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Application of flexible printed circuit board (FPCB) in personal computer motherboards


Focusing on mechanical performance_1_introduction




Flexible printed circuit boards (FPCBs) are being used extensively in current electronics devices because of their excellent flexibility, lightweight, and reduced thickness. In the present study, a proposed novel approach that utilizes FPCB as a substrate for motherboards is investigated. Deflection and stress are the key factors that determine the feasibility of FPCB motherboards. As flow velocity increases, flow-induced deflection and stress also become more prominent. The present study also explores several configurations of the motherboard, such as different fastening options and component layouts. This fluid–structure interaction (FSI) study is performed using the fluid solver FLUENT and the structural solver ABAQUS, coupled online using the Mesh-based Parallel Code Coupling Interface (MpCCI). A simple experiment was also conducted to substantiate the FSI simulation technique utilized in the present study.Although the present study on FPCB primarily focuses on the motherboard, the findings could also provide information for other FPCB applications.



1. Introduction


Flexible printed circuit boards (FPCBS) can be used as alterna- tives to rigid printed circuit board (PCB) in certain applications because of its excellent flexibility, twistability,and light weight.FPCB facilitates a space-efficient system design, reduces size and weight, improves the internal appearance of a system, and offers low-cost installation. Over the years,FPCBs have been widely used in numerous electronic devices,but their application as personal computer(PC) motherboards is still rare. Compared with rigid PCB, FPCB permits much higher deflection in operations, especially under flow conditions in typical PC systems. Therefore, durability and long-term performance are the primary concerns in the use of FPCB as motherboards. Numerous studies have focused on this area, and a review of previous research, which builds the back- ground for the current study, is presented.


Azar and Russell experimentally studied the effect of component layout and geometry on flow distribution in a circuit pack. They found that the flow in electronic enclosures was highly three dimensional, and the location and orientation of components with large aspect ratio can significantly affect flow distribution. A similar study was reported by Lee and Mahalingam, who used a computational fluid dynamics (CFD) tool to evaluate the velocity and temperature fields of air flow in a computer system enclosure and correlated the predictions with real-time experiments.


Dealing with component-PCB heat transfer, Rodgers et al. performed numerical and experimental analyses to assess the predictive accuracy of the CFD tool for the thermal analysis of electronic systems in natural- and forced-convection modes. A detailed characterization of the air-flow patterns around PCB-mounted elec- tronic components using few complimentary flow visualization techniques was also reported.


A numerical solution for the steady-state forced convection of air flowing through a horizontally oriented PCB assembly under laminar flow conditions was developed by Leung et al. Shank- aran et al. [8] proposed the ‘‘zoom-in modeling, a multi-level approach with an exact boundary condition import, for accurate and time-efficient  CFD design calculations for a populated system cooled by forced air flow. Leon et al. used the FLUENT code to obtain the optimal  layout for cooling fins in forced-convection cooling by uniquely considering both the heat flux and flow resis- tance. A model of forced-convection cooling was also reported by Baelmans etal.  who observed that fan-induced swirling flows were difficult to predict with standard k–ε turbulence models. In addition, Cole et al. investigated the aerodynamic and thermal interactions of ball-grid array (BGA) packages in a real environ- ment. The effects of air flow and PCB thermal conductivity on the junction temperature were analyzed.


Extensive experiments and modeling of flow and heat transfer in  axial flow fan-cooled electronic systems were conducted by Grimes et al. Experiments were conducted in fan-sucking (fan mounted at the system outlet), as well as fan-blowing (fan mounted at the system inlet), modes. The flow in the sucking mode using a laminar model was observed to be steady, uniform, and easily predictable, whereas in the blowing mode and using a turbu- lent model, the flow was unsteady, swirling, and too complex to predict with reasonable  accuracy.  However, heat transfer was enhanced by shifting the fan from the exhaust (sucking) to the inlet (blowing).


Fig.1. ASTM D638 test for PCB material.


For rigid PCB, Leicht and Skipor introduced a method of measuring  the  reliability of the mechanical bending fatigue of area-array packages. A global/local modeling of PCB mechanical loading  was  also proposed by Zhu et al. Using the finite element method (FEM) model, Shetty et al.and Shetty and Reinikainen investigated the durability of chip scale package interconnects when assembled on FR4 substrates. Lau et al. Studied plastic BGA package assemblies under a three-pointbending condition using experiments and simulations. Furthermore, Yuetal.analyzed the full-field dynamic responses of PCBs at the product level using a digital image correlation technique.


Few studies have focused on the material and structural characteristics of FPCB. Li and Jiao studied the effects of temperature and aging on the Young’s moduli of polymeric-based  flexible substrates, such as polyethylene naphthalate, polyester, and poly- imide, using the Dynamic Mechanical Analyzer. Barlow et al.  successfully demonstrated the feasibility and viability of FPCB for miniaturization purposes on integrated power modules. Combining FEM with  optimization analysis, Ptchelintsev developed an automated procedure for the fatigue life assessment of FPCB. The fatigue behavior of thin Cu foils for FPCB was also experimentally studied by Han et al.


Considering fluid–structure interaction (FSI), Arruda and Freitas  proposed a multiphysics simulation model to describe the interaction  between FPCB and its surrounding air. Huang et al. investigated the mechanical reliability of a BGA mounted on FPCB under drop impact using simulation and found that stress can be reduced by increasing the height of solder joints. Interest- ingly, Siegelet al. were able to fabricate several low-cost flex-ible electronic circuits on paper substrates.


Recently, Arruda et al. have studied the flex–rigid PCB (RFPCB) interface to evaluate the cracking phenomena during thermal cycling using FEM. They found that high stresses could result in micro cracks in the connection between rigid and flex boards, and the development of stress was attributed to the design of the PCB and the non-uniformity in thermal expansion of various parts. Sun et al. [32], using FE analysis, found that some risky places exist for potential failures, such as the corners of the connecting parts of RFPCB. In addition, Das et al. discovered that the flexibility of FPCB decreased with an increasing number of metal layers. They were   also   able to fabricate biocompatible FPCB on a poly- dimethylsiloxane substrate.


Few studies related to PC motherboards were also conducted. Pitarresi et al. and Yating et al. addressed the FEM is- sue regarding the dynamic responses of a typical PC motherboard. They locally modeled the large components as simple blocks to avoid expensive modeling and found that the predictions corre- late well with  experimental measurements. On the other hand, Darveaux et al.  investigated temperature distribution on laptop computers under  various conditions and observed that the operating temperature range is approximately 55–80 degrees. Tari and Yalcin suggested that  placing the CPU,motherboard, and RAM on the back side of the  laptop lid can passively cool the laptop.


However, to the authors’ knowledge, no literature has reported on the application of FPCB in PC motherboards. Therefore, the current work aims to study the possibility of using FPCB as moth- erboards. The present study is aimed at investigating the deflection and stress induced on the FPCB motherboard under different flow conditions. To ensure the mechanical feasibility of FPCB mother- boards, the present work uses various parameters, such as different flow velocities, fastening options, and component layouts. The fluid–structure interaction (FSI) problem is addressed using the fluid-flow solver FLUENT and structural solver ABAQUS, coupled online using the Mesh-based Parallel Code Coupling Interface (MpCCI). The flow is assumed to be three dimensional, laminar, incompressible, and unsteady. A simple experiment was also car- ried out to validate the FSI simulation technique. However, the consideration of electronic viability is beyond the focus of the pres- ent study. Although the present  findings are important to FPCB motherboards in future PC development, the contributions are also significantly valuable for other FPCB applications.


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