Another common misconception (at least with adhesive materials) is the concept of Tg (glass transition temperature). Most of the adhesives used in the electronics industry are thermosetting materials. As such they do not “melt” but have two physical properties that are related. The Tg which is the temperature at which the material softens and the decomposition temperature – the temperature at which the material breaks down. Materials can “function” above their Tg, but their physical properties will be altered. For example, silicone materials (such as bathroom caulk, or conformal coatings) often have Tgs that are below room temperature. Yet they still adhere, protect from the atmosphere (moisture), etc. Since most adhesives used in electronics (FR-4, underfill, SMA) are epoxies, it is useful to understand how these materials are affected above the Tg. The most significant effect on epoxies is that the CTE increases significantly (typically 2-3 times). That doesn’t necessarily mean that a device encapsulated/underfilled/coated with one of these materials will fail. There are other physical properties that come into play as well (especially when it comes to reliability) such as modulus, and adhesion. The best way to confirm that a material is suitable for the application is to test it using known established test methods.
what is glass transition temp in polyimides fr4 fr5
There is not any “single” FR4 or FR5 material. Depending on the formulation of the specific epoxy system used. These typically range anywhere from 135C to 180C (a few as high as 190C).
Polyimide materials used in higher frequency applications tend to be higher, on the order of 150C to 200C.
Another good web site on this topic is: http://www.arlon-med.com/Measuring%20and%20Understanding%20Tg.pdf
you talk about the CTE with epoxies and how it changes drastically, but is that above or below Tg?
The CTE is different for thermoplastic material (like
epoxies) above and below Tg.Â
These two values are commonly referred to as alpha 1 (a1) and alpha 2 (a2).Â Alpha 1 is the CTE below Tg and alpha 2 is the CTE above
Tg.Â If you only see one CTE
listed for a material on a TDS it is most commonly the alpha 1 Tg.Â (Unless the Tg is below room
temp, as is the case for silicones).Â
The CTE value can increase by 2 or 3 times. In conjunction with this is
also the fact that the modulus decreases – the material becomes softer.Â So even though it expands more, it is softer
and can deform more easily.
We’ve had fallout using an epoxy material with higher Tg (48 compared to the 30 of the material being proposed) – cracked capacitor. They’re both epoxies, and I’m wondering two things: 1. What other parameters should I compare to predict if this new eopxy will work, and 2. what testing will verify the new material. I’m aware of thermal shock/thermal cycling, but is there any other testing you would recommend? Thanks.
Why is glass transition considered as second order transition?
Are you seeing a cracked cap that is mounted on an FR4/epoxy board? If that is the case, then you need to look at the Z-axis CTE. The board could be expanding and pushing into the cap and cracking it. The best way to test this is to do thermal cycling and/or shock to determine if the expansion will crack the device in question.