Study on the effect of fixtures on deformation and warpage of the double-sided flexible printed circuit board through reflow using DIC

Flexible electronics provide new design options not afforded by rigid electronics in a variety of applications including wearable electronics, robotics and automotive systems. However, the processes for the manufacturing of complex electronic assemblies using fine-pitch components are not as well developed as those for rigid electronics. The lack of structural rigidity of flexible printed circuit cards requires the use of fixtures or palletes for component placement and subsequent reflow. In addition, mechanisms are needed to compensate for the deformation and warpage of the flexible substrate and components during assembly. In this paper, two different fixture options have been presented for the manufacture of flexible circuits. The first fixture option includes the use of a pallete with tensioners. The second fixture option includes the use of vacuum fixture during reflow of the flex circuit assembly. The deformation of the flex circuit assemblies during reflow has been studied using a combination of low-speed high-resolution cameras in conjunction with digital image correlation. The double-sided board used for the experiment is of BGA 256-144 combination with dummy components, A-PBGA256-1.0mm–17mm and A-CABGA144-1.0mm-13mm. Firstly, warping of the flexible circuit with only PBGA 256 on the front side was studied, then CABGA 144 was placed on the back side using the pick and place machine, and warping was studied again, both using DIC. Two low speed Point Grey Cameras were used to capture many frames of flexible circuit going through every stage in reflow and a DIC software Vic 3D 2007 was used to get the warpage measurements. Both of the warping data has been presented in this paper and compared with the results in rigid circuit boards.