PSI - Issue 18

Alexandru Falk et al. / Procedia Structural Integrity 18 (2019) 214–222 Falk A./ Structural Integrity Procedia 00 (2019) 000–000

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Keywords: Digital image correlation, PCB, strain, gauge rosette;

1. Introduction

Printed Circuit Boards (PCBs) are designed to mechanically support and electrically connect an electronic component assembly. Control of strain is very important due to the smaller dimensions of PCBs, in present in the automotive industry the limit of 700 microstrain is allowed. Admissible strain is becoming smaller due to smaller PCB sizes, shifting to environmentally friendly materials and more fragile due to transition from Lead Solder (SnPb) to lead-free solder paste, the use of increasingly complex electronic components. Thereby, admissible strain is becoming lower. PCB bending due to the screw assemblies, differences of surface level on which the PCB is placed, assembly process of electronic components, vibrations, temperature variation, and shock will induce a certain state of stress and strain on PCB. These can lead to some problems in microprocessors BGAs (Ball grid array) and routes damage of electronic component. Gu et al. (2017), Benabou et al. (2013), Kok et al. (2016), Sinkovics and Krammer (2016), Wong et al. (2000), Han at al. (2017), Amalu and Ekere (2012). To determine the strains on PCBs in present the resistive strain gauge method is used. This method allows to determine the strain only at certain points. The layout and type of strain gauge used to measure strains is presented in CPI JEDEC 9704A (2012). For the measurement of the strain, the most used strain gauge in practice are in the form of a rosette consisting of three gauge located at a certain angle (0°, 45°, 90 °) or (0°, 60°, 120°), Fig.1, Bin and Ueda (2011).

Fig. 1.Strain gauge rosette 0°, 45°, 90 °.

In this paper will be study the possibility of applying the digital image correlation method (DIC) for strain measurement. Digital image correlation is a non-contact optical measurement method for measuring strain across the surface. This method is used to measure strain of material surfaces and structures subjected to various stresses (such as mechanical stresses or thermal stresses). Thus, this method is used according to the documentation for a variety of applications such as: large deformation measurements Tarigopula at al. (2008), crack analysis Tung S.H. et al (2008), σ-ε diagram definition for new materials Mol’kov (2013), constructions - to evaluate certain displacements and cracks, measurement of strain caused by temperature variations. Wang, Pan (2016); Rosa et al. (2015); Ramosa et al. (2015); Liat al. (2017); Pan et al. (2014); Tekieli et al. (2017); Hild, Roux (2006); Malesa et al. (2013). In general, three steps are required to implement the DIC method: sample preparation by applying a random speckle pattern (Fig. 2), then recording the images in different phases (before, during and after the load), and in the last phase processing of recorded images. Image processing can be done with several correlation algorithms developed over time. The basic principle of DIC operation is that after collecting the images during the application of the various loads, a comparison of the images is made following what happens with the points of random pattern divided into several subgroups compared to the image in which the surface is undeformed, Fig. 2, Li at al. (2017), Malesa et al. (2013), Pan et al. (2009), Hung and Voloshin (2003), Lin et al. (2015).