As BGA pitch decreases and I/O count increases, stacked microvia designs become more prevalent However, there is little data in the public domain on the reliability of these stacked designs. Previous studies done by PWB Interconnect Solutions and EIT showed that there can be a significant reliability degradation of stacked microvias in certain constructions. This project is designed to evaluate 2, 3, and 4 stack microvias both “stand alone” and stacked on buried via and compare the data to a plated through holes (PTH) using Interconnect Stress Testing (IST) and TMA/DMA/TGA testing. The test vehicle will consist of three different thicknesses (.062, .093, and .125) with two different laminate materials and will simulate 0.8/0.4mm and 1.0/0.5mm BGA pitches. The extended process time at high temperatures in multiple lamination designs coupled with multiple assembly cycles and rework cycles may cause stress that are undocumented. Participants in this project will receive statistically accurate data on stacked HDI reliability.
Many industry professionals are worried about the impact of ROHS on long term PWB reliability. With peak temperature differentials of approximately 30 degrees C, laminate parameters like CTE, Tg, and Modulus of elasticity are of major concerns. The stress on vias due to differential expansion and PTH failure modes like foil cracks, post separation, barrel cracks and corner cracks are all worrisome.
The extended process time at high temperatures in multiple lamination designs coupled with multiple assembly cycles and rework cycles may cause stress that are undocumented. This project will attempt to provide statistically accurate data on multiple heat cycle stresses.
The below schematic diagram is a representation of the two different via systems that this project will evaluate
Stacked vias Staggered vias
Evaluate the reliability of multi-stacked microvias in Pb-free
A controlled experiment testing multiple lead free laminate survivablity with a non-lead free control.
Potential specifications are:
4 - lamination cycles
• 6X@288ºC – cross-sections
• 6X@260ºC Reflow – cross-sections
• Other testing? TBD
–Evaluate at 1mm and 0.8mm pitch
• Buried Via Array
• Through Via Array
• Stacked Microvias (2) on Buried Via Array