tailieunhanh - SMT Soldering Handbook surface mount technology 2nd phần 8

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 ) giải thích sự miễn cưỡng chung của ngành công nghiệp để áp dụng một công nghệ chì | 252 Component placement Positional accuracy With fine-pitch lead spacing the width of the footprints is half their distance. Fine-pitch below mm 20 mil and ultrafine-pitch with mm 10 mil spacing have become real demands. This means footprints mm 5 mil wide and lateral placement accuracies of mm mil. Pick-and-place equipment with a placement accuracy of mm 2 mil and a repeatability of mm mil is commercially available. Angular accuracy of placement also matters. With a QFP of dimensions 25 mm 1 in X 25 mm 1 in an angular twist of 1 means a lateral displacement of mm 8 mil at every corner. As a result with a fine-pitch layout about half the legs would sit on the wrong footprint Figure . Therefore with large fine-pitch components angular placement accuracy has to move into the bracket. Because BGAs and flip-chips are able to correct even massive misplacement by self-alignment as soon as the solder melts see section the accuracy of their placement is less critical. Component identity and functionality checks The number of placement errors is a measure of the reliability of a placement system. Until not so long ago wrong-value chips and melfs in blistertape or bulk packages contributed significantly to manufacturing reject rates. Smart placement systems which detect and correct such errors during placement on-the-fly brought a drastic improvement and have become a common feature with most automatic placement machines. Figure Angularaccuracyofplacement Component placement 253 Placement options A user must decide between two strategies of component placement on the one side are the manual and semi-automatic manual methods on the other the fully automatic ones which also fall into two categories the sequential and the simultaneous systems. The choice between them depends on several factors 1. A newcomer to SMD technology who operates on a small-to-medium scale will tend to opt for a manual or semi-automatic .