Due to the high density and high-speed signal requirements of eight-layer PCBs, the lamination quality directly affects signal integrity and product reliability. This article breaks down 16 key control points, spanning from design, material selection, and process to inspection, to help you precisely avoid pitfalls.
I. Design Phase: Laying a Solid Foundation
Consistent Inner-Layer Core Board Thickness
For PCBs with six or more layers, the warp and weft directions of each core board must be aligned to prevent bending and deformation after lamination.
Sufficient Spacing Reserved
Spacing for four-layer boards should be >10mm, for six-layer boards >15mm, with greater spacing for higher layer counts to avoid interlayer offset.
Precise Design of Positioning Holes
Four-layer boards require at least three positioning holes, while six-layer or more boards need five or more, with hole positions placed near the edges to minimize interlayer deviation.
Zero Defects in Inner-Layer Core Boards
No open circuits, short circuits, oxidation, or residual films; board surface cleanliness directly affects adhesion strength.
II. Material Selection: Matching Requirements
Selection of Prepreg (PP)
Four-layer boards use 7628/7630, while six-layer or more boards prioritize 1080/2116, with 7628 used for thickening dielectric layers.
Copper Foil Configuration
Select 1oz/2oz copper foil based on current requirements, ensuring compliance with IPC standards to avoid impedance deviations.
III. Inner-Layer Processing: Enhancing Adhesion
Black Oxide Treatment
A 0.25-0.50mg/cm² magnesium oxide film is formed on the copper foil surface to improve resin wettability.
Brown Oxide Treatment
An organic film covers the copper surface to prevent corrosion of the copper layer by resin curing agents at high temperatures.
IV. Lamination Process: Strict Parameter Control
Multi-Stage Temperature Curve Control
Heating rate: 2-4℃/min to avoid internal resin stress.
Curing temperature: 160-170℃ to ensure complete resin reaction.
Hot plate temperature: 180-200℃, adjusted based on heat transfer efficiency.
Multi-Stage Pressure Application
Initial low pressure: Expels interlayer air to prevent bubbles.
Mid-stage high pressure: 15-35kg/cm² to promote resin filling of gaps.
Avoid excessive pressure: Prevent resin extrusion leading to thickness deviations.
Vacuum Level ≥ -0.095MPa
Vacuum is applied during the heating phase and maintained during the holding phase to thoroughly remove volatiles.
Precise Time Matching
Pressurization time, heating time, and gel time must be coordinated to avoid weak bonding interfaces.
V. Stack-Up Structure: Symmetrical Design
Classic Stack-Up Options
Option 1: S1-S2-GND-S3-PWR-S4-S5-GND
Good symmetry and excellent EMC performance, but only one power layer, requiring caution for complex systems.
Option 2: S1-GND-S2-PWR-GND-S3-PWR-S4
Dual power and dual ground layers for excellent impedance control, but high cost and asymmetry.
Option 3: S1-GND-S2-PWR-S3-S4-GND-S5
Cost-effective and suitable for scenarios with moderate signal integrity requirements.
Reference Planes Adjacent to Signal Layers
High-speed signals should preferably run on inner-layer striplines (e.g., S2/S3/S4) to reduce crosstalk and radiation.
VI. Inspection and Post-Processing: Ensuring Quality
Interlayer Alignment Accuracy ≤ ±8μm
An X-ray positioning system is used to compensate in real-time for differences in the coefficient of thermal expansion (CTE) of materials.
Comprehensive Inspection
AOI Inspection: For visual defects (e.g., bubbles, delamination).
TDR Testing: For impedance consistency.
3D X-ray: For via stubs and interlayer alignment.
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