For non-plated through-holes (NPTHs), the decision to use one drilling operation or two is determined primarily by the hole diameter and the PCB manufacturer's ability to cover and protect the hole with dry film during plating. The choice also depends on positional accuracy, process capability, production efficiency, and cost.
What Are One-Drill and Two-Drill Processes?
One-Drill Process
In a one-drill process, the finished PCB panel is drilled only once after copper-clad laminate has been laminated. The drill creates the final NPTH diameter directly. Because the hole is not intended to receive copper plating, the manufacturing process must physically shield or seal the hole during subsequent plating operations.
For this method to work reliably, the dry film used during the plating process must completely cover the PCB NPTH and remain intact. If the dry film breaks or fails to seal the opening, plating solution can reach the hole wall. Copper, tin, or tin-lead plating may then be deposited on the hole wall, causing the hole to become partially or completely plated. The result is a plated through-hole rather than the required NPTH.
Two-Drill Process
A two-drill process uses two separate drilling operations. The most common approach is to drill a smaller pilot hole before plating, complete the required plating steps, and then use a larger drill after plating to enlarge the hole to its final diameter.
- Drill a pilot hole smaller than the final NPTH size.
- Complete the normal plating process while the smaller opening can be covered more reliably.
- Perform a final drilling operation with a larger tool to reach the specified finished diameter and remove the plated material from the hole wall.
Because the second drilling operation cuts away the plated hole wall, this method can provide a more reliable copper-free NPTH, particularly when the final hole is too large for the dry-film process to protect effectively.
Another two-drill arrangement places the second drilling operation at a different point in the overall fabrication sequence, such as before routing or profiling, or before etching. This approach may be used for special NPTH structures, for small-diameter holes, or when a one-drill process creates problems that cannot be controlled consistently. The exact hole position may vary slightly between operations, so positional tolerance must be considered during design and manufacturing planning.
Key Differences Between One-Drill and Two-Drill Methods
One-Drill Manufacturing
The one-drill method is straightforward: the final hole diameter is drilled immediately after lamination, and no later enlargement operation is required. This reduces the number of process steps, shortens production time, and lowers manufacturing cost.
Its main limitation is the requirement for reliable dry-film hole coverage. During PCB plating, dry film is used to protect areas that should not be plated, including NPTH regions. The film must remain properly adhered over the opening throughout exposure, development, cleaning, and plating. If the hole is too large, the film may not bridge the opening adequately. It can crack, lift, or rupture under process conditions, exposing the hole wall to the plating chemistry.
Once the hole wall receives metal deposition, the finished board may no longer meet the NPTH requirement. This is not simply a cosmetic defect. It can affect electrical isolation, mechanical clearance, assembly behavior, and the intended function of the hole.
Two-Drill Manufacturing
The two-drill method introduces an additional operation but provides a way to separate the requirements of plating protection and final hole size. The pilot hole is kept small enough to improve dry-film coverage during plating. After plating has been completed, the larger final drill removes the plated material around the opening and establishes the final NPTH diameter.
This process is particularly useful when a large NPTH cannot be reliably sealed by the available dry-film process. It can ensure that the finished hole is free of copper even when the initial plating sequence would otherwise create a risk of metal deposition.
The additional operation, however, increases drilling time, tooling requirements, process management, and production cost. It also introduces a second alignment requirement. If the final drill is not accurately registered to the first hole, the finished opening may shift from its intended location or produce an uneven annular condition around the hole.
Why Is a Second Drilling Operation Necessary?
Dry-Film Hole-Coverage Capability
The most direct reason for choosing two drilling operations is the limited hole-covering capability of dry film. During a positive-working plating process, the dry film must protect non-plated areas while allowing plating to occur in the designated circuit regions. A large NPTH opening is more difficult to cover than a small one because the film spans a larger unsupported area.
When the opening exceeds the practical capability of the dry-film process, the film can fracture or separate from the surface. The exposed hole wall may then be plated. In this situation, drilling the final large opening only after plating is a practical way to remove the unwanted metal and restore the required NPTH structure.
Problems Associated with Very Small NPTHs
Two-drill processing may also be selected for small NPTHs when other manufacturing problems are difficult to control. For example, solder mask ink can enter a small hole and remain trapped inside. If the ink cannot be properly developed out, it may remain in the opening and contribute to defects during solder coating or other surface-finishing operations.
Such conditions can lead to solder or tin beads being retained in the hole. Residual palladium or other process-related deposits may also remain in or around the NPTH and contribute to unwanted metal attachment during subsequent finishing operations, including gold plating. When these defects cannot be controlled effectively with a one-drill flow, a later drilling step can be used to clean the hole and establish the required final opening.
How Manufacturing Capability Affects the Decision
1. Hole Diameter
Hole diameter is usually the most immediate factor. A small or moderate NPTH may be suitable for one-drill production if the dry film can cover it reliably. A larger opening may exceed the manufacturer's film-tenting capability and require a pilot-hole and final-hole sequence.
The decision should therefore be based on the actual process window rather than on the hole diameter alone. The required result depends on the complete manufacturing flow, including lamination, drilling, dry-film application, exposure, development, plating, and final finishing.
2. Dry-Film Process Capability
Dry-film quality and hole-sealing capability vary among PCB manufacturers and process lines. A manufacturer with stronger film adhesion, better exposure control, and more stable plating preparation may be able to produce larger NPTHs using one drilling operation. A process with less margin may need two-drill processing for the same design.
This means that a hole size that is acceptable for one factory should not automatically be treated as acceptable for every factory. The selected process should be confirmed against the manufacturer's documented capabilities and the specific board construction.
3. Positional Accuracy
Two-drill processing requires accurate registration between the first and second drilling operations. Any alignment error can cause hole offset, an uneven final opening, or insufficient clearance from nearby copper features.
This consideration is especially important when the NPTH is located close to pads, traces, copper planes, mechanical edges, or other holes. If a manufacturer cannot maintain adequate second-drill alignment, improving the one-drill dry-film process may be preferable to introducing an additional registration-sensitive operation.
4. Cost and Production Efficiency
One-drill production generally provides a simpler and more efficient manufacturing route. It reduces the number of drilling operations and avoids the additional handling and alignment associated with a second drill. When the quality requirements can be met, manufacturers will normally prefer the lower-cost process.
Two-drill processing increases the number of operations and therefore adds cost and production time. Nevertheless, avoiding the second drill solely to reduce cost can create a greater risk of plated NPTHs, hole contamination, or dimensional nonconformance. The correct decision is the process that achieves the specified hole condition with sufficient manufacturing margin.
Design and Manufacturing Considerations
When an NPTH is included in a PCB design, the designer should identify it clearly in the fabrication data and distinguish it from plated holes. The finished hole diameter, allowable positional tolerance, and required copper-free condition should be communicated consistently across the drill file, fabrication drawing, and other manufacturing documentation.
Designers should also avoid assuming that every NPTH can be produced by a one-drill process. The selected fabrication shop may use different dry-film materials, exposure conditions, plating sequences, or drilling strategies. Early communication is particularly important for large NPTHs, small NPTHs, holes near copper features, and designs with tight mechanical clearances.
The one-drill versus two-drill decision is therefore a balance between process capability, dimensional control, quality risk, cost, and production efficiency. One drilling operation is simpler and less expensive when the dry film can protect the hole reliably. Two drilling operations are justified when the hole size, plating sequence, contamination risk, or process limitations make a guaranteed copper-free result difficult to achieve with one drill.
In short, the choice is not determined by the PCB design file alone. It is a manufacturing decision based on the relationship between NPTH geometry and the fabricator's actual process capability. A reliable design should leave enough margin for dry-film coverage, drilling registration, and final inspection rather than relying on a nominal hole size without considering the complete fabrication flow.
