The conclusion first: the way a connector mounts decides which process step owns its requirements. "Make this hole accurate" means three different things. For a surface-mount connector it is about pad geometry and local flatness. For a through-hole solder connector it is about drilling, hole-wall copper and solderability. For a press-fit connector it is about the finished hole diameter and the hole-wall copper after plating — results that belong to plating, not to drilling.
That last point is where most translation errors happen. A connector datasheet states a finished hole size. If the fabrication drawing lists a single hole diameter without saying whether it is the drill size or the finished size, the fabricator reads it one way, the designer meant the other, both hit their own target, and the connector still will not go in.
So this article does one thing: map the three mounting styles onto the manufacturing requirements they actually create. Which datasheet numbers to transcribe as-is, which not to, who owns positional relationships, and how these holes should be verified.
1. Where each mounting style lands
Surface-mount connectors. No holes involved. Everything lands on the pad side: pad geometry and its relationship to the solder mask opening, the solderability of the surface finish, and the local flatness of the area the connector sits on. Local is the operative word. Board-level flatness and the condition of one small footprint area are not the same measurement. A board that passes overall can still let the ends of a long connector lift. What needs to be communicated is "this area must be flat," not a copy of the whole-board figure.
Through-hole solder connectors. Holes are involved, but their job is to accept a pin and let solder wick up. Requirements land on pin-to-hole clearance, annular ring width, continuity of hole-wall copper, and coverage of the surface finish inside the hole. Tolerances here are comparatively relaxed because solder fills the clearance — the clearance is part of the design.
Press-fit connectors. Holes again, but their job has changed completely. The hole wall must form a lasting mechanical and electrical contact with the pin, with no solder to make up the difference. Two requirements now dominate, and both only exist after plating:
- the finished hole diameter, meaning what remains after copper plating and surface finish;
- the integrity and continuity of the hole-wall copper, because the pin scrapes that wall on insertion and the copper has to survive it without being pushed aside or peeling.
Treating either as a drilling matter assigns the requirement to the wrong process step. Drilling is only the starting point: hole-wall preparation, electroless copper, electroplated copper and then surface finish each change the final diameter and wall condition. The plating-side control logic is covered in electroplating quality and release criteria; this article only establishes ownership.
2. Which datasheet numbers to transcribe
What a connector datasheet says about the board falls into three groups, handled very differently.
Group one — transcribe as-is. Finished hole diameter range, the requirement that the hole be metallized, minimum pad or annular ring requirements, and restrictions on surface finish type (some press-fit connectors exclude certain finishes). These are direct constraints the connector vendor places on the board. Copy them into the drawing and state explicitly that they are finished dimensions.
Group two — convert or qualify; do not copy. Datasheets commonly show a recommended land pattern, recommended silkscreen and recommended mask opening. Those are suggestions, not manufacturing requirements — they have to become actual geometry in CAD first, and the geometry is the manufacturing input. Some datasheets give a board thickness range, but board thickness is also tied to your own stackup and impedance targets and cannot be set by the connector alone.
Group three — does not belong on a board drawing. Insertion force, retention force, withdrawal force, contact resistance, mating cycles. These are connector and assembly-process performance figures, not board manufacturing requirements. A fabricator can neither achieve nor verify "retention force"; it can only guarantee that hole diameter and wall condition fall inside the stated requirement. Putting such figures on a PCB drawing leaves the fabricator unable to commit while the customer believes the requirement has been placed.
One test covers it: anything measurable only after the pin is pressed in is not a PCB manufacturing requirement; only what can be measured on the bare board as it ships belongs on the drawing.
3. Multi-position connectors: who owns alignment
An accurate individual hole does not mean the connector will fit. A multi-position connector engages the collective position of a row of holes; one hole out of place loads the whole pin row unevenly. Two separate things are at work:
- hole-to-hole relative position, determined by the drill program and the repeatability of the drilling step;
- the position of the hole group relative to the board outline and other datums, determined jointly by datum definition and profiling.
The second is the one that gets missed. If the connector must also line up with an opening in an enclosure panel, then "hole group relative to board edge" matters more than "pitch within the hole group" — and that has to be stated with explicit datums rather than left for the fabricator to infer. How tolerances are written on drawings treats this subject directly; the reminder here is simply that any hole mating with an external part needs its datum spelled out.
4. Verification is not a single diameter measurement
Measuring one hole diameter answers "how big is the hole," not "is the hole usable." Verification focus shifts with mounting style.
Through-hole solder holes: hole-wall copper continuity and solderability. Voids or breaks in the wall copper show up at soldering as incomplete fill or a cold joint inside the barrel. These need cross-sectioning and in-hole examination; the methods are covered in drilled hole quality inspection.
Press-fit holes: three things at once.
- Finished hole diameter, sampled across different areas of the panel — plating distribution is never perfectly uniform, so measuring one location assumes uniformity rather than confirming it.
- Hole-wall copper thickness uniformity and adhesion, visible in cross-section and invisible to optical inspection. The entire reliability of a press-fit joint rests on that copper not being pushed aside by the pin.
- Hole-wall morphology — whether the roughness, burrs and resin smear left by drilling were properly covered by plating, which directly affects how the pin loads the wall on insertion.
Surface-mount footprint areas: the object is not a hole but flatness and pad condition, measured over the area the connector actually sits on. A board can pass overall warpage and still fail a long connector locally.
One point that gets overlooked: verification requirements have to match what the drawing says. If the drawing states only a finished hole diameter, a hole-wall copper thickness report cannot suddenly be demanded at incoming inspection. State it on the drawing, and the fabricator will build the corresponding inspection and release step into the flow.
5. Problems the board cannot solve
Some assembly problems persist no matter how accurately the holes are made:
- Mismatched pin and hole selection. A press-fit pin and its hole diameter range are a matched pair. Choose an incompatible combination and the joint will be poor even with the hole dead on nominal.
- Board thickness outside the connector's range. Press-fit pins have an applicable board thickness range; outside it the engaged contact length is wrong. That is a selection issue, not a manufacturing one.
- Stress from assembly sequence. Whether the connector goes in before or after the screws, and where the board is supported, changes how the board is loaded during insertion. This belongs to assembly process engineering, and no increase in hole accuracy will change it.
- Panel opening and hole group not lining up. This is a dimensional chain problem that needs margin allocated during mechanical design; it cannot be pushed entirely onto fabrication accuracy.
Identifying these early avoids a long loop of repeatedly asking the fabricator for tighter tolerances while the symptom stays the same.
6. What a complete handoff to the fabricator contains
Following the division of responsibility above, the drawing and notes should carry at least:
- a purpose label for each hole class (solder hole / press-fit hole / mounting hole / tooling hole) — different purposes carry different requirements, and one hole table with diameters alone is not enough;
- hole diameters explicitly marked as finished dimensions, given as a range rather than a single value;
- whether the hole wall must be metallized, and any separate requirement on in-hole copper;
- the surface finish type, and whether the connector excludes any finish;
- explicit datums and positional requirements for hole groups that mate with external parts;
- local flatness requirement and its measurement area for surface-mount connector footprints, if any;
- which verification records must ship with the order.
Every item on that list is something measurable, judgeable and recordable before the board leaves the factory. Insertion force and retention force stay with assembly process validation and off the PCB drawing. For the process window around connector holes and pads on two-layer boards, start from double-sided PCB; tolerance notation is covered under tolerance, verification and release under quality, and surface finish and base material choices under materials.
FAQ
Q: Should the drawing state the drill size or the finished size?
A: For connector holes, state the finished size and say on the drawing that it is the finished size. The fabricator works back to a drill size from its own plating distribution — that is a process decision. A bare number with no qualifier is the single most common source of this class of problem.
Q: What is the fundamental manufacturing difference between a press-fit hole and a through-hole solder hole?
A: In a solder hole the clearance is filled by solder, so diameter tolerance is comparatively relaxed. In a press-fit hole the wall copper contacts the pin directly, mechanically and electrically, with nothing to make up the difference, so both finished diameter and wall copper condition determine reliability. One hole accommodates; the other contacts.
Q: Do insertion force and retention force belong on the PCB drawing?
A: No. Both can only be measured after the pin is inserted, so a fabricator can neither guarantee nor verify them in isolation. What the board side can commit to is finished hole diameter and hole-wall condition. Insertion and retention force belong to assembly process validation.
Q: A multi-position connector will not fit the panel opening. Is that a hole position error?
A: Not necessarily. First separate "relative position within the hole group" from "hole group relative to the board outline." The latter involves datum definition and profiling, and is often a dimensional-chain margin problem — panel opening, board outline and hole group tolerances have to be calculated together rather than pushed onto hole position accuracy alone.
Q: The ends of a surface-mount connector lift, but the board passes its warpage spec. Is that a contradiction?
A: No. Whole-board warpage and local flatness over the connector footprint are two different measurements, and long connectors are especially sensitive to local unevenness. The fix is a separate flatness requirement for that area with a stated measurement region, not a tighter whole-board figure.
Q: Can a hole-wall copper thickness report be requested at incoming inspection if the drawing does not ask for one?
A: It can be asked for, but writing it into the drawing up front is far more reliable. With the requirement on the drawing, the fabricator schedules the corresponding inspection and release action and keeps records. Requested afterwards, you often get only that lot's sampling result, and there may no longer be a sample available to cross-section.
Q: Does drilling roughness and burr still matter after plating?
A: Yes. Roughness, burrs and resin smear from drilling get covered by later steps but do not disappear. They change how well the wall copper adheres and how the pin loads the wall on insertion. That is why press-fit hole verification looks at wall morphology, not just the diameter number.