An practical study on improving the lifetime of FPC with controlling the viscoelastic properties of adhesives

在传统结构材料疲劳研究工作的基础上，对柔性印刷电路FPC功能紊乱的疲劳可靠性进行了实验研究。特别考察了复合材料的尺寸效应和胶粘剂的力学性能对FPC NG寿命的影响，在应变重复作用下，FPC的电阻Nf值提高到原值的10%。通过循环弯曲试验发现，减薄铜箔和胶粘剂可以改善Nf，而减薄基材的Kapton则相反。在对数-对数图中，Nf与铜的最大应变epsilon呈直线关系，即epsilon * Nf。然后，可以理解为铜导体和聚酰亚胺基的尺寸效应是由于铜的应变，epsilon。同时，从胶粘剂的拉伸性能与Nf的关系来看，为了提高FPC的使用寿命，胶粘剂应同时提供柔韧性和强度。扫描电镜断口形貌分析表明，铜箔的疲劳过程可分为三个阶段：(1)裂纹萌生阶段，(2)裂纹密度增大阶段，(3)裂纹张开阶段。则Nf对应裂纹萌生阶段(1)。此外，实验还发现，通过控制玻璃化转变温度、胶粘剂Tg和超级工程塑料覆盖膜的拉伸性能，可以改变FPC的寿命。通过提高胶粘剂Tg或控制粘弹性，可显著提高FPC的使用寿命。

The fatigue reliability on the functional disorder of flexible printed circuit, FPC was experimentally studied upon the basis of the conventional works on the fatigue of structural materials. Especially, the size effect of component materials and mechanical properties of adhesives on NG life of FPC, Nf that is the electric resistance rising up to 10% of the original value, were examined under strain repetition.It was found by cyclic flexural testing that Nf could be improved by thinning the copper foil and also adhesives but inversely by thinning Kapton of the base material. Nf was related to the maximum strain of copper epsilon as the straight relation of epsilon * Nf in the log-log diagram. Then, it can be understood that the size effects in the copper conductor and the polyimide base are due to the strain of copper, epsilon. While, from the relation between the tensile properties of adhesives and Nf, it is suggested that the adhesive agent should be provided either flexibility and strength in order to improve FPC life.The fractography with SEM showed that the fatigue process of copper foil could be divided into three stages as follows:(1)the crack initiation, (2)the crack density increase, (3)the crack opening. Then, Nf was corresponded to the stage of (1), the crack initiation.Furthermore, it was experimentally found that the lifetime of FPC could be varried through controlling the glass transition temperature, Tg of adhesives and the tensile properties of cover films of super engineering plastic. By increasing Tg of adhesives or controlling its viscoelasstic property, the life time of FPC can be remarkably improved.