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Lựa chọn và giám sát các thông số hoạt động Điều kiện của dòng Cho rằng sự lựa chọn của thông lượng được giải quyết, nội dung của fluxer tại mọi thời điểm phải phù hợp với mật độ và / hoặc giá trị axit được quy định trong bảng dữ liệu của nhà cung cấp. Mục 4.2.2 thảo luận chi tiết yêu cầu này có thể được đáp ứng, bởi | 210 Reflowsoldering sophisticated and flexible must be the control system of the oven with emitter parameters having to be adjusted for different types of board to suit their specific heating requirements. Even so smaller components get hotter than large ones and in consequence temperature differences of up to 30 C 55 F can occur between their respective joints which results in widely differing microstructures.1331 A further cause of uneven heating under infrared radiation is the effect of the geometry of a solid body on its heat take-up through which edges and corners get hotter than flat surfaces see Figure 5.17 Section 5.5.2 . This is a further reason why most of today s reflow ovens operate with hot-air or hot-gas convection. 5.6.2 The physics of convection reflowsoldering Convection heating in a moving stream of hot air or gas is an equilibrium situation. The oven temperature is precisely defined and measurable being that of the circulating atmosphere. Whether the soldered goods fully attain the temperature of that heat transfer medium depends only on whether they remain in it for long enough. In practice all the joints on a board reach the same end temperature to within 5 C 10 F. The rate of heat transfer between a flowing gas and a solid surface is mainly proportional to the temperature difference between the gas and the surface the square of the speed at which the gas flows across that surface. Other factors enter into it as well such as the mode of flow at the interface the more turbulent it is the higher is the efficiency of heat transfer. Thus a densely populated board with a highly complex topography which produces local turbulence in the hot air flow over it picks up the heat better than a plain one the design of every heat exchanger is based on this principle . Thermal conductivity of the oven atmosphere and its specific heat per volume also matter see Table 5.13 with nitrogen having a 20 edge over air as regards heat content per volume. Flame .
