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Thermal Response of Electronic Assemblies During Forced Convection-Infrared Reflow Soldering in an Oven with Air Injection∗

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Thermal Response of Electronic Assemblies During Forced Convection-Infrared Reflow Soldering in an Oven with Air Injection∗ 865 Thermal Response of Electronic Assemblies during Forced Convection-Infrared Reflow Soldering in an Oven with Air Injection∗ Young-Seok SON∗∗ and Jee-Young SHIN∗∗∗ The thermal response of electronic assemblies during forced convection-infrared reflow soldering is studied numerically. Soldering for attaching electronic components to printed circuit boards is performed in a process oven that is equipped with porous panel heaters, through which air is injected in order to dampen temperature fluctuations in the oven. Forced convection-infrared reflow soldering process with air injection is simulated using a two- dimensional numerical model. The multimode heat transfer within the reflow oven as well as within the electronic assembly is simulated. Parametric study is also performed to study the effects of various conditions such as conveyor speed, air injection velocity, and electronic as- sembly emissivity on the thermal response of electronic assemblies. The results of this study can be used in the process oven design and selecting the oven operating conditions to ensure proper solder melting and solidification. Key Words: Numerical Analysis, Thermal Response, Electronic Assembly, Forced Convection-Infrared Reflow Soldering, Air Injection, Parametric Study is equipped with infrared panel heaters. The energy from 1. Introduction panel heaters to the card assembly melts the solder, and Recently, electronic equipments are under rapid then the electronic components are attached to the PCB progress with the electronic components being developed permanently by solidifying the solder in the cooling zone toward the trend of small size, high performance, and high near the oven exit. density. Therefore, reliable high-density mount technol- ogy is required to attach many electronic components on the limited area of printed circuit board (PCB). To meet this requirement, surface mount technology (SMT) where soldering is performed on the surface is widely used re- cently(1), (2). Infrared reflow soldering has been a common technique used in SMT, but it is recently known that forced convection-infrared reflow soldering is easier to control heating and cooling of the electronic assemblies(3). Figure 1 shows schematic of infrared reflow ovens. (a) In a conventional infrared reflow oven shown in Fig. 1 (a), unsoldered card assemblies are supported at their edges by a conveyor, which moves them through the oven that ∗ Received 7th March, 2005 (No. 05-5026) ∗∗ Department of Mechanical Engineering, Dong-Eui Uni- versity, 995 Eomgwangno, Busanjin-Gu, Busan 614–714, Korea. E-mail: [email protected] (b) ∗∗∗ Department of Mechanical Engineering, Dong-Eui Uni- versity, 995 Eomgwangno, Busanjin-Gu, Busan 614–714, Fig. 1 Schematic of (a) a conventional infrared reflow oven and Korea (b) a forced convection-infrared reflow oven JSME International Journal Series B, Vol. 48, No. 4, 2005 866 This study is concerned with the forced convection- thermal response of electronic assemblies during forced infrared reflow oven shown in Fig. 1 (b). In this modi- convection-infrared reflow soldering in an oven with fied oven that has porous panel heaters instead of infrared air injection are predicted numerically using a two- panel heaters, the process air is introduced to, and ex- dimensional (2D) model. The advantage of forced tracted from the oven through porous panel heaters. In convection-infrared reflow soldering is identified by com- this scheme each card assembly will be subject to simulta- paring the results with those of conventional infrared re- neous radiative and convective heating and cooling, which flow soldering. The effects of oven operating conditions is not inherent in the design of Fig. 1 (a). Therefore, lo- and property of electronic assembly on the thermal re- cal excessive heating that is the main weakness of the sponse of electronic assemblies are also analyzed. conventional infrared reflow oven could be prevented by Nomenclature the additional convection effect. Moreover, selective up- flow or downflow can establish augmenting inertial and e : emissivity buoyancy forces within the oven air. This augmentation, k : thermal conductivity [W/m·K] in turn, may stabilize the air flow, which might minimize Q : heat transfer rate per unit length [W/m] product temperature fluctuation during soldering through T : temperature [K] conjugate interaction. V : velocity [m/s] Even though experimental studies on the thermal as- x : horizontal coordinate pects of the electronic components during infrared reflow y : vertical coordinate soldering process have been performed(4) – (6), heat trans- Subscripts fer mechanism related to thermal behavior has not been b : blowing (injection) closely examined. It has been required that the develop- c : conveyor ment of a numerical model that can predict thermal re- conv : convection sponse of various electronic assemblies in various pro- rad : radiation cesses because electronic packaging and soldering tech- 2. Analysis nique are under rapid progress. A simple numerical model which predicts thermal response of a electronic assem- 2. 1 Numerical model bly during infrared reflow soldering using radiative heat In this study, the flow in a forced convection-infrared transfer effect and convective heat transfer correlations has reflowovenwithairinjectionisassumedtobe2Dbycon- been developed(7), (8). To improve inaccuracy of the sim- sidering a simply populated board which consists of uni- ple model, a detailed numerical model which predicts heat form rows of electronic components. The first thing to be transfer characteristics by solving conduction, convec- considered in the geometrical modeling of the oven and tion, and radiation heat transfer equations has been devel- the card assembly is the relevant length scale. The length oped(9), (10), and analysis using this numerical model has of a typical infrared reflow oven is several meters, but that been performed(11). A numerical study reflecting the effect of solder in a card assembly which connects the lead of of forced convection by attaching a fan only near the exit a electronic component and PCB is several millimeters. of the infrared reflow oven has been performed(12).How- Since relevant length scales span approximately three or ever, comprehensive analyses on the forced convection- four orders of magnitude in a geometrical model, it is im- infrared reflow soldering process and the effect of the op- possible to resolve local solder temperatures by applying erating conditions on the thermal response of the elec- a single model to the entire oven or excessive computation tronic assemblies have not been performed sufficiently yet. time is required due to the tremendous number of meshes It is important to control the temperature in the oven in order to resolve local solder temperatures accurately. during reflow soldering process in order to achieve re- To solve this problem, two models, oven model and card liable soldering and to minimize thermally-induced card model, are introduced in this study. The oven model pre- assembly stresses which can cause lead-solder misalign- dicts the large scale velocity and temperature distributions ment and card assembly damage. The thermal response of within the oven, while the card model uses the results of the card assembly depending on the oven operating con- the oven model and estimates the small scale thermal re- ditions, which is governed by three coupled heat transfer sponse of a typical card assembly(10). modes of conduction, convection, and radiation, must be Figure 2 shows the oven and card models used in provided for proper thermal control. Especially stabiliza- the forced convection-infrared reflow soldering. The oven tion of air flow and minimization of temperature fluctu- model shown in Fig. 2 (a) does not account for the detailed ation in the oven by forced convection effect should be geometric features of the card assembly, but assumes a confirmed and the thermal response of the card assembly plane slab whose dimensions and thermophysical proper- should be predicted. ties are equivalent to those of the actual populated card as- In this study, heat transfer characteristics and the sembly. By defining effective thermophysical properties, Series B, Vol. 48, No. 4, 2005 JSME International Journal 867 Table 1 Velocities of injecting air at porous panel heaters (mm/s) (a) conduction equation(13),anda2Ddifferential, diffuse-gray radiative exchange analysis(14) are applied to the inside of the oven and card model domain shown in Fig. 2. The top and bottom panels of the forced convection- (b) infrared reflow oven with air injection are composed of 6 porous panel heaters, and each panel is 300 mm long Fig. 2 Coordinate system and geometry used in (a) the oven with an surface radiative emissivity of 0.95. The size of model and (b) the card model for the forced convection- the oven and the temperature distributions of the porous infrared reflow oven panel heaters are shown in Fig. 2 (a), and specific temper- atures used are based upon actual values taken from in- consistent oven and card model predictions can be gener- dustry. The unsoldered card assembly at 300 K enters the ated. The oven model predicts the large scale thermal re- oven at a velocity of 6 mm/s. The surfaces of the extended sponse in the oven, and generates the velocity and temper- domains at the inlet and outlet of the oven model are held ature boundary conditions for the card model as it moves at 300 K and are impermeable. through the oven. The oven model generates instantaneous The air injection velocities of inflow and outflow air velocity and temperature distributions throughout the through each porous panel heater are listed in Table 1, and computational domain of the oven, in addition to the distri- the temperatures of the injected air are the same as those of bution along the vertical planes located at the plane card’s the porous panel heaters shown in Fig. 2 (a). Three cases leading and trailing edges of the plane card. are considered in this study: air injection up, air injection The card model shown in Fig.
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