Identify why the via belongs in the pad
Start by distinguishing a fine-pitch signal escape from a thermal connection under an exposed pad. Both can place a hole within a solderable region, but the land dimensions, solder requirements and review priorities differ. A dense package may need a planar filled and capped structure, while a particular thermal-pad design may follow a different attachment recommendation.
Compare the in-pad option with moving the connection outside the land. An external via may consume routing space or lengthen a connection, but it can simplify the solderable surface. Make the choice using the actual package drawing and layout constraints rather than assuming that in-pad routing is inherently better on every board.
Specify the cross-section instead of an ambiguous shorthand
A useful drawing identifies the hole, fill, copper cap where required, and final solderable surface. Mask tenting does not describe the same result as filling and plating over a hole. Nor does a filled hole automatically define the cap, local flatness or acceptance conditions needed for a component land.
Separate these elements in the fabrication notes and request engineering confirmation of the combined process. If a process description uses a standard terminology, state the intended structure and applicable revision rather than relying on an unexplained abbreviation. The objective is for layout, fabrication and assembly teams to visualize the same physical land before production begins.
| Structure | What it establishes | What still needs review |
|---|---|---|
| Open hole in land | An opening remains in the solder region | Solder migration and package guidance |
| Mask-covered opening | Mask treatment on specified side | Coverage and assembly compatibility |
| Filled hole | Specified material occupies the hole | Cap, flatness and surface acceptance |
| Filled and copper-capped land | Defined metallized solder surface | Process limits and inspection |
| Via outside land | Separate connection and solder area | Routing space and electrical path |
Coordinate paste deposition with the hole treatment
Paste volume and distribution are part of the package attachment design. An exposed-pad stencil may use segmented apertures, while signal lands follow their own geometry. Do not copy a thermal-pad paste pattern onto a different package without checking its attachment guidance. Likewise, changing a via treatment can warrant revisiting the stencil rather than leaving the assembly files untouched.
Texas Instruments discusses solder loss and package-specific thermal-via choices in its QFN developer’s guide. The practical lesson is to review the relevant package and process together; a recommendation made for one land geometry is not a universal via diameter, spacing or filling rule.
Assess thermal and electrical goals separately
A via under a thermal pad is part of a larger heat path through the package, solder, copper, board and surrounding system. The via’s location alone does not establish junction temperature. Similarly, a short electrical connection may still need a suitable return path and a transition model for a demanding signal.
Document the goal that justifies the feature. For thermal use, provide package guidance, estimated dissipation and the intended connection to other copper or a heat sink. For signal escape, identify the relevant net class and routing constraint. This prevents a geometrically convenient solution from being accepted without verifying the performance question that motivated it.
Define inspection before hidden joints become inaccessible
Once a component is assembled, the land and solder joint may be difficult to inspect directly. Establish the required fabrication and assembly evidence before release. The plan may include verification of the specified filled structure, appropriate assembly inspection and a functional or electrical check suited to the device.
Do not equate an inspection method with an unlimited ability to detect every defect. Ask what feature will be evaluated, how acceptance is defined and which records identify the board revision. If the design is new or the process has changed, use the prototype stage to resolve the specific uncertainty rather than assuming an unchanged netlist means unchanged assembly risk.
Create one coordinated fabrication and assembly package
Highlight the affected lands, identify the via type and add a section view when useful. Include package drawings, mask and paste layers, and any special acceptance note. Keep the board and stencil revisions aligned. If only selected vias require special treatment, identify them clearly so the instruction does not unintentionally apply to unrelated holes.
Review via terminology, BGA assembly and stencil requirements alongside the capability directory. Use Request a Quote to request an engineering review of the actual in-pad construction. Availability, geometry and acceptance must be confirmed for your project rather than inferred from this explanation.
Via-in-pad review package
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Frequently asked questions
Must every thermal via under a pad be filled and capped?
No universal rule applies. The component's attachment guidance, pad geometry, solder process and acceptance requirements determine the appropriate treatment. Confirm the specific package and construction before release.
Can solder mask replace a copper cap?
It does not create the same structure. Mask coverage, hole filling and copper capping perform different roles, and the required solderable surface must be defined explicitly.
Does via-in-pad always improve thermal performance?
The total heat path determines thermal behavior. Evaluate the package, copper, board, interfaces and cooling conditions; placement of a via by itself is not a verified thermal result.
What files should change when via treatment changes?
Review fabrication notes, via identifiers, mask and paste layers, stencil instructions and inspection requirements. Update the affected files together and preserve the approved revision relationship.
Can an assembly photograph prove the buried structure is correct?
Usually it cannot establish the complete internal construction. Define the relevant fabrication and assembly evidence in advance, including the method and acceptance conditions for the feature being checked.