posted on 2024-01-04, 08:03authored bySagar M. Doshi, Alexander Barry, Alexander Schneider, Nithin Parambil, Catherine Mulzer, Mobin Yahyazadehfar, Aref Samadi-Dooki, Benjamin Foltz, Keith Warrington, Richard Wessel, Lei Zhang, Christopher Simone, Gregory S. Blackman, Mark A. Lamontia, John W. Gillespie
As
the need for high-speed electronics continues to rise rapidly,
printed wiring board (PWB) requirements become ever-more demanding.
A typical PWB is fabricated by bonding dielectric films such as polyimide
to electrically conductive copper foil such as rolled annealed (RA)
copper and is expected to become thinner, flexible, durable, and compatible
with high-frequency 5G performance. Polyimide films inherently feature
a higher coefficient of thermal expansion (CTE) than copper foils;
this mismatch causes residual thermal stresses. To attenuate the mismatch,
silica nanoparticles may be used to reduce the CTE of PI. A nodulated
copper surface can be used to enhance the Cu/PI adhesion by additional
bonding mechanisms that could include a type of mechanical bonding,
which is a focus of this study. In this investigation, a 90°
peel test was used to measure the peel strength in copper/polyimide/copper
laminates containing nodulated copper and polyimide reinforced with
0, 20, and 40 wt % silica nanoparticles. The influence of silica nanoparticles
on the peel strength was quantitatively evaluated. Laminates incorporating
polyimide films lacking silica nanoparticles had a ∼3.75×
higher peel strength compared with laminates reinforced with 40% silica.
Their failure surfaces were analyzed by using scanning electron microscopy
(SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron
spectroscopy to identify the mode of failure and to understand bonding
mechanisms. The key bonding mechanism, mechanical interlocking, was
achieved when the polyimide surrounded or engulfed the copper nodules
when the laminate was created. Post-testing failure surface analysis
revealed the presence of copper on the polyimide side and polyimide
on the copper side, indicating mixed mode failure. An analytical
model was developed to determine the impact of applied pressure, temperature,
and time on the polyimide penetration and mechanical interlocking
around the copper nodules. The model was validated by measuring the
peel strength on another set of specimens fabricated using increased
temperature and pressure that showed a 3× increase in peel strength
compared to lower temperature/pressure processing conditions. This
enhanced adhesion resulted from the lower polymer material viscosity
at higher temperatures, which fosters deeper and more complete penetration
around the copper nodules during processing at higher pressures for
longer durations. The methodology of combining peel testing, viscosity
and CTE measurement, SEM/EDX, surface chemical analysis, and penetration
depth calculation developed herein enables the calculation of the
desired processing parameters to enhance functionality and improve
adhesion.