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Significantly Increased Machining Difficulty
Drilling
Increased risk of drill bit breakage due to greater hole depth, difficult chip evacuation, and high friction, making tool failure highly likely.
Drill Drift
Excessive diameter-to-thickness ratios cause drill drift, resulting in inaccurate hole positioning and significant misalignment between inner and outer layers.
Poor Hole Wall Quality
Prone to drill contamination, rough surfaces, and “flared holes” (enlarged exit diameters).
Forming/Outline Machining
V-Cut Failure: For thick boards requiring panelization, traditional V-cuts have limited depth (typically not exceeding 2/3 of board thickness), failing to fully separate layers. Forced separation causes edge chipping and cracks.
Milling Difficulty: Accelerated cutter wear leads to burrs, delamination, or corner chipping on edges.
Lamination and Thermal Stress Control
Resin Insufficiency
During lamination of multilayer boards, resin within prepreg must flow to fill gaps between inner layer traces. When inner layer drill holes require PP filling, increased board thickness lengthens resin flow paths. This often causes “resin starvation” or voids in central areas, reducing interlayer bonding strength.
Bubbles and Delamination During lamination of thick boards, trapped gases and volatiles are harder to expel. High temperature and pressure can form bubbles, posing a delamination risk upon cooling.
Warpage and Deformation The different CTE (Coefficient of Thermal Expansion) values of each layer material (copper foil, core board, PP) cause accumulated thermal stress to manifest more significantly in thick boards during the high-temperature lamination and cooling processes. This readily leads to board warping, complicating subsequent SMT placement.
Electroplating Process Challenges
Hole Copper Uniformity: Electroplating solutions struggle to uniformly penetrate deep holes in thick boards. This results in sufficient copper thickness at both ends of the hole wall but insufficient thickness in the middle (creating a “dogbone” or “waist drum” phenomenon). This severely impacts current-carrying capacity and reliability, potentially causing open circuits.
Blind/Buried Via Filling: For HDI thick boards, the difficulty of via filling electroplating increases exponentially.
Through-Hole Insertion and Soldering Issues
Poor Solder Penetration: During wave soldering or manual soldering of through-hole components, solder struggles to climb the entire hole wall via thermal conduction. This often creates voids or insufficient solder penetration in the hole center, compromising joint mechanical strength and electrical reliability.
Component Instability
Heavy Components: On thick boards, components relying solely on solder joints for fixation are prone to fatigue failure under vibration.
Equipment and Material Constraints
Equipment Compatibility
Many automated conveyors, exposure machines, and etching lines impose upper limits on board thickness (e.g., typically ≤3.2mm).
Special Material Requirements
High-reliability substrates with elevated TG and low CTE are necessary to withstand thermal stress, incurring higher costs.
How to overcome these challenges?
Optimize Machining Processes
Drilling
For deep holes, employ segmented drilling using the “peck drill” technique (drill-and-retract), facilitating chip removal and heat dissipation.
Use high-quality specialized drills with superior wear resistance and rigidity (e.g., tungsten carbide drills), and optimize drilling parameters (spindle speed, feed rate).
For step drilling, first create pilot holes with smaller drills before enlarging with larger ones.
Strictly control the length-to-diameter ratio. Generally, through-hole ratios should not exceed 10:1 (exceptional factories may achieve 15:1). If designs exceed these limits, consult with clients to modify specifications.
Forming
Eliminate V-CUTs; switch to Routing or stamp holes for panel connections. ✒︎ Employ slower milling speeds with multiple passes, using sharp new milling cutters.
For extremely thick sheets, consider depth-controlled milling.
Improve Laminating Process
Use low-flow or high-resin-content PP sheets to ensure sufficient resin fills long gaps.
Implement multi-stage temperature/pressure ramping in the pressing program. Extend dwell time at lower temperatures to allow resin to flow fully and gases to escape slowly before applying high pressure for curing.
Add buffer material and flow channels. Place material along the edges of the pressing steel plate to guide excess resin and gases out.
Incorporate a post-lamination baking process to further relieve internal stresses and minimize warpage.
Upgrade Electroplating Capabilities
Adopt pulse plating or horizontal plating technology. Both techniques significantly improve electrolyte exchange efficiency within deep holes, greatly enhancing copper plating uniformity.
Employ stronger agitation and vibration devices, such as ultrasonic agitation, to facilitate electrolyte penetration into deep holes.
Extend plating duration and monitor parameters
Ensure minimum copper thickness compliance through rigorous destructive cross-section analysis.
Design-Manufacturing Collaboration
This represents the most effective approach to overcoming thick board challenges. Early coordination should address:
Relax design tolerances Increase line width/spacing, enlarge via diameters and pad dimensions.
Optimize hole design to avoid excessively small apertures, allowing space for plating.
Increase process margins and use locating holes to enhance positioning and clamping accuracy during production.
Consider back drilling for high-speed signals. Remove unused copper stubs from vias—though costly, this improves electrical performance.
Strict Quality Control and Inspection
100% electrical continuity testing: Critical for thick boards.
Increase slice analysis frequency: Conduct regular sampling to monitor via copper quality, laminate bond strength, etc.
Employ 3D X-ray inspection: Non-destructive testing for internal via copper and layer alignment.
Perform thermal stress testing (e.g., 288°C immersion solder test): Validate plated-through-hole and laminate reliability.
Manufacturing PCBs thicker than 3.0mm presents challenges as increased geometric dimensions drastically reduce the uniformity and controllability of physical/chemical processes. To overcome this, consult PCB manufacturers with proven thick board production experience and capabilities as early as possible when thick boards are required. Submit design files for manufacturability analysis. Note that production lead times and costs for thick boards typically exceed those of standard thicknesses significantly, necessitating realistic expectations.