tailieunhanh - SMT Soldering Handbook surface mount technology 2nd phần 5

yêu cầu một hàn với một điểm nóng chảy khác nhau hoặc có thể là một tỷ lệ phần trăm nhất định của bạc, người dùng sẽ được khuyên không bao giờ thay đổi các đặc điểm kỹ thuật hàn của mình. Những biến động đó sẽ gây ra bằng cách thay đổi từ hàn thiếc-chì chì (mục ) | Wavesoldering 131 demand a solder with a different melting point or maybe a certain percentage of silver the user will be well advised never to change the specification of his solder. The upheaval which would be caused by changing from the standard tin lead solder to a lead-free one Section explains the general reluctance of the industry to adopt a lead-free technology unless forced to do so. 4. The standard wavesoldering temperature of250 C 480 F plus or minus a few degrees is like the conveyor angle the result of over four decades of practical wavesoldering experience. Without a compelling need it is advisable not to depart from it. Choosing and monitoring operating parameters Condition of the flux Given that the choice of flux is settled the contents of the fluxer should at all times match the density and or the acid value which is specified in the vendor s data sheet. Section discusses in detail how this requirement can be met by automatic equipment if required. It is worth restating at this point that the success of wavesoldering depends critically on the consistent quality of the flux and that this constancy is assured more easily with sprayfluxers than with foamfluxers. Amount of flux per unit of board area This parameter also affects the soldering success though to a lesser degree than the density and the activity of the flux. Too much flux means more solvent in the flux cover and unless the preheater is adjusted accordingly a risk of boiling and solder-prill formation as the board passes through the solderwave. If boards have to be cleaned after soldering too much flux reduces the cleaning efficiency. Too little flux uneven fluxcover or worse unfluxed patches inevitably cause soldering faults such as bridges icicles solder adhering to the board and open joints especially with low-solids fluxes. With these the margin of error is much narrower than with high-solids fluxes. The thickness of the flux cover can be controlled to some extent .