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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Reflow glue and past.

Charlie

#7566

Reflow glue and past. | 8 February, 2000

I'm looking for information from both those who do cure glue and reflow past with the same profile and those who have tried and decided against it. What were the problem? What are the limitations?

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Travis Slaughter

#7567

Re: Reflow glue and past. | 8 February, 2000

I have never tried this and wouldn�t really even want to consider it. Epoxy generally cures at around 160c in a fairly short amount of time, solder paste requires much more time and a peek temperature of about 210c or more in some cases. Curing your epoxy at this high temperature would subject your assembly to an unnecessary high heat cycle, surely putting added stress on the components and shortening the product life.

Travis S

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dean

#7568

Re: Reflow glue and past. | 8 February, 2000

Generally speaking with most electronic grade epoxies:

1. Cure time is a function of temperature. 2. Polymer bonds begin to quickly break down above certain temperatures (product specific). I believe aprox 170-180C. The strength of the bond deteriorates rapidly above the plastic point. (Wave solder guys know this well).

Exposing epoxy to reflow temperatures may weaken the bonds holding the part to the board. Then the wave solder people take the blame for missing SMT parts! I reccomend working very closely with your epoxy vendor and find out what their chemists reccomend for time vs. tempeature curing. If not careful this could blow up in your face...

But then again...why would you want to "stress" your assembly, thermally when you can generate a proper low temp cure profile in seconds. How does your customer feel about this?

Oh yeah, watch out for the hydroscopic nature of epoxy...your preheat may do you in if not careful.

Dean

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#7569

Re: Reflow glue and past. | 9 February, 2000

Charlie: People cure glue using their paste reflow profile all the time. And most of them are happy.

Personally, I�m with Dean and Travis. It�s possible to as you suggest, it�s appealing not to mess with your oven profile, but it just doesn�t seem shrewd. A coupla things in supporting their comments: � Loctite 3609 cures in 60 seconds at 150�C (90 seconds at 125�C) � not paste reflow-like. � Loctite 3616 can be ramped up to 6�C, although most adhesives have a 2�C maximum ramp � not paste reflow-like. � In "Microcanyons", Circuits Assembly magazine, December 1998, D Pauls & T Munson describe adhesive on boards that was cured too rapidly and formed a skin that trapped volatiles and solvents. The volatiles and solvents created long voids in the adhesive, while out-gassing. � Pauls & Munson (ibid.) talk about adhesives being stronger when cured at temperatures lower than the manufacturer�s recommended maximum.

My2�

Dave F

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#7570

Re: Reflow glue and past. | 9 February, 2000

All: I don�t think this isn�t what Charlie was asking about, but since he�s got our juices flowing on the topic, does anyone have a sense about running paste and adhesive at the same time, beyond the issues that we spoke about in responding to Charlie? I couldn�t find a source (maybe I imagined it in one those fitful periods of sleep between Kafka dreams and "grinding" about a Delbert boss, maniac production workers, obsessive customers, or whatever). Anyhow, in this episode someone talked about the reaction between the volatiles and solvents of a paste and an adhesive when reflowed/cured together.

Thanks

Dave F

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Travis Slaughter

#7571

Re: Reflow glue and past. | 10 February, 2000

I have had cases when it was necessary to run epoxy and paste at the same time. The only good reason I know of to even think about messing with going to this trouble is if the surface tension of the solder melting pulls the part out of alignment. The epoxy cures, then the solder melts, this works well for holding the part in place long enough to be soldered. I have never seen any issues come up as far as volatiles contaminating or having any kind of affect at all on the solder quality.

Travis S

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