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16 PCB007 MAGAZINE I OCTOBER 2023 The Material Science of PCB Thermal Reliability Printed circuit board (PCB) reliability test- ing is generally performed by exposing the board to various mechanical, electrical, and/ or thermal stimuli delineated by IPC stan- dards, and then evaluating any resulting failure modes. ermal shock testing is one type of reliability test that involves repeatedly expos- ing the PCB test board to a 288°C pot of mol- ten solder for a specific time (typically 10 sec- onds) and measuring the number of cycles it takes for a board's copper layer to separate from the organic dielectric layer. If there is no delamination, fabricators can rest assured that the board will perform within expected tem- perature tolerances in the real world. From a materials perspective, what is hap- pening to the copper and dielectric during thermal shock testing? What causes copper to separate from the organic dielectric layer in the first place? To answer these questions, it is necessary to take a closer look at each component at the molecular level during testing. Subjection to intense thermal fluctuations causes different rates of expansion and contraction in different materials. is can generate stresses and ulti- mately lead to cracks or delamination between the copper and dielectric parts of the test board. e coefficient of thermal expansion (CTE) is defined as a material's dimensional change in length over a measured temperature range. Typical CTEs are listed in Table 1. Cop- per exhibits a Z-axis CTE of ~18 ppm/°C from 20–25°C while organic materials like epoxy Material Insight Feature Column by Preeya Kuray, Ph.D., AGC MULTI MATERIAL AMERICA

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