PCB007 Magazine

PCB007-July2019

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10 PCB007 MAGAZINE I JULY 2019 Feature by Jerry Magera and J.R. Strickland MOTOROLA SOLUTIONS INC. Abstract Microvias connect adjacent copper layers to complete electrical paths. Copper-filled micro- vias can be stacked to form connections be- yond adjacent copper layers. Staggered micro- vias stitch adjacent copper layers with paths that meander on the layers between the micro- vias. Both microvia configurations are formed by essentially the same sequential operations of laser drill, metallization, and patterning, us- ing various chemical, mechanical, and ther- mal treatments to form each layer, one over the other. Stacked microvias must be filled while staggered microvias do not. Process spe- cifics differ from manufacturer to manufac- turer. Stacked microvias fracture during reflow assembly while staggered microvias do not. Assembly reflow subjects the printed wiring board (PWB) to the greatest temperature ex- cursion. Stacked microvias with a weak inter- face fracture during assembly reflow and are a hidden reliability threat. This phenomenon was reported in IPC-WP-023 [1] in May of 2018. IPC-TM-650 Method 2.6.27A is a performance- based PWB acceptance test that detects frac- tured microvias. SEM pictures are presented to initiate discussions in the search for the root cause. Included are cross-section images of completed microvia structures, SEMs after la- ser drill, and after electroless copper. Not all stacked microvias fail. To learn why, microvia samples were collected from different PWB suppliers. Microvias drilled by UV lasers are compared to microvias drilled by other laser configurations. The images show that microvia structure was influenced by laser type. This ar- ticle discusses the various laser-drilled micro- vias and presents SEM photographs to begin the search for the root cause of weak copper interface. Introduction The search for root cause in this article fo- cuses on laser drill. Not all stacked microvias fail. To learn why, microvia samples drilled by UV lasers were visually compared to microvias drilled by other laser configurations. Samples

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