PCBgogo

As smartphones move toward lighter, thinner bodies and multi-camera architectures, HDI PCBs have become the backbone of mobile electronics. With line width/spacing ≤ 0.1 mm and blind/buried via diameters ≤ 0.2 mm, HDI boards require extremely precise micro-via fabrication.

However, the blind & buried via process is the main determinant of HDI yield. Industry data shows that without process optimization, void rates can exceed 8%, pushing mainboard rework rates above 12%. One smartphone brand even scrapped 5,000 PCBs in a single day due to plating voids—causing over $30,000 in direct losses.

PCBGOGO has specialized in mobile HDI board manufacturing for 8+ years and delivered 20 million+ mobile HDI PCBs, achieving 99%+ blind via yield stability. This article breaks down the core process challenges, critical parameters, and best-practice quality control to help consumer electronics manufacturers achieve high-volume, high-reliability HDI production.

Understanding Blind & Buried Via Challenges in Mobile HDI PCBs

HDI blind/buried via fabrication must strictly follow IPC-6012 Section 3.6 for micro-via qualification. The core challenge is achieving perfect via wall quality in extremely small diameters.

1. Drilling Accuracy

• Typical blind via diameter: 0.15–0.2 mm

• Position deviation requirement: ≤ ±0.02 mm

• Deviation above ±0.03 mm increases the open-circuit rate by 15%

(Defined in IPC-2226 Section 4.2)

2. Via Wall Roughness

• Required roughness: ≤ 1.5 μm

• Roughness >2 μm increases plating void rate by 20%

• Tested per IPC-TM-650 2.2.17

3. Copper Plating Thickness

• Via wall copper thickness: ≥ 18 μm

• Uniformity tolerance: ±2 μm

• Copper thickness <15 μm increases signal loss by 8%

(According to GB/T 4677 Section 5.3)

Recommended HDI base material:
Shengyi S1130 (Dk 4.3±0.2, thickness 0.1–0.3 mm) — ideal resin flow for via filling.

For drilling, PCBGOGO uses FANUC CNC micro-drilling machines (precision ±0.005 mm). For plating, an acid copper system ensures excellent copper uniformity.

Four-Step Blind & Buried Via Process Optimization

Below is the complete optimization process used in PCBGOGO’s HDI production lines.

Step 1: Drilling Parameter Optimization

• Via diameter: 0.2 mm

• Spindle speed: 180,000 rpm

• Feed rate: 50 mm/min

• Retract speed: 80 mm/min

PCBGOGO testing shows:

• When spindle speed rises above 200,000 rpm, via wall carbonization reaches 12%.

Equipment: FANUC DR-300
Inspection: Optical microscope spot-check 10 panels per hour.

Step 2: Via Wall Pre-Treatment (Desmear + Micro-Etching)

• Desmear temperature: 80°C ± 2°C, time: 6 min

• Micro-etching amount: 1.5–2 μm

• Ensures via wall roughness ≤ 1.5 μm

Equipment: JPE-Desmear-600
Standard: IPC-TM-650 2.3.11

Step 3: Electroplating Parameter Optimization

• Acid copper plating

• Current density: 1.8 A/dm2

• Temperature: 25°C ± 1°C

• Agitation: 300 rpm

• Time: 25 min

Ensures:

• Copper thickness ≥ 18 μm

• Uniformity ≤ ±2 μm

Inspection: 50 panels per batch using JPE-Cu-500 copper thickness tester.

Step 4: Via Filling & Curing

• Filling material: Epoxy resin (viscosity 5000 cP @25°C)

• Curing temperature: 150°C ± 5°C, time: 40 min

• Filling completeness: ≥ 98% (IPC-A-600G Class 2 requirement)

Inspection: X-Ray (JPE-XR-900)
Requirement: Via void rate ≤ 0.5%

Mass Production Quality Control Strategy

1. First-Article Verification (FAI)

• Via position deviation ≤ ±0.02 mm

• Via copper thickness ≥ 18 μm

• Void rate ≤ 0.5%

Only approved lots can enter mass production.

2. In-Process Monitoring

Every 2 hours:

• Sample size: 20 panels

• Monitor:

• Plating current deviation: ≤ ±0.1 A/dm2

• Filling temperature deviation: ≤ ±2°C

Triggered automatically by PCBGOGO’s Process Early-Warning System when abnormalities appear.

3. Final Inspection

• Void rate ≤ 0.8%

• Via copper thickness ≥ 18 μm

• Position deviation ≤ ±0.02 mm

• Batch pass rate ≥ 99%

Failures are analyzed by PCBGOGO’s HDI defect analysis team, which provides a root-cause report and corrective action.

Conclusion

Optimizing blind & buried vias in mobile HDI PCBs requires precise control over four key factors:

Drilling accuracy → Via wall preparation → Copper plating uniformity → Resin filling completeness

PCBGOGO provides a full-process HDI manufacturing solution, including:

• DFM analysis with JPE-DFM 7.0

• FANUC micro-drilling + automated plating lines

• Complete X-Ray inspection of via quality

• 99%+ stable mass-production yield

This closed-loop approach—spanning design, manufacturing, and quality assurance—ensures mobile device manufacturers can scale HDI production with high reliability and minimal defect rates.