Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Adhesive on hot PCBs

Ken Daniels

#8264

Adhesive on hot PCBs | 8 December, 1999

I have a problem dispensing chip adhesive while boards are still hot from the topside process. Has anyone else seen this? Is it a matter of my material, or the machine (Fuji GL 5) or the mask. When I try to dispense I get a great deal of stringing, but after the boards cool things work perfectly. Thanks for any help.

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#8265

Re: Adhesive on hot PCBs | 8 December, 1999

Hi Ken Given the small mass of the dot I'm guessing that the adhesive is being flash cured by the heat of the board and the resulting semi-solid is unable to release from the nozzle like a liquid thereby causing your strings. Placing components on cured or semi-cured (skinned) adhesive is likely to increase your defect rate as well. Obviously the easiest answer is cooling. Could be as simple as a fan over the conveyor. Enclosed, chilled conveyors are available. Check with your adhesive manufacturer. A different formulation or cure type could work in you present application. Good luck. John Thorup

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Brian W.

#8266

Re: Adhesive on hot PCBs | 8 December, 1999

I think the number is about 80 degrees C. If the board is hotter than that, glue problems start to occur. John is right. The adhesive starts its cure process and its release from the nozzle is inhibited. Fans and coolers are one way yo go. If you are already using them, buffer conveyors are another alternative.

Brian W.

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shane.M

#8267

Re: Adhesive on hot PCBs | 8 December, 1999

We encounter a similar problem, whereby stringing becomes a real problem when board temperatures are raised. We tried slightly larger dots, but this did nothing more than block the nozzles of the pick & place machine. You basically have only two options, cool the PCB before dispensing, or change your type of adhesive. The way we overcame it was to install a series of fans on the inlet conveyor, we installed a sensor and reprogrammed the conveyor PLC to halt the board for a period, then conitunes on. It's essentially just buffering that board for 20-30 seconds. It works very well for us.

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Curtis T.

#8268

Re: Adhesive on hot PCBs | 13 December, 1999

Ken, we always build the bottom side of the board first. That also insures that there isn't any problems with the actives being reflowed and falling off in the glues curing process. By the way we do both paste and glue on the bottom of the boards to prevent low volume solder joints due to shadowing at the wave. If you have a chance get rid of the air over system and get a positive displacement pump, they are sooo much better. Curtis T.

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