For via-in-pad, the choice between copper fill and resin fill follows the type of via, not a single BGA-pitch rule. Laser-drilled microvias are usually filled solid with electroplated copper, which gives a flat, conductive pad and is required when microvias are stacked. Mechanically drilled through-hole and buried vias under pads are usually filled with a non-conductive epoxy resin, planarized and plated over with a copper cap, the construction IPC-4761 calls Type VII (filled and capped) and the industry calls VIPPO. In a via in pad copper fill vs resin fill decision, copper fill buys heat conduction and stackability on small vias; resin fill with a cap is the practical choice for larger, deeper holes.
| Copper fill | Resin fill + copper cap | |
|---|---|---|
| Typical via | Laser microvia, blind | Drilled through-hole or buried via |
| How it is made | Electroplating fills the hole | Epoxy plugged, cured, planarized, then plated over |
| Conductive core | Yes, solid copper | No (unless conductive paste is used) |
| Stacking another via on top | Yes | Only on the cap, per design rules |
| Heat conduction | Highest | Through the plated barrel only |
| Main defect to control | Dimple or void in the fill | Cap dimple, resin voids, cap separation |
| Relative cost | Higher per via on deep holes | Lower for through-holes |
What Is Via-in-Pad and Why Does It Need Filling?
A via placed inside a component pad rather than beside it, so the trace can drop straight down. It frees routing space under fine-pitch BGAs and shortens the path to ground for power and RF parts. An open hole in a pad would wick solder away from the joint during reflow and trap air or flux, so the hole has to be filled and covered with a flat, solderable surface.
IPC-4761, the design guide for via protection, defines seven types from tented to filled and capped: Type V is filled, Type VI filled and covered, and Type VII filled and capped with metal (IPC-4761 contents). Type VII is the via-in-pad construction. The type says the hole is filled and capped; it does not say what the fill is, so the drawing must name copper or resin separately. IPC’s revision table lists IPC-4761 (2006) as no longer maintained, but the type numbers remain the common language. For tenting and plugging, see our comparison of tenting, plugging and filling.
When Should a Via Be Copper Filled?
When it is a microvia, when another microvia will be stacked on it, or when it must carry heat. Electroplating fills a small blind hole from the bottom up with special additives; Atotech gives an example of filling a 60 µm diameter, 30 µm deep blind via with a surface dimple under 5 µm using its InPro SAP3 process (Atotech InPro SAP3). That flat, solid copper is what allows one microvia to sit directly on the next in a stacked HDI build.
Copper fill becomes impractical as holes get larger and deeper, because plating a deep through-hole solid takes very long and tends to leave voids. That is why copper fill is mainly a microvia process, and why acceptance for it in IPC-6012F appears as a separate requirement on dimple and protrusion of copper-filled microvias (IPC-6012F contents, Table 3-14). For stacking rules, see our guide to stacked and staggered microvias.
When Is Resin Fill With a Copper Cap the Better Choice?
For drilled through-holes and buried vias under pads, which is most via-in-pad on standard multilayer boards. The process plugs the plated hole with epoxy, cures it, grinds the surface flat and plates copper over the top, so the pad looks and solders like a normal pad.
The resin is a non-conductive, low-expansion filler, so current and heat still travel through the plated barrel. One via-fill resin maker lists a Tg of about 120 °C, expansion of about 40 ppm/°C below Tg and 150 ppm/°C above, hole diameters from about 0.1 to 2 mm depending on aspect ratio, and a cure of at least 45 minutes at 150 °C (Peters PP2795 data). Fill resins are chosen to expand less than ordinary laminate resin so the cap does not crack during reflow.
The cap plating is where quality shows. IPC-6012F gives cap plating requirements for filled holes in Table 3-13 and covers the dielectric fill itself in 3.2.11. Ask for microsections showing the cap thickness, flatness and absence of voids under it.
Is Conductive Fill Better Than Non-Conductive Fill?
Usually not, for through-holes. Conductive (silver- or copper-filled) epoxies conduct some heat and current through the core, but their expansion is closer to the laminate’s than copper’s and they cost more. The plated barrel already carries most of the current, so a non-conductive fill with a good cap is the common default. Choose conductive paste only when a thermal via array needs the extra heat path and the fabricator has a qualified process for it; for real heat conduction in small vias, solid copper fill is the stronger option.
Which Defects Should You Look For?
| Defect | Where | Effect | Check |
|---|---|---|---|
| Dimple | Copper-filled microvia or cap surface | Solder voids, poor stacking | Dimple depth on microsection |
| Void in fill | Resin or copper core | Trapped gas, cap cracking | Microsection |
| Cap separation | Cap to barrel or fill | Open circuit after reflow | Thermal stress, microsection |
| Resin protrusion | Above pad before plating | Uneven pad | Planarity check |
| Thin cap | Cap plating | Cracking, poor solderability | Cap thickness on section |
Most of these show up only in a cross-section, so make microsection evidence part of the order for via-in-pad boards that matter. Aspect ratio drives several of them; our article on microvia aspect ratio shows how it is calculated.
How Should You Specify Via Fill on the Drawing?
| Note | Example |
|---|---|
| Via type and location | “Via-in-pad under U1–U4 per drill file VIP” |
| Construction | “IPC-4761 Type VII, non-conductive resin fill, copper capped” |
| Microvias | “Laser microvias copper filled; stacked per stackup” |
| Planarity | “Cap flush with pad; dimple and protrusion per IPC-6012 Class 3” |
| Evidence | “Microsection of filled vias per lot” |
| Other vias | Tented, plugged or open, stated separately |
Put via-in-pad holes in their own drill file or tool so CAM cannot miss them, and state the class; Class 3 limits on cap plating and dimples are tighter than Class 2.
How Can We Help?
QueenEMS builds via-in-pad with resin plugging and electroplated copper capping (IPC-4761 Type VII, VIPPO), and makes stacked and staggered microvias for HDI boards. Send the stackup with the via-in-pad locations and the fill you want; the quote, back within 2 hours for bare boards, confirms the construction and the microsection evidence we will supply. For the wider HDI picture, see our HDI PCB guide.
Conclusion
| Via | Fill |
|---|---|
| Laser microvia, single | Copper fill (or resin per stackup) |
| Stacked microvias | Copper fill |
| Through-hole via-in-pad under BGA | Resin fill, copper cap (Type VII) |
| Thermal via array under a power pad | Resin fill and cap, or conductive fill if qualified |
| Via outside pads | Tent, plug or leave open |
Decide the fill by via type, name the fill material as well as the IPC type, and ask for microsections. For a quote, send us the Gerber files, the stackup, the via-in-pad drill data and the quantities.
FAQ
When should you use via-in-pad? Under fine-pitch BGAs where there is no room to fan out between pads, and under power or RF parts that need the shortest path to a plane.
What are vias filled with? Electroplated copper for microvias, and non-conductive epoxy (sometimes conductive paste) for drilled vias, usually with a copper cap on top.
What is a copper-capped, resin-filled pad? A via-in-pad whose hole is filled with resin, ground flat and plated over with copper, IPC-4761 Type VII.
Is IPC-4761 Type VII copper filled? Not necessarily. Type VII means filled and capped; the fill can be resin or copper, so the drawing must say which.
Does via-in-pad cost more? Yes. Filling, planarizing and capping add process steps, so use it only where the layout needs it.
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