Selective Soldering with Precise Amounts of Liquid Solder

Selective soldering methods are established in the market for joining electric or electronic components with printed circuit boards (PCBs). Despite the lower productivity of these methods compared to mass soldering methods e.g. wave or reflow, several applications still require selective soldering methods concerning economical or technical issues. Examples for these types of applications are single and scattered solder joints on already soldered printed circuit boards, joints which have to be re-soldered, components which are difficult to access, and temperature sensitive components. The productivity of selective soldering methods can be increased by automation. When increasing the automation degree, the control of the process, particularly the control of the temperature and the heat input, has a high impact on the formation, the characteristic, and the durability of the joint and it is the main important issue for the quality. However, these parameters are difficult to control, also with established selective soldering methods. Common soldering methods heat up the solder and the solder joint with one single heat source. Thus the heat input into the joint or into the solder often exceeds the required heat input. Especially for reliably soldering of temperature sensitive components an adequate heat- and temperature control is required to avoid an excessive heat input and to prevent damages to the substrates. Therefore a new selective soldering process, called duothermal soldering, was developed which applies the heat input into the joint by employing low and controlled process temperatures and precisely dispensing liquid solder. This process is based on separating the application of heat to liquefy the solder and to heat the elements to be joined. Using this new soldering method, the process temperature can be regulated by the temperature of the joint and the solder as well as by the quantity of heated solder. To evaluate the duothermal soldering process, a prototype including a liquid solder dispenser has been realised for controlled heating of the solder in a special vessel with a heating spiral and for controlled heating of the solder joint with a hot air stream. The functions of the liquid solder dispenser are to liquefy the solder and to dispense it in defined quantities onto the joint. To implement this method, it was tested on common PCBs and on temperature sensitive flexible printed circuits and flexible flat cables