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SMT electronics assembly manufacturing forum.


Loctite Adhesive Viscosity

John

#6580

Loctite Adhesive Viscosity | 14 May, 2001

We have been running a product for the past several months using Loctite 3615 in our adhesive dispenser (Camalot 3800). The program the machine has been running has gone unchanged for quite some time now and everything has run well. Today, it appeared that we were putting on much more glue than we had last week. It caused alot of stinging, and glue contaminated the solder pads on several boards. The operator put a new tube of glue on the machine today. Has anyone had problems with the viscosity of Loctite's adhesive being inconsistent? What method do you use to monitor it?

Thanks John

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#6585

Loctite Adhesive Viscosity | 14 May, 2001

Which direction is the glue stringing, relative to the dispense head movement?

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John

#6591

Loctite Adhesive Viscosity | 15 May, 2001

The stringing followed the head movement, but I also saw dots that were large enough that adhesive squeezed out when the component was placed. It is very unusual as we've been running for several months without any changes. The temperature in that area remained between 76 and 78 degrees yesterday which is very normal for our plant.

Thanks John

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#6592

Loctite Adhesive Viscosity | 15 May, 2001

The viscosity is only consistent over a small temperature range. If your room temperature has changed by a few degrees, it can make all the difference.

You may have to increase the settling time.

We did not check the viscosity, but measured the consistency of glue dot sizes, by spreading the dots out between two glass plates.

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#6593

Loctite Adhesive Viscosity | 15 May, 2001

John, we were responding almost to the same time and I did not see what you were writing. If the glue dots are not consistent within one syringe, than I would blame it on the pump spindle. Do you have an extra one to put on, or clean the spindle with Acetone and try again.

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#6594

Loctite Adhesive Viscosity | 15 May, 2001

Stephan makes good points. While monitoring the temperature in the plant is good, it might be more appropriate to measure the temperature in the cabinet of your dispensor. A sure indicator of temperature change is process changes over time. Consider placing witness marks on each board. These marks can be used as part of your first piece and for checking the process through time.

Are you positive the splats are the result of component placement? Other possible solutions to this problem are: * Reduce the suspension travel of the clearance holder * Reduce the acceleration of the Z axis

Other solutions to the "stringing in the direction of travel" problem are: * Decrease temperature and / or pressure * Purge system 1-3 minutes * Clean clearance holder / nozzle

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John

#6595

Loctite Adhesive Viscosity | 15 May, 2001

I should have been more clear;the temperature that I indicated was measured inside the dispenser cabinet. We setup a very simple system to monitor and maintain the temperature inside the cabinet. By comparing our temperature to our production results we found our best performance to be in that temperature range. I fixed the problem by reducing the shot size (in milliseconds on the Camalot) for all shots. This produced dots that were of the same size as we normally run and the problems stopped. I'm just trying to understand what caused the change. The tube of glue was from the same lot that we've been running for the past three weeks. We'll take a look a the pump. Unfortunately, I don't have one in inventory to replace it with. Do you know of anyone doing a viscosity check on a regular basis? Is there a simple volume measurement? Are there other controls we should be using that we haven't mentioned? By controlling the ambient temperature, we had achieved very good results, and this issue caught us off guard.

Thanks again, John

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#6596

Loctite Adhesive Viscosity | 15 May, 2001

you can buy a visco meter for $ 143.44 from McMaster-Carr http://www.mcmaster.com but to my expirience the viscosity was very consistent from batch to batch. The glue application itself has so many variables, temperature, pressure, needle diameter, spacer, shot time and settling time, which makes it difficult to control the entire process. Sometimes the cause of stringing is not so obvious. For example, I moved the spacer into a different position. In this postion the spacer touched some traces, which are slightly higher than the board. I found more strings on this board than before. This leads me to another problem with glue application. You never know if you are just at the edge of good glue dots without stringing, meaning that only little changes mess up your boards.

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Justin

#6599

Loctite Adhesive Viscosity | 15 May, 2001

Another possibility could be the moisture monster. SMT adhesives are extremely hydroscopic. Was it one syringe in particular or your entire lot going haywire? Are you in a humid area? EG if you're manufacturing in AZ you may not need to worry about this as much as I might over here in Florida... I've never run the 3615 so i'm not sure about it's resistivity to accumulating moisture...

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Scott B

#6600

Loctite Adhesive Viscosity | 15 May, 2001

We had a similar problem a while back and found it to be an operator messing with the air pressure valve (of course no-one admitted to doing it). When cranked up enough, there was enough back pressure in the syringe to increase the volume of material dispensed causing stringing where there was none before.

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John

#6602

Loctite Adhesive Viscosity | 15 May, 2001

I don't think the syringe pressure has been adjusted, but I'll watch over the next few days to see if it changes (by itself of course). We don't control the humidity in our plant and don't really track it. However, there have been some significant humidity changes over the past few days. Would a more humid environment cause the adhesive to be more or less viscous?

Thanks again, John

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Justin

#6603

Loctite Adhesive Viscosity | 15 May, 2001

Again, I'm not familiar with your material but SMT Adhesives as a collective group are typically hydroscopic. An increase in humidity in your facility will lower the viscosity of certain materials. I don't want to say who's material I've had bad experiences with but I will give you an easy test to determine if your material is good or bad. Dispense some material onto a glass slide. Apply another slide on top to make a yummy glue sandwich smashing the adhesive into a lovely polymeric pancake. Cure this glass slide in your oven and then see if you have a honeycomb like structure in between the glass slides. Use a microscope at about 10x. If you see the honeycombing, it's caused by moisture pulled into the adhesive and the subsequent "evaporation / outgassing" will leave this structure.

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