tailieunhanh - SMT Soldering Handbook surface mount technology 2nd phần 4

cùng với thông lượng theo thử nghiệm, được lưu trữ trong một bầu không khí ẩm ướt, ở 40 ° C/645 ° F và 91% để 95% độ ẩm tương đối, trong ba ngày. Ăn mòn được coi là đã xảy ra nếu dư lượng thông lượng đã thay đổi màu sắc, hoặc nếu đốm trắng đã xuất hiện trong đó. ISO 9455-5 | 94 Wavesoldering The cylinder need not be removed during rest periods and overnight. The flux on it will of course dry but rotating the cylinder for about 15 minutes before starting work again will clear it. For longer breaks in production the cylinder is removed from the fluxer and cleared of flux with an appropriate thinner which as a rule is supplied by the flux vendor. Rotating-brush sprinklers Figure explains the working principle a rotating cylindrical brush carrying fairly stiff nylon bristles and of a length corresponding to the width of the solderwave is arranged at right angles to the travel of the circuit board conveyor. The lower portion of the brush dips in a container of flux. The sense of rotation is contrary to the direction of travel of the board conveyor. Somewhat before the bristles reach the apex of their rotation they pass the straight edge of a blade which can be pushed into the path of the bristles so as to bend them backwards. Having passed the blade the bristles spring forward and fling the flux they have picked up from the reservoir upwards against the underside of the circuit board which passes overhead. A sensor-actuated mechanism pushes the blade against the brush when a board arrives above the aperture ofthe sprinklerand retracts it as soon as the board has passed. The width of the spray is governed by the length of the blade which is adjustable to match the width of the boards to be fluxed. The amount of flux delivered is governed by the controllable speed of rotation ofthe brush while the depth ofimmersion of the bristles in the flux determines the size of the flux droplets to some extent. It is customary to keep the brush rotating during short breaks in production. During longer breaks the brush is removed and stored in a container filled with thinners and provided with a well-fitting lid. Should the bristles harden by being left to dry in air a brief period of rotation in the fluxer will soften them again. Figure Working .