tailieunhanh - SMT Soldering Handbook surface mount technology 2nd phần 6

giải thích sự miễn cưỡng chung của ngành công nghiệp để áp dụng một công nghệ chì, trừ khi buộc phải làm như vậy. 4. Các tiêu chuẩn wavesoldering nhiệt độ 250 ° C/480 ° F cộng thêm hoặc trừ đi một vài độ, giống như góc tải, kết quả của hơn bốn thập kỷ kinh nghiệm wavesoldering thực tế. Nếu không có một nhu cầu hấp dẫn, nó được khuyến khích không để khởi hành từ. | 168 Reflowsoldering the solder in the paste print-down has melted but not later than 20 sec. after it has been launched on the solderbath the test-coupon is lifted off and allowed to cool. DIN 32 513 stipulates a waiting period of 1 hour and another one of 72 hours between print-down and melting and a solderbath temperature of 215 C 420 F. This test is so simple to perform that it can and should be used on the shopfloor every time a fresh tin of paste is opened or before paste which has been recovered from the printing frame at the end of a run is used again for printing. Some vendors of paste can supply stencils for the solderball test. Alternatively a stencil can be punched from a sheet of metal. More simply punching a hole through a selfadhesive paper label folded double upon itself with a normal office paper-punch will produce a stencil aperture for a paste printdown with a diameter of mm and a thickness of mm which is adequate for a reproducible practical test result Figure . The test specimen can be heated by placing it on a hotplate though for preference it should be floated on a small solderbath which is thermostatically held at the test temperature. In actual paste-printing and reflowsoldering practice test temperatures tend to differ somewhat from the prescriptions of ANSII PC-SP-J-STD 005 if the paste is to be used for reflowsoldering in an infrared oven a test temperature of 250 C 452 F is preferred. If reflowsoldering is carried out in a vapourphase installation the test temperature will be the same as the vapour temperature . 215 C 419 F. It is normal practice to commence using the tested paste for production as soon as the solderball test has been carried out and proved satisfactory. It is wise however especially when starting with a new delivery of paste or when testing an alternative product to set some test specimens aside and heat them after the maximum time which will elapse between printing down the Figure The solderballing .