Disputes over resin-plugged vias almost always start in the same place. The drawing says "all vias resin filled, must be fully filled." At first-article review the fabricator measures surface planarity at the via mouth, the customer sections the board and looks at fill inside the barrel. Both data sets are real, and neither answers the other side's question.
The root cause is not inspection capability. It is that the requirement was never split by inspection plane. A resin-filled via has at least two physically distinct planes to judge: the condition inside the barrel (voids, how far the fill extends) and the condition at the via mouth (dimple, protrusion, whether it is capped). Different operations create them, different methods measure them, and the documentation should state them separately.
1. Start with what the fill is supposed to achieve
Resin plugging is not about "closing the hole." It provides a stable, machinable base with no residual cavity for the operations that follow. Three purposes are common: giving a via-in-pad land a flat enough surface, preventing trapped air or process chemistry during sequential lamination, and reducing downstream dielectric risk in thick-board or small-hole structures.
The first practical consequence is that different purposes justify different strictness. When the fill serves via-in-pad, mouth planarity dominates. When it serves sequential lamination of buried vias, the presence of a connected cavity inside the barrel matters far more. Applying one criteria set to both scenarios usually over-inspects what is harmless and under-inspects what actually fails.
The second consequence is terminology. Resin plugging and solder-mask plugging are not interchangeable: the material system, whether curing and planarization occur, the function served, and the acceptance focus all differ. Confirm the process routes and applicable structures on the resin plugged via PCB page, then let engineering confirm against your specific stack-up.
2. Fields that must appear in the documentation
An inspectable fill requirement needs the following in controlled documents. Any missing item turns into a verbal agreement at inspection time.
- Via list: which holes are filled and how they are designated (diameter, layer pair, net, or a dedicated fill layer). If the note says "all vias," it must also state whether component holes, test points and tooling holes are included.
- Fill type: resin or solder mask; screen printing or vacuum fill; whether complete fill is required.
- Capping: whether cap plating (copper over the fill) is required, one side or both, and the target land condition after capping.
- Target mouth condition: dimple allowed, protrusion allowed, or near-flush required, with numeric limits taken from the approved documents.
- Acceptance basis: applicable industry class and revision, project-specific additions, and which document prevails in a conflict.
These fields must agree across Gerber, drill data, the stack-up table and the fabrication drawing. A hole that carries one diameter in the drill file and maps to a different layer pair in the stack-up table is the most common documentation conflict in fill projects. For the overall data package, the Gerber file preparation checklist applies; fill-specific fields must be declared on top of it.
3. Planarity, dimple and protrusion need a measurement datum
The via mouth involves three distinct quantities: dimple (resin below the copper plane), protrusion (resin above it) and local planarity (height variation across an area). Their limits are not interchangeable, and neither are the measurement methods.
To make the clause executable, state four things: which surface is the datum (adjacent copper, the laminate surface, or the land after capping), what equipment measures it (contact profilometer, optical interferometry, microscopic measurement or microsection), the sampling rule (critical via-in-pad locations, the densest area, board edge versus board center), and the sample size with a batch definition.
The numeric limit itself should come from the approved documents rather than a generic figure. The reason is practical: the tolerable dimple depends on what happens next. If the mouth will be cap plated and then surface finished, the acceptable as-filled dimple is not the same as when solder mask goes directly over it. Align finish types on the surface finish page first, then converge on numbers in the project documents.
4. Void criteria and microsection locations
Internal voids are the substantive risk in fill projects and are entirely invisible to surface inspection. The hazard is not the absolute volume but the location and connectivity. A small void near the mouth may only affect appearance and downstream planarization, while a through-going cavity or one near an interlayer interface can become a path for gas and chemistry during lamination, thermal stress or surface finishing.
Void criteria therefore need three layers of information. First, the quantification basis: how a void is expressed (area ratio in a cross section, maximum single void dimension, or a qualitative clause such as "no connected cavity"), and on which section plane it is evaluated. Second, the microsection locations: chosen for engineering relevance rather than at random, typically the smallest diameter, the highest aspect ratio, via-in-pad areas, one dense and one sparse region, and interlayer interfaces in sequentially laminated or mixed-press structures. Third, quantity and lot rules: coupons per lot, sections per coupon, and the escalation and disposition path when a section fails.
Do not put an unsourced percentage on the drawing. A void limit that does not come from a customer-approved document or a mutually agreed specification becomes a number with no origin the moment it is disputed. The sound structure is: reference the industry class for the level, write project-specific values into the drawing or technical agreement, and state the order of precedence.
5. Interfaces with solder mask, surface finish and SMT lands
Plugging is not an isolated operation; downstream processes redefine what its result means. Three interfaces need joint review.
Solder mask: residual dimple at the mouth makes mask thickness uneven over that area, while protrusion reduces the real margin on dam width and opening size below the drawing nominal. Confirm whether mask openings are defined against the pre-fill or post-fill surface.
Surface finish: an unsealed micro-cavity can draw in and retain finishing chemistry, creating a reliability risk. That is the engineering reason internal voids must be judged before surface finishing, not merely a cosmetic preference.
SMT lands: the point of via-in-pad is that the component land prints and reflows normally. Acceptance there should map to assembly needs (printable land area, planarity) rather than only the plugging operation's own pass line. For inspection methods and controls on the assembly side, see PCB quality inspection methods and the quality assurance page; the governing criteria remain the project documents.
6. Lot evidence and traceability
Fill quality evidence comes in two kinds: process evidence (fill parameters, cure conditions, planarization records) and verification evidence (surface measurement data, microsection images and read-out conclusions). A releasable lot has both, and both map to a specific production lot and panel identity.
SKPCB operates full-chain QR-code traceability from incoming material to finished board, so inspection records can be tied to lots. Which records ship with the order, in what format, and how long they are retained is a project-level agreement that belongs in writing before production, not after. Align the overall manufacturing and inspection scope on the capabilities page.
On acceptance level, SKPCB can organize inspection to IPC-6012 Class II and Class III; the applicable class, revision and additional requirements are set by the project documents. The standard supplies the framework, while fill-specific project values still have to be written down.
7. Release checklist
Confirm before production: filled via locations are designated in a controlled way, with component and tooling holes explicitly included or excluded; fill type is stated (resin or mask, screen or vacuum, complete fill or not); capping requirement and number of sides are stated; the target mouth condition separates dimple, protrusion and planarity and names the measurement datum; equipment, sampling rule and sample size are defined; void quantification, section plane and microsection locations are documented; mask openings are defined against the post-fill surface; internal judgement occurs before surface finishing; via-in-pad acceptance maps to assembly requirements; the scope of process and verification records to be submitted is agreed; the applicable industry class and order of precedence are declared.
With the checklist closed, submit Gerber or ODB++, drill data, the stack-up table, fill layers and acceptance criteria through the quote request so engineering can confirm the route against complete data. Most fill defects cannot be reworked, which makes defining the inspection planes during review considerably cheaper than arguing about data provenance after shipment.
FAQ
Q1: Is "resin filled, must be fully filled" enough on a drawing? No. It is not inspectable. Split it into internal fill condition and mouth surface condition, each with its own measurement method, datum and documented source for the limit.
Q2: Can resin plugging and solder-mask plugging be swapped? No. The material system, whether curing and planarization occur, the function and the acceptance focus all differ, so swapping them mismatches both process route and inspection plan.
Q3: What void limit should the drawing state? Not an unsourced generic number. Reference an industry class such as IPC-6012 Class II or III for the level, and take project-specific void criteria and values from the customer-approved document or a technical agreement, naming the evaluation plane.
Q4: Why must internal judgement come before surface finishing? An unsealed micro-cavity can retain finishing chemistry inside the barrel and create a downstream reliability risk. Sectioning after the finish assigns blame; it does not prevent the failure.
Q5: What governs via-in-pad fill acceptance? Assembly feasibility. Printable land area and planarity for reflow are what matter, so the datum should be the land after capping or surface finish rather than the surface immediately after plugging.