Ink residue inside small non-plated through holes (NPTHs) is a common PCB manufacturing defect, particularly when the hole diameter is 1.0 mm or smaller. During solder mask printing or spraying, solder mask ink can enter the hole. Because of surface-tension and capillary effects, the ink may not flow back out easily, and the developer may not completely remove it.
This issue is most likely to occur when the hole has a small diameter, the board is relatively thick, or the solder mask process is not optimized for effective fluid exchange inside the hole. The result may be visible ink contamination, a partially blocked aperture, or a ring of residual ink around the hole wall. The following sections summarize the main causes, influencing factors, and practical countermeasures.
Why Do Small NPTH Holes Retain Solder Mask Ink?
During solder mask processing, an NPTH has no copper on its inner wall. Instead, the wall consists of exposed laminate material, primarily glass fiber and resin. Compared with a copper surface, this relatively smooth base-material surface can provide stronger adhesion for certain solder mask inks. Several additional mechanisms make small NPTHs especially vulnerable.
1. Capillary Action
As the hole diameter decreases, capillary action becomes more significant. The ink is drawn into the aperture and can spread along the inner wall. Once it has entered a small hole, its limited flow area makes it difficult for gravity or subsequent rinsing to remove it completely.
Capillary action is also affected by the relationship between hole diameter and board thickness. A small-diameter hole in a thick board forms a long, narrow passage. Ink can enter this passage during printing, but fresh developer and rinse water may not reach the entire surface effectively.
2. Restricted Developer Flow
During development, the ink inside a small NPTH is difficult to displace with fresh developer. The fluid exchange inside the hole is limited, and the developer may not reach all portions of the ink layer with sufficient velocity. As the hole becomes smaller, this problem becomes more pronounced.
If the developer spray pressure is too low, or if the spray direction does not adequately reach the hole openings, the liquid inside the aperture may remain stagnant. The outer surface of the solder mask may appear properly developed while residual ink remains inside the hole.
3. Solder Mask Ink Characteristics
Photosensitive solder mask ink begins to polymerize during the pre-bake stage. If too much ink has accumulated inside an NPTH, the layer may become too thick for the developer to dissolve completely. Ink viscosity also affects the result: a high-viscosity formulation is less likely to drain from the hole before exposure and development.
Ink flow, adhesion, and curing behavior must therefore be considered together. Reducing viscosity may improve drainage, but excessive dilution can increase uncontrolled spreading or allow ink to enter other small openings.
4. Process and Screen Design
Some production flows use open-screen printing rather than a local ink-blocking pattern in the screen or artwork. In this case, solder mask may be deposited directly over the NPTH area, increasing the amount of ink that enters the aperture.
A local ink-blocking mesh, dot-blocking pattern, or comparable masking method can reduce the quantity of ink deposited above the hole. This does not eliminate the need for proper pre-bake and development settings, but it reduces the initial ink load that the process must remove.
Factors That Increase the Risk of Residue
- Smaller hole diameter: The probability of residual ink rises sharply as the NPTH diameter decreases.
- Greater board thickness: A longer and narrower hole passage makes it more difficult for developer to flush out the ink.
- Higher solder mask viscosity: High-viscosity ink drains less easily and is more likely to remain on the hole wall.
- Incorrect pre-bake time or temperature: Excessive pre-baking can partially cure the ink before development, making removal more difficult.
- Insufficient development pressure: Low pressure may not provide enough force to remove ink from the hole.
- Improper spray-nozzle angle: If the spray does not enter the hole effectively, fluid exchange inside the aperture remains poor.
- No local ink-blocking pattern: More ink enters the NPTH when the screen or artwork does not restrict deposition in the hole area.
- Air entrapment: Bubbles in the ink can cause localized accumulation and prevent uniform development.
These factors can interact. For example, a small NPTH in a thick board may be acceptable under one ink and development condition but fail when the ink viscosity, pre-bake condition, or spray pressure changes. The defect should therefore be evaluated as a process interaction rather than attributed to hole diameter alone.
Design-Stage Countermeasures
The most direct solution is to increase the NPTH diameter wherever the mechanical and electrical design allows it. A diameter of at least 1.5 mm is recommended in the process guidance described here because the larger opening improves ink drainage and developer access.
If a hole smaller than 0.5 mm is unavoidable, one possible option is to drill the hole again after solder mask processing. This approach requires careful review of the hole-position tolerance and the customer's acceptance criteria. It should not be treated as a universal substitute for process control, because the secondary drilling operation adds cost and may slightly deform the hole.
Designers should also identify small NPTHs early and communicate their location, finished diameter, board thickness, and solder mask requirements to the fabricator. A hole that is technically manufacturable may still require a special solder mask strategy when it is positioned in a dense pattern or close to other openings.
Solder Mask Process Adjustments
Use a Local Ink-Blocking Pattern
An ink-blocking mesh, local dot-blocking pattern, or similar screen feature can be applied over the NPTH region. The purpose is to reduce direct ink deposition above the aperture, thereby limiting the amount of material that can be drawn into the hole by capillary action.
This method is particularly useful when many small NPTHs are located in the same area. It reduces the burden on the development process and improves consistency across the panel. The blocking pattern must nevertheless be checked against the required solder mask coverage and surrounding features.
Control Ink Viscosity
Reducing solder mask viscosity can help the ink flow out of the hole before it begins to cure. A suitable thinner may be added when permitted by the ink process specification. The adjustment must be controlled carefully, because excessive thinning can reduce coating stability, increase spreading, or cause the ink to enter other small openings.
Viscosity control should be combined with routine monitoring rather than judged only by appearance. Changes in ink temperature, mixing condition, storage time, and material batch can affect flow behavior during printing.
Optimize the Pre-Bake Condition
The pre-bake temperature and time should be adjusted so that the ink remains sufficiently developable. Excessive heating or an excessively long pre-bake can partially cure the ink inside the hole, making it resistant to the developer.
The objective is not simply to lower the temperature or shorten the time as much as possible. The process must still provide adequate handling strength and coating stability before exposure. The correct setting should be established through process verification for the specific ink, board thickness, and panel structure.
Improve Development and Rinsing
Increasing development pressure can improve the removal of ink from small holes. An upper-and-lower oscillating spray arrangement can also improve fluid exchange at both sides of the board. Where appropriate, the development time may be extended to allow the developer to penetrate and replace the liquid trapped inside the aperture.
Spray-nozzle orientation is important. A nozzle angle that reaches the hole openings directly is more effective than a flow direction that only washes across the board surface. However, excessive pressure or development time may damage other solder mask features, so the adjustment should be verified against the complete panel pattern.
Secondary Drilling After Solder Mask Curing
If process adjustments cannot reliably eliminate the residue, the NPTHs may be drilled a second time after the solder mask has been completed and cured. In this approach, ink is allowed to enter the holes during the normal solder mask process, and the affected holes are subsequently re-drilled.
The second drill diameter is specified as 0.05–0.1 mm larger than the original hole diameter. This removes the residual solder mask ink and a portion of the material on the hole wall. It can be an effective corrective method when the holes are mechanically suitable for the operation and the resulting dimensional change is acceptable.
The disadvantages are the additional drilling cost and the possibility of slight hole deformation or positional variation. Before adopting this method, the manufacturer and end customer should confirm the allowable hole-position tolerance, finished-hole size, and any clearance requirements around the NPTHs.
Inspection and Production Control
The quality requirement described here is that NPTH holes should have no visible solder mask residue. A slight annular residue may be acceptable if it does not obstruct the effective hole diameter. Inspection should therefore distinguish between a cosmetic ring and residue that reduces the aperture or interferes with mechanical assembly.
For mass production, the preferred process combination is generally a local ink-blocking pattern, controlled pre-bake conditions, and sufficiently strong development with effective spray coverage. If the available design clearance is inadequate or the residue remains unstable, secondary drilling can be considered after confirming the hole-position and dimensional requirements.
Controlling NPTH ink residue requires coordination between PCB design and manufacturing. Increasing the hole diameter is the simplest design measure, while screen design, ink viscosity, pre-bake, development pressure, spray direction, and optional secondary drilling provide process-level solutions. The most suitable combination depends on the hole size, board thickness, solder mask material, and acceptance criteria for the finished PCB.
SEO Title: How to Prevent Solder Mask Ink Residue in Small NPTH Holes Meta Description: Learn why small NPTH holes retain solder mask ink and how PCB designers and manufacturers can prevent residue through screen design, viscosity control, pre-bake optimization, development adjustments, and secondary drilling.
