, air pressure to the machine, ambient temperature, epoxy temperature, etc. | One other thing that can cause problems, is the glue check function...it's a good concept ( I guess), but I've seen more problems with it than it actually solves. It's supposed to automatically adjust the amount of glue dispensed based on a binary image of a few trial glue dots that it spits out on the board at the beginning of a program... | so what happens if there's a bubble in the syringe when the test dots get dispensed? You see my point? I never used it... | Hope this helps a little... | -Steve Gregory- | I've sung the glue stringin' blues, too. Here's the needle size/standoff ratio. .4 mm nozzle, .1 mm standoff .5 mm nozzle, .2 mm standoff .6 mm nozzle, .3 mm standoff Great rules of thumb, I'm actually using a .3 standoff on a .5 nozzle, but that's just a peculiarity with my "Special (Siemens 80G!)" process. A few other string-makers: cured adhesive inside the nozzle. A little solvent and ultrasonic cleaner can fix that right up. Low departure angles. You know, when there's two dots really close to each other and instead of Z-ing up far enough to break the string, the machine begins its X-Y motion to the next dot. Then when the string breaks and gravity takes over, the Hershey's kiss look like it sat in the sun. Dented nozzles or needles. We have a well-thought out process where we insert all the axials, radial, and DIPs before attaching bottomside SMT. The machine is contantly tweaking nozzles off of leads, which slowly but surely dents the heck out of them and wreaks havoc. Heat. A 3-4 degree Celsius change in the glue temp will cause a 10% change in the viscosity of the adhesive. Internal machine temps can change quickly - our camera is mounted on a dispense head ganrty and the heat it gives off can make terrible strings on that one head. In the summer we actually open the machine up during changeovers and blow a fan in there. Good luck!

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