Issue link: https://iconnect007.uberflip.com/i/1172746
64 SMT007 MAGAZINE I OCTOBER 2019 expensive and take considerable time and effort to design and fabricate. Solder- defect levels can be high because solder fountains change a wave's flow dynamics. This is not a very common process. 3. Site-specific or "dancing wave," in which a robotic carrier moves the solder foun- tain, is most common since the numbers of through-hole components on boards have been declining over the years. It is a perfect process if you have only a few components on the board. In this method, the board remains stationary, but the solder fountain travels to protruding leads. The solder fountain solders each lead or row of leads individually. These site- specific soldering machines have a built-in fluxer, preheater, and solder fountain, and tend to simulate the standard wave soldering process. Such machines are flexible and do not use fixtures and use entirely different soldering profiles, includ- ing much higher solder fountain tempera- tures than the standard wave soldering process (e.g., 290–300°C for selective soldering vs. 255–265°C for wave). You will need a separate machine or separate wave pots for tin-lead and lead-free. 4. There are various other selective soldering methods, such as hand soldering, focused light soldering, and diode laser soldering in unique applications for cost and tech- nical reasons. For example, diode laser soldering is a newer technology, but not everyone feels comfortable being on the leading edge. 5. Solder pot for selective soldering or rework is used as well. However, the lead- free solder in solder pot also has been found to leach or erode away most of the copper from the plated through-holes if the duration of soldering or lead removal for rework tends to be long (i.e., almost 60 seconds, which is way longer than the approximate 5 seconds during wave sol- dering). Thus, one needs to be mindful about copper erosion when using solder pot, especially for rework (removal and replacement) of through-hole components. Conclusion What is the best method for soldering in general and selective soldering in particu- lar? It depends on your application and capi- tal budget. To select the right soldering option for your application, examine your product volume, mix, and complexity; then, weave through both technical and business issues to come up with the right soldering option for your products. You will need at least two dif- ferent soldering options to handle both SMT and through-hole components in your mixed- assembly products. SMT007 Ray Prasad is the president of Ray Prasad Consultancy Group and author of the textbook Surface Mount Technology: Principles and Prac- tice. Prasad is also an inductee to the IPC Hall of Fame—the highest honor in the electronics industry—and has decades of experi- ence in all areas of SMT, including his leadership roles implementing SMT at Boeing and Intel; helping OEM and EMS clients across the globe set up strong, internal, self- sustaining SMT infrastructure; and teaching on-site, in- depth SMT classes. He can be reached at smtsolver@ray- prasasd.com and has an upcoming SMT class October 21–23, 2019. More details at www.rayprasad.com. To read past columns or contact Prasad, click here.