High Frequency Base Materials: Laser-Material Interaction for HDI/ICP Applications
Authors: Ryder Christopher , Lott Geoffrey , Chen Ruolin , Falletto Nicolas 
Published: December 2018

Abstract
High frequency (HF) applications are nothing new. Nor are the multitudes of materials and processes in electronics dedicated to such applications. But with the rising applications and production volumes associated with 5G, IoT (Internet of Things), autonomous driving, and the like, a whole new emphasis is needed to characterize expected base material performance in terms of its processability. Laser-based manufacturing, and in particular laser via drilling, plays a large role in the interconnect and packaging concepts needed for the typically miniaturized design rules, but given the diversity of materials and laser types that can potentially be employed, some clear results-driven guidelines would be beneficial to manufacturers looking to optimize a cost/quality/throughput balance.
In this paper we explore laser-based via drilling applications with base materials intended for use in high frequency end applications. There are several HF base material sets employed in HDI (High Density Interconnect)/ICP (Integrated Circuit Packaging) manufacturing, each with their own range of bandwidth and applicable dielectric constant. However, agnostic of the particular material application specifications, the aim of this analysis is to explore potential manufacturing benefits and/or trade-offs for diverse laser/material interactions, while focusing on via-drilling for the HDI and ICP industries in particular. Benefits and trade-offs are characterized by practical and quantifiable elements, precisely: throughput (vias per second), quality (roundness, burr, dimensional integrity, etc).
In combination with various laser types, in particular UV (nanosecond), CO2 (microsecond) and green (femtosecond) wavelengths, we analyze the fundamental interaction with materials common to high frequency applications, namely LCP (Liquid Crystal Polymer), PTFE (Polytetrafluoroethylene), HF FR4, and PI (Polyimide) in an effort to attain practical guidelines for quality and manufacturability. Thus, evidence is provided to potentially increase manufacturing yield with a typically cost-inhibitive material set. Given the, as yet, novel usage of such materials for broadband HDI/ICP applications, this work aims to explore new or re-affirmed baselines suitable to the existing production landscape. Given the broad scope of potential DoE parameters, this work focuses on target via size (75um) applicable to multiple industrial applications, including but not exclusive to, handheld communications, automotive and IoT. Furthermore, the materials is clad with a 12um Cu foil to offer insight into ablation capabilities of each laser type. As a test vehicle default, the vias are drilled in a standard BGA (Ball Grid Array) grid with a pitch of 0.6mm.
The results of this work offer-a scored matrix breakdown of our predetermined criteria (throughput, quality) to the laser-material subset analyzed. Given the non-exhaustive nature of this study, the conclusions do not aim to resolve laser-material dilemmas for all forms and factors of high frequency applications and material configurations, but rather offer conclusive evidence that a) high-frequency materials typically require special attention when processing with laser, b) not every laser type works at the same rate of efficiency and efficacy for each of the chosen HF materials and c) a cost/quality balance can be sought by cross referencing results from various laser sources for the intended application.