Bluetooth Earphone Module — 8L 2-Step HDI, Stacked Via

8-layer 2-step HDI · 6.52 × 5.21 mm · 0.75 ± 0.08 mm · ENIG · Resin plug · Stacked laser via · White solder mask · 75 μm solder bridge · SMT assembly · Build-to-print from Gerber

Product Specifications

8-Layer 2-Step HDI Stack-Up

• 2-step sequential build-up — two lamination + laser drill + plate cycles on a 4-layer core

• Step 1: L2–L7 core lamination + L1–L2 and L7–L8 laser blind vias

• Step 2: Second lamination + L1–L3 and L6–L8 stacked laser vias

• Stacked via (叠孔): L1–L2 aligned on top of L2–L3 — copper-filled for via-in-pad routing

• Total 8 conductive layers: 3 signal + 2 ground + 2 power + 1 mixed, in 0.75 mm

• 0.75 ± 0.08 mm tolerance — tight thickness control for consistent acoustic cavity clearance

Resin Plug + Stacked Via

• Resin plug: epoxy-filled laser vias before second lamination — flat surface for stacked via landing

• Vacuum-assisted fill — void-free plug verified by X-ray, < 2% void rate

• Planarization: mechanical scrub after plugging — coplanar surface for next build-up layer

• Stacked via reliability: copper-to-copper interface between stacked vias — no pad between, direct connection

• Via-in-pad: stacked vias land directly under 0.4 mm pitch BGA pads — zero escape routing needed

6.52 × 5.21 mm — Ultra-Compact Design

• 34 mm² board area — among the smallest Bluetooth earphone modules in production

• 8 layers in 0.75 mm — 93 μm average dielectric per layer

• Stacked via saves space — no staggered-via real estate penalty, all via-in-pad

• High panel utilization — estimated 50+ boards per 100 × 100 mm sub-panel

• Laser routing singulation — clean edge, no burr, tight board outline tolerance

White Solder Mask + 75 μm Bridge

• White solder mask — LED-compatible reflectivity for status indicator, clean cosmetic finish

• 75 μm minimum solder mask bridge — between adjacent pads, prevents solder bridging on fine-pitch

• LDI solder mask imaging — ± 15 μm registration, critical for 75 μm bridge definition

• Photo-imageable solder mask — high resolution, consistent dam height, no bleed onto pads

• Matte white surface — good contrast for AOI and visual inspection

ENIG Surface Finish

• Ni 3–6 μm / Au 0.05–0.12 μm — IPC-4552 compliant immersion gold

• Flat pad surface — coplanarity suited for 0.4 mm pitch BGA and 0201 passives

• Multiple reflow cycles — module assembly reflow + customer board-level reflow

• Solder joint integrity — Ni₃Sn₄ IMC formation, Au dissolves fully into solder

• Shelf life > 12 months — immersion gold resists oxidation, no pre-solder dip needed

Manufacturing Notes

• SMT assembly ready — ENIG pads + white mask + laser-routed outline, pick-and-place compatible

• Build-to-print from Gerber RS-274X — impedance coupon on production panel

• 100% flying probe electrical test — continuity + isolation, all nets including stacked vias

• 100% AOI — 2 μm resolution, 75 μm bridge inspection on all layers

• X-ray per lot — resin plug void check + stacked via alignment verification

• Cross-section per lot — via plating thickness, stacked via interface, dielectric thickness

• Lead-free RoHS compliant — ENIG + white mask, no lead in finish or laminate