TU933 Dielectric : Dk values after lamination - Page 1
Forum section: electronics / rf-microwave

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: March 01, 2023, 10:14:09 am
content: Friends,I am designing an RF board (<6GHz RF) which has several high speed serdes lanes (<=32Gbps, NRZ (2 level)). Being cost sensitive, we have planned to use the TU933 dielectric on the board, which is currently at 12 layers.The base material has a Dk of 3.1 with DF = 0.002 at 10GHz. This is the data from the material manufacturer. But the PCB manufacturer gives us a value of Dk = 2.6 for the same. His feedback was that the material changes properties after lamination.Not convinced, we tried to get info from another PCB manufacturer. But they responded that the Dk of the material does not change after lamination.Has anyone had any experience with the TU933 material? Is it reasonable to assume that the Dk values change so much after lamination? (From 3.1 to 2.6)Note : We have sent an email to TUC (the dielectric material manufacturer based in Taiwan) but have not heard back from them.Material detailshttps://www.tuc.com.tw/en-us/products-detail/id/3Regards

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 01, 2023, 07:06:40 pm
content: I haven't worked with this material and I can't open their datasheets without registering, but based on what you wrote it is possible to have such different values.First you have to differentiate between cores and prepreg layers in your stackup. Cores are already cured before lamination and don't change their internal composition, so might be better in this regard. Unfortunately unless your outermost layers are on cores there is no way to route signals without being covered by a prepreg at least from one side.The prepreg datasheet should have a table describing the different glass weave types, starting and final thicknesses and corresponding resin ratio. The resin and the glass fibers in the material have wildly different Dk. The volume ratio of the two determines the effective dielectric constant. Depending on the copper to etch ratio on the two conducting layers enclosing the prepreg a different amount of resin gets squished out into the gaps between traces. The glass stays in the middle but the total thickness of the resulting dielectric layer changes due to the resin being forced into the etch gaps during lamination. Since the glass remains but the resin amount changes between two conductors the ratio of the resin to glass changes thus will the effective Dk of the material.Another factor to consider is that the material manufacturer's numbers are coming from some sort of standardized test. E.g. IPC TM650 etc. Some of these can have significant differences compared to what your lines (or the fab house's tests coupons) would "see". Even if you calculate the final resin ratio, the distribution of the glass and the resin is not homogeneous. Neither along nor across. Depending on your line geometry your fields would see more or less resin and glass thus you'd observe a different Dk value.I'm not aware of any one size fits all mitigation technique, other than using an expensive material, which is selected for these reasons. Cost saving can be beneficial on high volume runs, but you might be forced to do multiple iterations during development to get these sort of issues flushed out by trial and error, since there is a practical limit how accurately you can predict the result.Hope this helps.