About this document
Glass vias metallized by double-sided sputtering bond only at the entry and exit, leaving an adhesion gap in the middle. This Technical Note uses a Corning patent and thermal cycling literature to examine how that gap becomes a damage path under repeated thermal deformation, and compares it with a PDA full-length bonding structure that extends the bond to the via center.
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Summary
Even in a fully filled copper via, if the middle of the inner wall is not bonded, the load from the copper–glass thermal expansion mismatch concentrates at the bond boundaries. In a Corning patent's thermal analysis, full adhesion raised the largest crack-free cylindrical via diameter from 25 μm to 30 μm, and 3,000-cycle thermal cycling literature showed that interface delamination and glass cracking can occur while resistance stays stable.
Conclusion
In vias filled by plating after double-sided sputtering, the top and bottom ends provide the metal connection and anchoring that secure initial conduction and shape. But because sputtering does not reach the deep inner wall, an adhesion gap remains in the middle of the via even after filling. Copper and glass expand differently: on heating (125 °C) copper expands and protrudes, creating shear and tensile stress at the bonds, and on cooling (−55 °C) the interface restrains copper contraction, repeating delamination and shear loads at the bond boundaries (Demir et al. 2017, via-end FEM). With the middle unbonded, this repeated load concentrates at the boundaries of the unbonded section. Corning patent US11171094B2 compares the same fully filled cylindrical via with only the adhesion region changed: partial adhesion, bonded at the top and bottom with no adhesion layer in the middle, and full adhesion, with the adhesion layer continuing through the middle. In the patent's Table 1 thermal analysis predictions, the largest crack-free via diameter was larger with full adhesion for all three shapes: cylindrical 25 to 30 μm (a 20% increase), tapered 17 to 19 μm and narrow-center 25 to 27 μm. The stress analysis (Fig. 12–13) likewise showed partial adhesion concentrating stress near the center and surface, while full adhesion spread thermal deformation through the thickness over the longer bonded length. Demir et al. (IEEE TCPMT, 2017) thermally cycled 24 daisy chains (192 vias) of 30 μm diameter conformal vias in 100 μm glass for 3,000 cycles at −55 to 125 °C with 15-minute dwells. Room-temperature four-wire resistance stayed stable, yet cross-section SEM of the same samples showed copper–glass interface delamination around via entries and surface wiring and glass cracks at via corners. The deep inner wall away from the via ends showed no delamination or cracking, which was attributed to low local stress and retained sidewall adhesion. Long-term life must therefore be judged on the initiation and growth of delamination and cracking, not resistance alone. Cools proposes PDA full-length bonding, which forms a continuous glass–PDA–copper interface all the way to the via center. Bonding the deep inner wall removes the central adhesion gap and restrains copper from separating from the glass, and the load that concentrated at the ends of the unbonded section is instead carried along the full-length interface, a design intended to suppress the initiation and spread of interface damage. This comparison is a design direction based on the Corning patent's analysis and literature observations; thermal cycling measurements of the PDA full-length bonding structure are not included in this document.
References
- 1. Corning, US11171094B2 (2021)
- 2. Demir et al., IEEE Transactions on Components, Packaging and Manufacturing Technology 7(6), 829–837 (2017)
Topics
- Glass substrates
- Plating
