Application of lead-free solders in practice
Hot air leveling (HAL) with leaded solders has been the most common electronic surface treatment process for almost 30 years. There were several reasons for this: On the one hand, it was a very inexpensive process, and on the other, the achievable surface properties met the requirements of numerous applications. Finally, HAL surfaces have very good soldering and storage properties compared to other finishing processes. When the lead ban became effective in electronics processing, all relevant equipment and its peripheral components had to be converted to the use of lead-free solders. This posed huge challenges for the entire industry, as the process now needed to be managed at significantly higher temperatures with significantly higher thermal stress for all materials and a much more aggressive solder than metals. Also, the preferred solder systems SnAgCu and SnCuNi behaved very differently with regard to the service life of the solder bath as well as the processability, wettability and surface gloss. Finally, the flux had to be adapted to the increased process temperatures in two respects. On the one hand, they themselves were supposed to be more temperature-stable; on the other hand, their increased reactivity due to higher temperatures should not lead to increased chemical material stress.
All these parameters could be adapted and optimized within a research project at PENTAGAL Chemie und Maschinenbau GmbH (Bochum) for the different applications. It could be shown that the solder joints obtained are equivalent to those made with conventional lead / tin solders and can be used by the printed circuit board industry after adjusting the conditions. The process can be adopted for all industries that have already used the HAL process.
The two mentioned solder systems have their advantages and disadvantages: The silver-containing solder is used between 250 to 255 ° C. Thus, the thermal load of the printed circuit boards is about the same as in lead / tin. The disadvantage of this solder is the higher price and the larger copper removal, especially in the upper temperature range. The solder pot control must work extremely accurately and must not overshoot. Furthermore, the temperature difference between melting point and operating point is lower (23 ° C) than the tin / copper solder (43 ° C). This results in the long dive times and problems with cold system parts such as retaining clip and cold air knife. The copper-containing solder is used at a temperature of 270 ° C. The thermal load of the printed circuit boards is higher compared to Pb / Sn solder and
Ag solder. The copper removal is, however, lower than with the Ag solder. The biggest advantage of the solder lies in the high residual heat capacity. Between melting point and operating point, the temperature difference is 43 ° C. This makes the HAL process easier and the plant operation is comparable to that with lead / tin solder. But the high working temperature also brings disadvantages. The most common errors that result from this are delamination, discoloration of base material in certain circuit board materials, such as e.g. CM 1.
For both solders, the copper accumulation of the bath is faster than with the eutectic lead / tin solder, and the copper depletion, ie the freezing of the intermetallic phase Cu6Sn5, is more difficult, but is now well manageable. Experience has shown that the PENTA machines make a very good separation from about 240 ° C downwards, whereby it should be noted that in this procedure the bath pump must be switched off so that a different temperature prevails inside the bath as indicated by the thermocouple. With regard to the depletion cycles, experience must be gained by trial and error. It should be noted that the copper enrichment does not increase linearly with the same bath load time, since the dissolving power of the tin for copper is slower with increasing copper content. The excreted needles are up to over 10 mm long and can be easily get out with a sieve trowel, in which case the smaller ones are also removable in this composite.
When adding tin, it should be noted that not only the tinning process itself but also the removal of the needles requires adequate addition of the corresponding additional metals nickel, silver and possibly germanium. The freezing of the copper / tin compound naturally requires that the tinning process is interrupted for hours. The bath can be cleaned much faster if the process is carried out externally in the independently operating PENTA separator. For this purpose, a part of the solder bath is pumped off and immediately replaced by new or previously cleaned tin, so that after about 10 minutes interruption can continue working while externally the depletion process is running.