Copper paste is widely used for via filling in HDI PCBs, TGV substrates, multilayer circuits and advanced electronic packaging. However, one common manufacturing problem is that a via appears completely filled before curing, but becomes recessed, develops voids or loses volume after curing.

Why does this happen? In most cases, copper paste shrinkage is related to solvent evaporation, organic binder changes, copper particle packing, trapped air and an unsuitable curing process. Understanding these mechanisms can help manufacturers achieve more stable and reliable via filling.

1. Solvent Evaporation Is a Major Cause of Copper Paste Shrinkage

Conventional conductive copper pastes normally contain copper particles together with solvents, binders and other organic components.

Before curing, these components occupy part of the total paste volume. During drying and thermal curing, volatile solvents gradually evaporate. The material remaining inside the via therefore occupies less volume than the original wet paste.

This is one reason a via that looks completely filled immediately after printing may become lower than the surrounding surface after curing. This recessed surface is commonly described as dishing.

2. The Binder Changes During Curing

Copper paste is not simply loose copper powder. A binder system holds the particles together before and during processing.

As temperature increases, the binder may polymerize, decompose, crosslink or otherwise change its structure depending on the paste formulation. These changes can cause dimensional changes inside the filled via.

Some controlled shrinkage is not automatically a defect. In conductive materials, moderate densification can bring conductive particles into closer contact and help form a continuous conductive network. The real problem is excessive or uneven shrinkage.

3. Copper Particle Packing Affects Final Volume

The size, shape and distribution of copper particles strongly affect how densely the paste can fill a microvia.

If the particles are poorly distributed, large spaces can remain between them. During curing or sintering, the particles move closer together and the structure becomes denser, resulting in measurable dimensional change.

A carefully designed particle-size distribution can allow smaller particles to occupy spaces between larger particles, helping improve packing density while maintaining suitable paste rheology.

4. Trapped Air Can Be Mistaken for Shrinkage

Not every recessed or incomplete via is caused purely by chemical shrinkage.

Air can become trapped inside deep or high-aspect-ratio vias during printing, dispensing or squeegee filling. After heating, the trapped gas can expand or migrate, leaving internal voids.

From the surface, the defect may look similar to material shrinkage, but the root cause is incomplete filling.

5. An Incorrect Curing Profile Can Make Shrinkage Worse

Heating the paste too quickly may cause volatile components to escape before the material can stabilize. This can increase the risk of bubbles, internal voids or surface collapse.

On the other hand, insufficient curing may leave the conductive network incomplete and lead to unstable resistance or weak mechanical properties.

The recommended curing temperature, heating rate and holding time should therefore be matched to the specific copper paste formulation and substrate.

What Problems Can Excessive Via Filling Shrinkage Cause?

How to Reduce Copper Paste Shrinkage During Via Filling

Use a High-Solid or 100% Solid Formulation

Reducing volatile solvent content is one of the most direct ways to reduce volume loss during curing. A high-solid formulation leaves more functional material inside the via after processing.

Optimize Paste Rheology

The paste must be fluid enough to penetrate the via while maintaining sufficient stability to prevent particle sedimentation or material flow-out. Suitable thixotropy and viscosity are especially important for fine and high-aspect-ratio vias.

Remove Trapped Air

Degassing the paste and using vacuum-assisted processing can reduce the possibility of internal air pockets.

Consider Multi-Step Filling

For deep vias, one-pass filling may not always provide sufficient filling density. A controlled fill, dry and refill process can compensate for volume changes and improve final via geometry.

Follow the Recommended Cure Profile

Temperature should be increased according to the material's recommended process window rather than simply applying the highest possible temperature. Controlled curing helps organic components evolve more gradually while supporting the formation of a stable conductive network.

What Should You Check When Choosing a Via Filling Copper Paste?

Instead of looking only at initial conductivity, engineers should evaluate the complete filling process.

Frequently Asked Questions

Final Thoughts

Copper paste shrinkage during via filling is not caused by one single factor. It is the combined result of paste formulation, volatile content, binder behavior, particle packing, trapped air, via geometry and the curing process.

For HDI, TGV and advanced packaging applications, controlling these variables is essential for achieving dense filling, low resistance and long-term reliability.

Choosing a copper paste specifically designed for via filling can significantly simplify process optimization and reduce defects such as dishing and internal voids.