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How does baking PCB's and FR4 change their toughness and tensile strength?
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libralect:
FR4 is one of the most physically resistant materials for a given weight, and the process uses some kind of oven post-processing where at about 125 degrees for many hours.

Do you know how the chemistry of the resin changes and how the physical properties of the resin toughen from high temperatures?
Benta:
You need to look into 2-component epoxy adhesives for that.
The simple answer is, that longer chains/nets are formed at higher temperatures.
I've worked a lot with "UHU plus 300 Endfest", which will only reach its maximum strength of 3000N/cm2 if cured at 180 C (I use FR4 for model racing boats. Excellent for motor mounts, reinforcements etc.).

The "baking" process you describe will not add to strength, but might remove warp.
tooki:

--- Quote from: Benta on May 25, 2023, 08:32:02 pm ---… "UHU plus 300 Endfest", which will only reach its maximum strength of 3000N/cm2 if cured at 180 C (I use FR4 for model racing boats. Excellent for motor mounts, reinforcements etc.).

--- End quote ---
That isn’t actually what the datasheet says. ;) (It says temperatures from 70-180C can be used, with the shortest cure time at 150-180C. So 180C is not required, it’s actually the maximum permissible, since it also says that at 200C, damage occurs.)
Kleinstein:
For tensile strengh in the normal FR4 the relevant part is the glass fiber. Here baking does not make a difference. Baking can make a difference for epoxy part and this is in 4 ways:
1) completing the chemical reaction
2) relief of stress by making the epoxy part a bit more liquid so that it can flow
3) removal of humidity
4) relaxation of the amophous structure of the expoxy to a more dense structure

The last part is because the expoxy part is a kind of glass like structure. Above the glass transistion there is a kind of exquilibrium degree of additional free volume that makes the structure more open and less viscose. Below the glass temperature it takes a relatively long time to get the more quasi (still amourphous) equilibrium density. With baking somewhat below the glass temperature and slow cooling one can make the structure more dense and thus higher viscosity. For the part it is about finding the right temperature or really slow cooling to have an effect.

The baking of the epoxy part, especially the last part may increse it's tensile strenght, but chances are the toughness would go down. So hardness goes up, but it also gets a bit more brittle.
Chances are the FR4 boads one buys are already bakes to some degree so that they done bend or shrink much on use.

For epoxy glue some baking to about it's glass temperature can make sense to get better strength.
Benta:

--- Quote from: tooki on May 26, 2023, 07:00:43 am ---
--- Quote from: Benta on May 25, 2023, 08:32:02 pm ---… "UHU plus 300 Endfest", which will only reach its maximum strength of 3000N/cm2 if cured at 180 C (I use FR4 for model racing boats. Excellent for motor mounts, reinforcements etc.).

--- End quote ---
That isn’t actually what the datasheet says. ;) (It says temperatures from 70-180C can be used, with the shortest cure time at 150-180C. So 180C is not required, it’s actually the maximum permissible, since it also says that at 200C, damage occurs.)

--- End quote ---
It is exactly what the datasheet says (p.2, p.3). Read and understand (it's even a Swiss page):
https://www.swiss-composite.ch/pdf/t-UHU-Plus-endfest300.pdf
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