Failure Analysis On The Surface Finish And Rigid-Flex Delamination Of Printed Circuit Boards

PCB is the core component of all electronic products, playing the role of supporting, interconnection and signal transmission. To adjust to the requirements of miniaturization, environmental friendliness, and multi-functionalization, more PCBs tend to be high density, lead-free and additively processed. As long as there is any oversight in design, material selection, fabrication, packaging and storage, hidden trouble will be left to cause premature failures. In terms of the PCB industry in China, the qualified rate of conventional PCB is between 95%-98%, while this figure drops to 95% when it comes to lead-free process. What’s worse, the qualified rate of high density multilayer PCB is no greater than 90% and that of rigid-flex multilayer PCB stays below 65%, far from the internaltional standard. Calculated by the defective rate of 5% out of the PCB annual output of 152 billion RMB in 2011, the annual great loss exceeds 7.6 billion RMB! Therefore, the improvement of PCB reliability is a most pressing task that bears far-reaching significance. What’s more, surface finish is a key connecting process during PCB assembly and rigid-flex board is a representative high-end PCB, so their failure analysis is especially both challenging and vital. Like conventional metallic materials, the failure of PCB involves five failure modes: fracture, distortion, corrosion, wear and attenuation. But owing to the characteristics of diversified materials used, complex process and miniaturized structures, its failure mechanisms and defect location are much more difficult than those of normal metallic materials. As a result, only by carrying out comprehensive analysis of the defective areas even the whole failure sample in terms of mechanical, physical, chemical and electic performances and combining the effect analysis of every link in PCB process and management can the failure modes, defects, mechanisms and causes be found out.In this dissertation, the structure, process and developing trend of PCB, the techniques of mounting devices and chip interconnection were fistly introduced; the research progress of PCB surface finishes and rigid-flex delamination problem were overviewed; the significance, characteristics and methods of PCB failure analysis were summarized. Then three PCB failure anslysis cases were systematically elaborated. The first case dealt with two kinds of PCB failures with electrolytic Ni/Au as surface finish. One was the poor bondability of gold wires to Ni/Au pads and the other was "dark gold" and weak solder wettability. The second case was gold plate peeling from electrolytic Ni/Au pads in 3M tape test and the third case discussed the delamination of rigid-flex multilayer PCB after reflow soldering. Accoding to the different failure mode of each case, a series of characterization methods such as three-dimensional stereo microscope (3D-SM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray fluorescence spectrometry (XRF), X-ray computer tomography (X-CT), Fourier transform infrared spectroscopy (FTIR), thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out to conduct in-depth analysis of the surfaces, interfaces, internal macro and micro morphology, element composition of the pads and subtrates, plating thickness and the thermal performances of the substrates. Combined with the specific process of the failure samples, the failure causes of each case were found out and detailed failure mechanisms were put forward and discussed. Finally, corresponding countermeasures were proposed for prevention.This study not only solves three real reliability problems of PCB which bother the manufacter a lot and proposes countermeasures for prevention of the reoccurrence of similar problems, but also discusses the effect of various links and parameters during process and storage on the quality and reliability of PCB as well as the systematic approach to PCB failure anslysis in detail, which has great practical significance in PCB reliability analysis and control in manufacturing practice.