2025 Multilayer Stackup Change
In 2025, Isola has depreciated their FR408-HR material, long used in our 4 layer stackup. As a replacement, we’ll be transitioning to EM-528 as the replacement. This material should be equivilent in most use cases, meaning few or no design changes.
However, we know how demanding our customers are, and we wanted to provide as clear of a path forward as possible. This page will be updated to the latest information available to us to help you migrate designs and plan any design work.
- last updated 2025-08-28 21:00PST, Firmer support dates provided
- last updated 2025-08-27 21:00PST, clarifying substrate availability timelines
Comparison of substrates
This is a work in progress; We’re still working with our manufacturer to get information that is critical to certain fields. Our service page and this document will be be updated as information becomes available to us.
| Value | FR408HR | EM528 | - |
|---|---|---|---|
| Datasheet | FR408-HR | EM528 | |
| Volume Resistivity | 4.4*10^7 | >10^10 | Mohm-CM |
| Surface Resistivity | 2.6*10^6 | > 10^9 | Mohm |
| Td(TGA) | 360 | 420 | °C |
| Tg (DMA) | 230 | 250 | °C |
| Tg (TMA) | 220 | °C | |
| Tg (DSC) | 190 | °C | |
| Thermal Conductivity | 0.4 | 0.6 | W/mK |
| Water Absorption | 0.061 | 0.11 | % |
| Flame Resistance | UL-94 V-0 | UL-94 V-0 | |
| Substrate color | white | red |
We’re currently requesting information for the following specs
- Outgassing information
- Dielectric Breakdown
- Arc Resistance
- CTI
- RTI
- Impedence tables for 50 ohm single ended + 90/100 ohm differential.
If there’s additional material information not listed here or on the EM528 datasheet, but critical to your application, please contact us at [email protected] .
Expected 4 Layer Stackup
May be subject to change
| Layer | Material | Dk | Df |
|---|---|---|---|
| 1oz copper | |||
| 7.67 mil prepreg | EM-528 2x3313-59% | 3.76 | 0.005 |
| 0.5oz copper | |||
| 39.00mil core | EM-528 5x7628 | 4.11 | 0.005 |
| 0.5oz copper | |||
| 7.67 mil prepreg | EM-528 2x3313-59% | 3.76 | 0.005 |
| 1oz copper |
Expected 6 Layer Stackup
May be subject to change
| Layer | Material | Dk | Df |
| —– | ——– | — | —v |
| 1oz copper | |||
| 4.56 mil prepreg | EM-528 - 2x106-77% | 3.35 | 0.004 |
| 0.5oz copper | |||
| 4.00 mil core | EM-528 - 1x3313-57.5% | 3.79 | 0.005 |
| 0.5oz copper | |||
| 21mil core | EM-528 - 3x7628 | 4.11 | 0.005 |
| 0.5oz copper | |||
| 4.00 mil core | EM-528 - 1x3313-57.5% | 3.79 | 0.005 |
| 0.5oz copper | |||
| 4.56 mil prepreg | EM-528 - 2x106-77% | 3.35 | 0.004 |
| 1oz copper |
Transitional Dates
Orders shipped before 2025/8/25
These were fabbed according to our longstanding FR408-HR stackup. All specifications for these will be archived at our 4 Layer FR408-HR Stackup Documentation and 6 Layer FR408-HR Stackup Documentation
A few Orders shipped on 2025/8/25
On this date, a small number of panels had an EM-528 core, with the prepreg layers remaining FR408-HR. This was a relatively sudden change due to stock availability issues ahead of the planned rollout, and affected customers were notified via email.
The impacted boards can be visually distinguished by the color of the core: FR408-HR is white, and EM528 is red. These boards will have a transitional stackup consisting of the legacy prepreg and new core configuration:
| Layer | Material | Dk | Df |
|---|---|---|---|
| 1oz copper | |||
| 7.67 mil prepreg | FR408HR 2113 | 3.61@1GHz | 0.0091 |
| 0.5oz copper | |||
| 39.00mil core | EM-528 5x7628 | 4.11 | 0.005 |
| 0.5oz copper | |||
| 7.67 mil prepreg | FR408HR 2113 | 3.61@1GHz | 0.0091 |
| 1oz copper |
Dual substrate Availability (projected ASAP)
At this time, we will maintain FR408HR as our preferred stackup.
We will be adding multilayer substrate selection to our site’s checkout process. This will allow you to mark boards with a preference for legacy FR408HR or the newer EM528 stackups.
During this phase
- Boards requring EM528 will be unable to select Swift, and may have extended turn times
- Boards requiring FR408 will be unable to select Swift, but will have standard turn times.
- Swift boards will be fabbed using FR408HR
- Medium Run boards will be able to select either substrate if required.
- Boards with no preference will typically be FR408HR
Soft Transition to EM528 (projected Oct 1)
At this time, we will swap our primary multilayer substrate to EM528, leaving options to select FR408HR.
During this phase
- Boards requiring FR408 will be ineligible for Swift, and may have extended turn times
- Boards requiring EM528 may select Swift, and will otherwise have standard turn times.
- Swift boards will be fabbed using EM528
- Medium Run boards will be able to select either substrate if required.
- Boards with no preference will typically be EM528.
FR408 phase out (projected Nov 1)
At this phase,
- All boards will be EM528, excepting ones selecting the FR408HR option.
- Medium Run boards will be able to select either substrate if required.
We are aiming to offer 4 layer FR408HR options up to Jan 1, with extended turn times; We may be able to maintain this longer depending on material availability and order volume.
We will not likely be able to offer 6 layer FR408HR beyond this date.
