How does baking PCB's and FR4 change their toughness and tensile strength?  - Page 1
Forum section: electronics / general

author: libralect
author_rank: Contributor
timestamp: May 25, 2023, 07:42:12 pm
content: 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?

author: Benta
author_rank: Super Contributor
timestamp: May 25, 2023, 08:32:02 pm
content: 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/cm2if 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.

author: tooki
author_rank: Super Contributor
timestamp: May 26, 2023, 07:00:43 am
content: Quote from: Benta on May 25, 2023, 08:32:02 pmThat 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 notrequired, it’s actually the maximum permissible, since it also says that at 200C, damage occurs.)

author: Kleinstein
author_rank: Super Contributor
timestamp: May 26, 2023, 08:36:36 am
content: 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 reaction2) relief of stress by making the epoxy part a bit more liquid so that it can flow3) removal of humidity4) relaxation of the amophous structure of the expoxy to a more dense structureThe 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.

author: Benta
author_rank: Super Contributor
timestamp: May 26, 2023, 10:18:17 am
content: Quote from: tooki on May 26, 2023, 07:00:43 amIt 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

author: tooki
author_rank: Super Contributor
timestamp: May 27, 2023, 02:27:11 pm
content: Quote from: Benta on May 26, 2023, 10:18:17 amOh, I missed the table of temperatures on the datasheet I saw. (Not an issue of “understanding”. To understand it you have to see it!) Sorry.