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When one considers the critical importance of solders and soldering in the fabrication of all types of electronic circuits, it is perhaps surprising that there is not a greater number of books available on the subject. For some time now one of the standard references has been ‘Soldering in Electronics’ by R. J. Klein Wassink, published in 1984. Unfortunately, the publication of this book coincided with a revolution in assembly technologies for electronic circuits with the advent of surface mount. Resulting from this there has been a complete revision of ‘Soldering in Electronics’ by the author to embrace the impact of surface mount technology.

Drawbridging or Stonehenge Effect of leadless components (i.e., the standing up on their end faces) has been investigated. An explanation is offered based on a straightforward theoretical model considering surface tension, sustained by a great amount of experimental evidence. The phenomenon is strongly associated with condensation reflow soldering, whereas the dimensions of the metallisation at the underside of the leadless components and the size of the solder lands on the board are major influencing factors. Practical hints are given to overcome the problem.

During reflow soldering the applied solder paste is melted and the components, previously placed on the solder paste, move into their final position. This process, however, may be accompanied by various unwanted movements of components and solder. Components may move horizontally along the surface of the board (this is called swimming or floating), or may move vertically and stand on their ends (this is called drawbridging or Manhattan effect). On the other hand, the molten solder may move to places other than those intended, e.g., into metallised holes (PTH) connected to the solder lands, or upwards along component leads away from the joint area; this effect is called solder wicking. Moreover, isolated small solder balls are often found on the board surface after melting of the paste. Experiments show that all these effects depend on the heating method, vapour phase soldering often being the most prone. The driving forces of the displacements can be explained in terms of forces and pressure caused by the surface tension of the molten solder, whereas the observed influences of the heating method are the result of the direction from which the heat is transported to the solder paste to be melted. From this, important conclusions for vapour phase soldering, infra‐red soldering and hot‐belt soldering may be drawn.

Some characteristics of the metallisation of leadless ceramic components are discussed, such as dissolution, reliability of joints, intermetallic compound formations and the migration of silver. It is concluded that it is not the type of the metallisation that is important, but the quality of the make.

Most footprints of surface mounted components have been calculated on the basis of the sizes and tolerances of their electrodes, taking into account the various inaccuracies of the mounting processes to be applied. The introduction of components with very small connecting areas or leads (e.g., fine pitch IC packages) does not only imply smaller solder land dimensions, but also a different approach for the calculation of the footprint. This approach tackles the question of the required insulation spacing and lead/land overlap, and the ‘optimum’ solder land dimensions and the available mounting (i.e., placing and fixing) freedom are then obtained. The reduction of mounting freedom with decreasing pitch distances is quantitatively demonstrated. The ratio width of lead/size of pitch proves to be an important parameter in this respect. Requirements for mounting quality are linked with certain criteria for inspection of soldered joints (for example, regarding shift and rotation and vice versa), and some of these criteria are discussed in more detail.

Several effects of the atmosphere in the soldering oven on both the soldering process itself and the soldering results are discussed. Experiments have been undertaken to compare the results of soldering in air and in nitrogen containing 10,100 and 1000 ppm oxygen, in which, e.g., discolouration, wettability, solderability after reflow, solder bridging and solder‐ball formation were investigated. Unmounted FR‐4 testboards with both an RMA solder paste of known high quality and a low‐residue paste were used. Mounted test boards were used to analyse the self‐alignment of components and to compare the levels of soldering defects obtained in air and in nitrogen. The test results show that a nitrogen atmosphere containing 1000 ppm of oxygen or less is sufficiently pure to realise improved soldering conditions for most types of components. For the low‐residue paste tested, 1000 ppm is too high, but 100 ppm is sufficiently low. All effects on the soldering process will depend on the amount of oxygen in the gas. To produce an oven atmosphere of nitrogen with a very low amount of O2 (e.g., <100 ppm) is rather expensive, if this oven is to work under production conditions. Will the extra cost of investment and gas consumption be worthwhile in view of a better production yield and higher product quality? The authors explain why they do not believe this to be the case.

Damage to components during soldering and degradation of soldered joints is determined to a large extent by the mechanical properties and the metallurgy of solder alloys and soldered joints. Knowledge of these properties is required for understanding of the mechanisms of damage and degradation. A compilation of this background knowledge is presented in this first article. It comprises the elastic, strength, creep and fatigue characteristics of tin/lead solders. Further, the metallurgy of tin/lead solders and soldered joints is discussed in terms of solidification structures, formation of intermetallic compounds, ageing of structures and effects of different solderable metallisations and soldering technology.

The paper provides a brief survey of the various aspects of soft soldering, in as far as they are relevant to the manufacture of soldered printed boards. The intention is that only a few lines will be devoted to each subject. It is mainly intended for those who only indirectly enter into contact with soldering, and who want a quick introduction to this field. Next, it may be of interest to those who occupy themselves in depth with soldering, because most aspects come under discussion, if only briefly. Soldering experts are not likely to find anything new, but they may still appreciate the overall survey provided here.

Components A distinction can be made between components with wire terminations (see Figure 28) and so‐called leadless components (see Figure 29). In between are the components with short terminations.

The wetting balance is used for the measurement of solderability of electronic components. The wetting force is measured dynamically and the technique gives information about both the degree and the speed of wetting. For practical quality assessment of electronic components, a simple‐to‐use index is required that incorporates the data on both degree and speed of wetting. The index must also have a uniform discrimination between different wetting properties, across the full range encountered in practical soldering. This paper reviews critically the numerous indices suggested in the literature, and supports with quantitative data the choices previously made subjectively in some soldering standards.