mSAP, or modified semi-additive processing, builds copper features from a thin starting conductive layer through patterned plating and subsequent removal of unwanted seed copper. It is a fabrication approach, not a synonym for every HDI board. A design should call for it because the required conductor geometry and process fit justify the choice.
Compare the conductor formation route
In a conventional subtractive approach, unwanted copper is etched away from a thicker starting layer. In a semi-additive family of processes, conductor formation relies more heavily on patterned deposition. The exact stackup, starting layer and sequence depend on the selected technology and supplier.
These differences can matter when fine traces and spaces are difficult to form consistently through the proposed subtractive route. They do not create one universal minimum dimension. Ask the fabricator for reviewed finished geometry, tolerances and evidence for the actual copper thickness and material system.
Questions for a process review
| Design input | Question to resolve |
|---|---|
| Trace and space targets | Are these finished dimensions or artwork dimensions? |
| Copper thickness | How does the proposed process achieve the required conductor cross-section? |
| Material construction | Is the surface and stackup compatible with the selected process? |
| Interconnections | How do plated conductors and via structures join within the approved sequence? |
Keep electrical and manufacturing models aligned
Fine features can increase routing density, but conductor thickness, sidewall shape and nearby reference structures still influence electrical performance. Use the expected finished cross-section in the appropriate model. Do not simply shrink a previous trace-width rule while keeping every other assumption unchanged.
Atotech describes equipment for HDI and mSAP processing. That establishes these as industrial process contexts; it does not establish ownership of such equipment or mSAP capability by this site's operator. Any supplier claim should be evaluated against current production evidence.
Decide when it is justified
Try a representative routing region with the proposed geometry and compare it with an achievable alternative. Record which nets or package rows require the finer process and whether another layer allocation or component package could solve the problem.
If mSAP remains the selected route, preserve the approved construction, acceptance requirements and change-control rules in the manufacturing package. Ask what changes would require renewed qualification or electrical verification. Procurement should compare suppliers on that same defined process rather than on the presence of “mSAP” in a capability list. Company-specific process availability, minimum dimensions and pricing are [NEEDS CLIENT INPUT].
Continue the engineering review
- HDI PCB manufacturing requirements
- Fine-pitch BGA fanout with microvias
- HDI PCB materials selection
- HDI PCB engineering hub
Source checked October 2, 2026: Atotech HDI and mSAP processing equipment. Supplier literature is context, not a claim of this site’s equipment or capabilities.
Review your HDI requirements
Share the board revision, proposed stackup, quantity and open questions. Include the requirements discussed in this guide so the project review starts with the same assumptions.