Polyethylene terephthalate (PET) is an interesting flexible substrate material for many applications in electronics since it is inexpensive and has good mechanical and electrical properties. Due to these good properties there is increasing interest in using PET in applications involving harsh environments. However, PET has relatively poor thermal properties, which may cause reliability concerns in demanding conditions. Above its glass transition temperature, Tg, PET is susceptible to hydrolysis, which may considerably restrict its use. Hydrolysis breaks the molecular structure of PET and makes the material mechanically unreliable. This paper reports the reliability of PET films in an LCD display application and in RFID tags in thermal and humid conditions. Additionally, neat PET films were studied. Several thermal cycling and humidity tests were used. The results clearly showed that above the Tg of PET the combined effect of humidity and high temperature caused PET to become brittle leading to considerable reliability problems. However, in dry thermal cycling conditions and in humid conditions below the Tg of PET, the reliability of PET was found to be excellent even under prolonged exposure.
Purpose
– The purpose of this paper is to examine the long-term reliability of an anisotropic conductive adhesive (ACA) attached polyethylene terephthalate (PET) flex-on-board (FOB) assembly for industrial application used in harsh environments. In addition, the possibility of reducing reliability testing time was studied.
Design/methodology/approach
– A−40/+125°C thermal cycling test with 5- and 14-minute soak times was used to study the reliability. To study the functionality of the FOB assembly during testing, a real-time resistance measurement was used together with a 90° peel strength test. Failure analysis was performed on samples using scanning electron microscopy and cross sectioning.
Findings
– No failures or noticeable increase in the measured resistance values were seen during testing. The peel strength, however, decreased significantly with both soak times used. The highest drop in the mechanical strength occurred at the start of the temperature cycling tests. The time spent at the high temperature extreme seemed to have a greater impact on the peel strength than the number of temperature cycles. The failure mode of peel tested samples changed due to temperature cycling from interfacial delamination to cohesive failure. The temperature cycling was also observed to induce voiding inside the adhesive.
Originality/value
– The paper illustrates the applicability of ACA attached PET flex in high reliability industrial applications. Additionally, testing methods for high reliability adhesive interconnections are discussed. Especially, the effect of temperature cycling soak time on peel test results and reliability testing time is studied.
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