Bottom line first: the yield of a castellation module board is not decided at drilling but at routing. The offset from hole centre to board edge, the number of passes of the rout slot and the residual burr on the barrel copper together set solder strength and cosmetic yield. Fixing those three in the drawing and the acceptance standard is far more effective than iterating parameters at sample stage, because castellation is a structure that a wrong design rule cannot recover once volume production starts.
What the process actually is
Castellation cuts a plated through hole along its centre line so that half of the barrel copper is exposed as the solderable face between module and motherboard. A typical flow runs: drilling, copper deposition and panel plating, via fill or open hole, imaging and solder mask, surface finish, profile routing that splits the hole, then deburr and cleaning. Control is lost at the last two steps. The mechanical stress of routing pulls and lifts the barrel copper, and chemistry trapped inside the open half hole causes solder blowholes during the subsequent reflow.
Four numbers to fix at design stage
- Hole diameter 0.6–1.0 mm: below 0.6 mm the barrel copper share is too low and solder strength is insufficient; above 1.0 mm the hole eats edge space on the module and chip evacuation during routing gets worse.
- Hole centre to board edge equals hole diameter / 2, tolerance ±0.05 mm: a 0.05 mm offset puts the cross-section about 10% off centre, one side keeps less than half its copper, and pull-off strength drops distinctly.
- Castellation pitch ≥0.8 mm: when the pitch is too small, adjacent barrels pull on each other during routing, which invites barrel tearing and solder bridging.
- Annular ring width ≥0.15 mm: an insufficient ring leaves too little copper exposed by the cross-section, and wetting after reflow is poor.
Three things the process must control
Route the slot in two passes. Cutting to final size in one pass is efficient, but cutting force concentrates and the risk of copper lift-off is high. For volume, rough out with 0.1 mm of stock left and finish to size in the second pass; a φ2.0 mm router with a 30–40k rpm spindle and 1.5–2.0 m/min feed holds the barrel tear rate very low. The rout path must also stay clear of the V-CUT line, because where the two overlap the castellation is torn away at depaneling.
Residual burr ≤50 μm. Routing must be followed by chemical softening plus mechanical brushing; burr above 50 μm lifts the component and forms a cold joint. Sample with 20× magnification or a cross-section, and look at the two cutting edges above and below the half hole rather than only inside the bore.
No chemistry left in the hole. Routing is a wet operation, and an open half hole wicks chemistry by capillary action. Clean with multi-stage ultrasonic plus high-pressure water rinse, and run an ionic contamination test before drying, holding it within 1.56 μg/cm² NaCl equivalent. Residual acid or plating bath bleeds out of the barrel during reflow and shows up as darkened joints and poor wetting, usually across a whole lot.
Surface finish and acceptance baseline
The castellation cross-section exposes bare copper, which oxidises readily after multiple reflows, so ENIG is the preferred finish: nickel 3–6 μm, gold ≥0.05 μm, good planarity and a long solderability window. Immersion tin is cheaper but withstands repeated reflow poorly, and suits only products that see a single reflow. Control barrel copper at 20 μm minimum and confirm by cross-section that there are no plating voids or copper separation. For reliability acceptance, run 1000 thermal cycles from −40 to 125 °C plus a castellation pull-off test, and freeze the result as the volume baseline instead of renegotiating it lot by lot.
Three things to do before ordering
- List hole diameter, edge distance, pitch and annular ring as a dedicated castellation structure table and submit it with the Gerber, so they do not live only in a note.
- In the panel drawing, confirm the position of the rout path relative to the V-CUT and the stamp holes, and leave enough bridge width so that depaneling does not tear the barrel.
- Ask the factory for first-article cross-section photos and ionic contamination data as the reference baseline for lot acceptance.
Where the cost actually sits
Most of the price difference between suppliers on this structure comes from the routing step and from the inspection discipline behind it, not from laminate. A supplier who quotes two passes, a controlled burr limit and an ionic contamination record is quoting a different process from one who quotes a single pass and a visual check. That difference is worth comparing line by line before comparing unit price.
Castellation is a structure where a design-rule error cannot be rescued in volume, so the safest route is to align every structural parameter with the fabricator before prototyping. AIPCB.cc supports 1–44 layers, a maximum panel size of 610×1100 mm, ±8 μm line-width accuracy and 24-hour expedited prototyping. For castellation module boards, send the stackup and panel drawing to info@ipcba.com.cn or call +86 138 2310 9230 for a manufacturability pre-review.
Send your Gerber files and stackup requirements; a 24-hour expedite channel is available.