Reliability Studies of Flexible Polyimide Substrates with Flip-Chip Attachments and Electrical Test Structures
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Miniaturization of modern electronic devices has greatly increased the demand for advanced substrates for high-density applications, fine pitch attachment processes, and high-speed applications. Flexible polyimide (PI) PCBs have been popular in electronics packaging due to their pliability and good reliability at elevated temperatures. However, high moisture absorption of PI films may cause reliability issues in harsh conditions. Therefore, it is critical to understand the behavior of PI PCBs under such conditions. The aim of this work was to study the reliability of PI substrates using three accelerated life tests (ALTs) to examine possible reliability issues of PI under demanding conditions. A thermal cycling (T/C) test (-40 °C/125 °C) was performed for 5000 cycles. A high humidity test (85 °C / 85% RH) was performed for 4000 hours and the effect of corrosive humid environment was studied in a salt spray test for 4500 hours. Four test structures were used to study the reliability of the PI substrates. The reliability of flip chip attachment was studied using anisotropic conductive adhesive (ACA). The reliability of vias in PI substrate was also studied. Additionally, the stability of the dielectric properties of PI was studied using a parallel plated capacitor structure. Finally, meander structures were used to study the reliability of wiring on the PI substrates. The results showed that copper wirings on the PI PCB were highly reliable under temperature cycling and humid conditions. This revealed that 40% failures of the via struc-tures in the thermal cycling test were most likely caused by the via structures instead of the wiring in between them. However, the vias were not affected in the humid condi-tion. This showed that application of PI film for multilayer PCBs, where the vias are needed to interconnect the layers, might undergo reliability issues under service condi-tions with temperature cycling. However, PI film for multilayer PCBs would show high reliability under humid conditions. Moreover, the PI film was found to be a reliable substrate for chip attachment applications under temperature and humidity stresses. That is, flexible PI substrate are promising PCBs for fine pitch application. In addition, PI film as a dielectric was found to exhibit better electrical properties with lower dielectric constant after exposure to temperature cycling condition. However, high moisture ab-sorption of PI deteriorated the dielectric property of PI film by increasing its dielectric constant value. This showed that the PI substrates in high-speed applications are reliable under service conditions with cyclic temperature stresses. However, the functionality of the PI PCB may be declined in humid conditions. In this study, the harsh condition of the salt spray test considerably deteriorated the reliability of all PI test structures.