Advanced PCB Material Selection Handbook: Mitigating Failure, Controlling Cost, and Optimizing High-Speed Signals
I am Eason Huang a person who has been around press machines AOI screens and too many late RFQs I want to talk about PCB Material the way I talk with friends on the line low Df PCB materials for 25 Gbps high Tg PCB materials for harsh environment and that tug of war of cost comparison Rogers vs Isola PCB materials
low Df PCB materials for 25 Gbps: DK/DF and flex vs rigid
When I look at a 25 Gbps lane I first touch the stackup notes checking Dk drift and Df at 10 and 28 GHz one small drift swings Z0 by a couple ohms and the eye closes before lunch ends
Standard FR4 lives around Df 0.015 to 0.02 at 10 GHz while Isola I Tera MT40 hangs near 0.003 and Rogers 4350B around 0.0037 source Rogers 4350B datasheet Isola MT40 datasheet
Flex PI I have measured at Dk 3.2 to 3.4 Df 0.006 to 0.009 low loss PI can sit 0.004 to 0.006 and LCP keeps Dk near 2.9 Df near 0.002 to 0.003 source IPC 2223E and Rogers ULTRALAM 3850HT
Maintaining Signal Integrity Under Flex
Every time a flex tail bends I picture Dk swelling a bit and Z0 dipping I write a note ΔZ0 Z0 about minus half of Δεr over εr and I keep bend radius above 100 times thickness per IPC 2223E because below that the AOI starts finding trouble
A 0.2 mm flex bent to 5 mm radius gave me one to two percent Dk shift roughly a one ohm drop around 50 ohms that tiny change still shows up in a 25 Gbps eye
My go to tweak is to widen the trace five to eight percent in the bend or route the bend perpendicular to the line it softens the dip source ECTC 2023 Su et al microvia bend reliability
Insertion Loss and Crosstalk Mitigation on Flex
On standard PI at 28 GHz I have seen 0.9 to 1.2 dB per inch with half ounce copper low loss PI cut that to about 0.5 to 0.7 LCP around 0.25 to 0.35 numbers close to Rogers ULTRALAM 3850HT app notes
Copper roughness bites RA copper gives me ten to fifteen percent better loss than ED on these coupons that shows up in the Huray fit and the VNA sweeps
To calm crosstalk on flex I keep spacing at least four times width add a ground mesh or stitch vias at lambda over twenty IPC 2223E hinted this and my coupons stayed near minus thirty two dB near end XT at twelve gig
Impedance Control and Ground Plane Strategy
I trust ground more than any fancy trick continuous ground keeps Z0 steady on flex I drop via fences at lambda over twenty for sub ten gig and lambda over forty if I am pushing 28 gig lanes source IPC 2223E plus my AOI notes
At rigid flex joints I taper copper and stagger vias so return current never jumps a gap hatched ground only works when the mesh pitch is tiny compared to wavelength otherwise I go solid and cut reliefs so it does not crack
For a 25 Gbps stripline on flex I like Dk around 3.0 to 3.2 and I fight to keep dielectric thickness within ten microns checked with FCT coupons and a TDR
high Tg PCB materials for harsh environment
When a board sits in heat and vibration I reach for high Tg and Td north of 340C to keep resin recession and CAF from waking up
Isola MT40 gives Tg about 200C Td near 360C Z CTE around 2.8 to 3.1 percent from fifty to 260C Rogers 3003 runs Td above 425C with Z CTE near 2.1 percent that is why I use it on radar boxes source their datasheets
On flex in rough spots LCP or a low moisture PI wins with uptake under 0.04 percent Dk stays put and CAF slows down source Rogers ULTRALAM 3850HT and IPC CAF notes
cost comparison Rogers vs Isola PCB materials
Cost drivers I watch
When I choose Rogers
When I choose Isola
In my 2025 quotes two fabs priced Rogers 4350B cores at about 1.7 to 1.9 times MT40 per panel hybrid stacks saved around eight to twelve percent against all Rogers on six layer 2s2p builds source fab RFQs Q3 2025
Case studies with sources
25 Gbps backplane eye height rescue
We pulled FR4 and went MT40 on a twenty inch channel the eye climbed from 280 mV to 410 mV at ten to the minus six BER same EQ loss slid from 1.05 to 0.62 dB per inch at ten gig source internal TDR VNA log and MT40 Df 0.003
Panel cost rose about fourteen percent but one retimer disappeared and payback showed up in under half a year on forty thousand units per quarter
mmWave 28 GHz antenna on flex
We ran LCP ULTRALAM 3850HT with RA copper 12 μm and saw 0.28 dB per inch at 28 GHz versus 0.65 on a standard PI coupon four mil by four mil geometry source Rogers app note plus our VNA sweep
A six mm bend radius over twenty thousand cycles showed no cracks on X ray and S21 moved less than 0.1 dB source ECTC 2023 Su et al and our AOI logs
Harsh env control unit high Tg choice
An automotive control unit shifted from mid Tg FR4 to MT40 for Z CTE and Td margin voids in lead free drop shrank threefold CAF shorts went to zero in a thousand hour 85 85 run source OEM qual 2024 and IPC TM 650 CAF
Thermal cycling minus forty to 125C for a thousand cycles showed under three percent via resistance drift with MT40 and VT 47 type prepreg stack source fab sheet and OEM DVT log
Material property table: PCB Material picks
| Material | Dk @10 GHz | Df @10 GHz | Tg / Td | Z-CTE 50–260°C | Moisture Absorption | Typical Use | Source |
|---|---|---|---|---|---|---|---|
| FR4 (high-speed grade) | 3.7 | 0.012–0.015 | 170°C / 320°C | 3.5–4.0% | 0.15% | Sub-10 Gbps general | IPC-4101 slash sheets |
| Isola I-Tera MT40 | 3.45 | 0.003 | 200°C / 360°C | 2.8–3.1% | 0.054% | 25 Gbps backplanes | Isola MT40 datasheet |
| Rogers 4350B | 3.48 | 0.0037 | 280°C / 390°C | 2.4–2.6% | 0.06% | RF 6–30 GHz | Rogers 4350B datasheet |
| Rogers 3003 | 3.00 | 0.0013 | 280°C / 425°C | ~2.1% | 0.04% | mmWave phased array | Rogers 3003 datasheet |
| ULTRALAM 3850HT (LCP) | 2.9 | 0.002 | NA / >500°C Td | <2.0% | <0.04% | Flex RF 24–40 GHz | Rogers ULTRALAM 3850HT |
| Low-loss PI flex | 3.2 | 0.004–0.006 | ~260°C Td | ~2.8–3.0% | 0.1–0.2% | Flex 10–25 Gbps | IPC 2223E, vendor data |
FAQ
Why does Df matter so much at 25 Gbps
Dielectric loss grows with frequency a Df drop from 0.012 to 0.003 can save about half a dB per inch at ten gig on stripline enough to skip a retimer
How do I pick between Rogers and Isola for mixed-signal
If RF runs above twenty gig I pick Rogers for phase stability if lanes stop near 25 Gbps and cost is tense MT40 usually carries it hybrid stacks balance both
What bend radius should I use on flex carrying RF
Keep bend radius above one hundred times thickness for dynamic bends and widen traces five to eight percent through the bend pick Dk stable stuff like LCP to hold Z0
How to control crosstalk on flex multilayer
Hold spacing at least four times width add ground mesh or fences at lambda over twenty and keep reference planes unbroken this kept my NEXT near minus thirty two dB at twelve gig on recent builds
Does copper roughness really matter
Yes RA copper compared to ED cuts loss by double digits at 28 GHz I measure ten to fifteen percent better using Huray fit and VNA coupons
Sources
IPC-2223E Flexible Circuit Design Standard radius and ground guidance https://shop.ipc.org/ipc-2223
IPC-4101 slash sheets FR4 properties https://shop.ipc.org
Isola I-Tera MT40 datasheet Dk/Df/Tg/Td https://www.isola-group.com
Rogers 4350B and 3003 datasheets https://rogerscorp.com
Rogers ULTRALAM 3850HT LCP datasheet https://rogerscorp.com
ECTC 2023 Su et al microvia bend reliability doi 10.1109/ECTC58653.2023.10125322
Huray model and copper roughness Keysight app note 5991-3963EN
Fab RFQ panel cost logs Q3 2025 internal dataset summary
Need a stackup and ROI walk-through
If you are weighing PCB Material for 25 Gbps or mmWave and juggling Rogers versus Isola while worrying about high Tg give me a note
Send lane targets bend limits and budget I will sketch a stackup with Dk Df callouts and a simple payback note
