Optimization of Flexible PCB Materials Micro-Drilling
Flexible Printed Circuit Boards (FPCBs), commonly referred to as flexible boards, are highly reliable printed circuit boards with exceptional flexibility, manufactured using polyimide or polyester film as the substrate material.
Compared to rigid circuit boards, flexible boards offer inherent flexibility and advantages such as ease of bending, lightweight construction, and thin profile.
Structurally, flexible boards are formed by bonding flexible copper-clad laminate (FCCL) and flexible insulating layers with adhesive, then laminating them together.
Advantages and Applications of FPCBs
They are printed circuits made from flexible insulating substrates, offering numerous advantages not found in rigid printed circuit boards.
For instance, they can freely bend, roll, and fold, allowing flexible spatial arrangement according to layout requirements.
They can move and expand/contract freely in three-dimensional space, achieving integrated component assembly and wire connections.
FPCBs significantly reduce the size of electronic products, meeting the industry’s demand for high-density, miniaturized, and highly reliable devices.
Consequently, FPCBs have found extensive application in aerospace, military, mobile communications, laptops, computer peripherals, PDAs, digital cameras, and related fields or products.
Micro-Hole Machining Techniques for FPCBs
Numerous methods exist for machining small holes in workpieces.
Among various micro-hole processing techniques, mechanical drilling is less susceptible to workpiece characteristics, facilitates thermal deformation cutting, and reduces post-drilling finishing processes.
Compared to laser drilling, mechanical drilling retains distinct advantages in micro-hole quality, through-hole processing, and deep-hole machining.
With the application of high-speed CNC drilling machines, the efficiency of mechanical drilling has significantly improved.
Its application in the field of ultra-fine micro-hole drilling will become increasingly widespread, making it a research hotspot in micro-hole drilling technology.
This paper investigates rigid-flex boards (i.e., rigid-flex laminates) provided by clients, optimizes the geometric parameters of micro-drills for PCBs, and derives relevant conclusions to provide theoretical support for micro-drill engineering design.
Introduction to FPC Materials
Flexible printed circuit boards (FPCs) fall into single-sided, double-sided, and multilayer types, primarily using polyimide-clad copper laminate as the substrate material.
Polyimide (PI) Film and Substrate Materials
Polyimide (PI) film stands as one of the most critical film materials in current FCCL manufacturing, accounting for the largest proportion among all film substrates used in FCCLs.
This material offers high thermal resistance and dimensional stability. A cover film laminates the board, providing both mechanical protection and excellent electrical insulation to form the final product.
Single-Sided, Double-Sided, and Multilayer FPC Construction
In double-sided and multilayer printed circuit boards, the surface and inner layer conductors are metallized to establish electrical connections between the inner and outer layers.
Single-sided boards utilize a single-sided PI laminate. After circuit formation, a protective film is applied to create a flexible circuit board with only a single conductor layer.
Standard double-sided boards utilize double-sided PI copper-clad laminate material.
After completing circuits on both sides, a protective film coats each surface, creating a circuit board with dual conductive layers.
Substrate-derived single-sided boards employ pure copper foil material. During circuit fabrication, a protective film sequentially coats both surfaces, resulting in a board with a single conductive layer and conductors exposed on both sides.
Substrate-based double-sided boards are created by laminating two single-sided PI-clad boards with bonding adhesive featuring windows at specific locations.
This produces a double-sided conductor board with locally bonded and locally separated layers, achieving high flex performance in the separated zones.
Multilayer boards utilize single-sided PI copper-clad laminate and adhesive as base materials.
Employing a process similar to double-sided board formation, multiple lamination cycles create a multi-layer conductor structure. Engineers can induce localized delamination to achieve high flexural performance.
Rigid-flex boards combine the flexibility of flexible boards with the structural support of rigid boards to form a versatile circuit board.
PCB Drilling Selection Principles
Rigid-flex boards feature complex structures, making optimal drilling parameters critical for achieving high-quality hole walls.
Use sharp drill bits to prevent burring on inner copper rings and flexible substrates.
For high-volume production or boards with numerous holes, replace drill bits promptly after drilling a set number of holes.
Spindle speed and feed rate are the most critical process parameters: excessively slow feed causes rapid temperature rise and excessive drilling residue; conversely, excessively fast feed risks drill breakage, laminate adhesion, and tearing or pinheading of the dielectric layer.
Secondly, select the drilling machine and optimize drilling parameters based on the board thickness and minimum hole diameter.
For small holes, higher rotational speeds yield better drilling quality. Simultaneously, the selection of cover plates and shims is crucial.
High-quality cover plates and shims not only protect the board surface but also provide effective heat dissipation.
Aluminum foil boards or epoxy phenolic boards are preferable for backing plates.
Avoid using paper backing plates, as their softness can cause severe burring during drilling.
Deburring these plates risks tearing or damaging the hole edges, complicating subsequent processes and compromising board quality.
Testing confirms that PCB drill contamination levels and thickness increase with rising drilling temperatures, accelerating above the resin’s glass transition temperature.
Consequently, some manufacturers have experimented with cryogenic drilling to reduce contamination by lowering board temperatures during processing.
Test Results Before PCB Micro-Drilling Optimization
Table 1 lists the customers’ on-site processing parameters, while Figure 1 shows the corresponding test results.
Table 1 Processing Parameters
Figure 1. Image showing the inspection of the hole wall after electroplating
Table 2. Statistical analysis of hole position accuracy
Table 3. Statistical analysis of borehole wall accuracy
On-site analysis findings:
(1) Post-plating hole walls exhibit nodules, primarily concentrated near the substrate layer. Single-stack hole walls show fewer nodules than double-stack configurations.
(2) Post-plating slightly compromises the perpendicularity of the hole walls.
(3) Regarding hole wall precision, the overall cross-section analysis revealed no internal quality anomalies. Both drill bit models demonstrated drilling performance without tearing the inner layer coating film.
The aforementioned processing fails to meet the customer’s precision requirements, necessitating drill bit re-optimization.
PCB Micro-Drill Optimization and Test Results
(1) Reducing the drill’s helix angle
This optimization further increases the internal clearance space based on previous designs to reduce heat generation during drilling, resulting in improved hole wall quality.
(2) Reducing the drill’s ligament width
Ligament width directly impacts hole wall roughness. A wider ligament increases the contact area between the drill and the hole wall during drilling, leading to greater heat generation and directly compromising hole wall quality. Reducing ligament width enhances hole wall quality.
Optimized test data (Tables 2, 3, 4, 5) demonstrate improved hole wall quality and enhanced positional accuracy.
Table 4 A129FP ⌀0.30 – 5.0 Hole wall accuracy
Table 5 A129FP ⌀0.35 5.5 Hole wall accuracy
The optimized results demonstrate significantly improved performance compared to pre-optimization, reflected in the following aspects:
(1) Wear condition: Both tools exhibit normal wear.
(2) Hole position accuracy showed concentrated distribution, with 3σ+ave ≤ 40μm < 60μm. Individual max values reached approximately 80μm, achieving CPK ≥ 2.5.
(3) Hole wall precision revealed no internal quality anomalies in overall cross-sections. Both drill bit models produced holes with intact inner coating films and straight entry points.
(4) Hole wall roughness is <25μm, and the drill head is <150%. The optimized A129FP ⌀0.30–5.0 and A129FP ⌀0.35–5.5 meet customer requirements.
Conclusion
Drilling tested a highly demanded rigid-flex board material for the electronics industry, provided by the customer.
Researchers optimized the geometric parameters of the PCB micro-drill. Results demonstrate that the optimized PCB micro-drill significantly improves hole wall precision and hole position accuracy, meeting customer requirements. This provides a theoretical basis for the engineering design of micro-drills.
