Differential Skew Mitigation
Uniformized glass removes large resin windows, keeping skew ≤5 mil across 10 in routes and easing COM budgets.
- Less phase conversion to common mode
- Allows longer straight routing
- Reduces need for serpentine compensation
Materials
Spread-Glass FR-4 Stackups
Spread-glass fabrics (1035, 1067, 2116, 3313, UTS) paired with VLP copper cut weave-induced skew and insertion loss enough to keep 10–28 Gbps channels in spec. We package the routing tactics, documentation prompts, and correlation workflow we use on RayPCB backplanes so SI teams can rely on the data.
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Why Use Spread-Glass FR-4
Predictable Dielectric Fields
Spread weaves hold Dk constant so solver results track TDR coupons within ±5% without over-guardbanding.
- Better alignment with field solvers
- Layer-to-layer uniformity
- Improved impedance stability
Lower Effective Loss
Spread fabrics plus VLP copper reduce high-frequency loss 3–5%, buying extra reach on 10–28 G lanes.
- Smoother current distribution
- Less local impedance bumping
- Better eye height in equalized links
Spread-Glass Fabrics We Specify
| Style | Construction | Use Case |
|---|---|---|
| 1035 / 1067 | Spread 1 oz warp/weft | Outer signal layers and stripline pairs |
| 2116 / 3313 | Dual-ply spread | Core layers balancing stiffness & skew |
| Spread 7628 | Macro spread heavy weave | Backplane cores needing rigidity |
| Ultra-Thin Spread (UTS) | Custom microspread | Premium skew-critical lanes |
| Spread Hybrid Stack | Mix of spread and standard | Cost-optimized builds |
Representative Dielectric Data
| Material Set | Dielectric Constant | Dissipation Factor | Notes |
|---|---|---|---|
| FR408HR + 1035 spread | 3.66 | 0.009 | Skew ≤5 mil with VLP copper |
| Megtron 6 + 1067 spread | 3.40 | 0.0020 | Used on 56 G lanes |
| Low-loss FR-4 + 2116 spread | 3.55 | 0.004 | Mid-loss builds |
| EM-826 + 3313 spread | 3.48 | 0.003 | Long reach 28 Gbps |
| Hybrid (spread outer / std inner) | Layer-dependent | - | Cost-optimized approach |
Values depend on resin and copper selection; reference laminate datasheets and lot certs for final solver inputs.
Spread-Glass Stackup References
12-Layer FR408HR 25 G Card
Spread 1035 outers and 2116 cores supporting PCIe Gen4/5 lanes.
- Layer-by-layer glass/foil callouts
- Backdrill residual <10 mil
- Coupon IDs etched per panel
18-Layer Backplane Mix
Spread 3313 core pairs for long differential runs, standard glass for planes.
- Routing angle map for critical nets
- Copper balance instructions
- COM/eye correlation pack
Hybrid Spread + PTFE
Spread-glass FR-4 digital sections tied to PTFE RF layers.
- Bondply and press windows logged
- Transition keep-out diagrams
- VNA/TDR overlay included
Controls for Spread-Glass Builds
Glass Documentation
Traveler lists style, supplier, and acceptable alternates per dielectric.
- Glass style etched in stack table
- Alternate approval workflow
- Traveler photo of lot labels
Routing Angles
We publish recommended routing angles or zig-zag patterns when spread is unavailable on specific layers.
- Angle guidance per layer
- Meander allowances in CAD rules
- Layout review checklist
Copper Roughness
Pair spread fabrics with VLP/HVLP copper and log Ra/Rz + micro-etch depth.
- Profilometer measurements
- Micro-etch SPC ±0.2 µm
- Huray parameters shared
Correlation Testing
TDR/VNA coupons placed on the same fabric orientation as product traces.
- Coupon rotation matches board
- ±5% impedance acceptance
- Insertion loss overlays stored
Application Snapshots
PCIe Gen5/6 Add-In Cards
Spread-glass FR-4 reduces skew on 16–32 GT/s differential pairs.
- Line matching ±1 mil
- VLP copper usage logged
- TDR coupon overlay
High-Speed Backplanes
18–30 layer backplanes with spread cores for 25–56 G lanes.
- Routing angle plan included
- Backdrill plan documented
- COM templates delivered
Telecom/5G Cards
Hybrid FR-4 + RF designs where spread glass keeps digital skew in check.
- Hybrid layer map
- Bondply and press notes
- Field-service coupon support
Spread-Glass Selection Prompts
Inputs needed before we lock spread-glass choices.
Skew Budget
Define allowable skew per channel to decide if full spread or mixed weave is required.
- Lane length
- Skew tolerance
Material Mix
Choose resin system (FR408HR, Megtron, low-loss FR-4) and spread style per layer.
- Resin selection
- Glass style priority list
Validation
Plan coupon placement, TDR/VNA scope, and solver correlation artifacts.
- Coupon orientation
- Required reports
Spread-Glass FAQ
Can you guarantee a specific glass style on every layer?
Yes, when specified in the stack table. We lock glass styles per layer and document approved alternates to avoid substitutions.
Do spread fabrics increase cost or lead time?
Cost impact is modest (≈3–5% per layer). We stock common spread styles and can pull specialty rolls within 1–2 weeks.
How do you prove skew performance?
We route skew test coupons alongside product nets, measure skew with TDR/VNA, and share overlays with solver predictions.