SELECTIVE SOLDERING GENERALLY uses two basic nozzle design types: One is the "round" nozzle, also known as a "bullet" or universal nozzle. It can approach any solder site from any direction, since it is completely radial in its design on the top.
The other is a component-specific wave or "letter-slot" nozzle, and its design is better suited to soldering rows of pins, such as found on an array. This nozzle shape is ideal for connectors, for example, a five or a six-row connector, with perhaps 40 to 50 pins in a row. One could use a round nozzle for such a job, but that would lead to a deviation in the overall soak time in the solder, because as the nozzle travels along parallel rows of pins, the pins in the center of travel dwell in the solder for the full diameter of the nozzle, but the pins on the outer edges--the outside rows--would see only a section of the diameter, basically an arc. As such, the dwell time under solder for the pins in the center actually could be twice as long as the exposure for the pins in the side rows. This can result in a variation in quality for the solder joints in the outside rows.
That's when we prefer to use the wave nozzle, because it is designed for the component shape we are soldering; in this case, all the rows would see the exact same dwell time, thus ensuring consistency in solder joint formation quality.
In selective soldering, it's important to get enough heat into the solder site, as rapidly as possible, to overcome the heat sink characteristics of the board and get good wetting and filling of barrels right up to the top side of the board. Accommodate longer pins, if encountered. To do this requires flexibility in the amount of "freeboard" of molten solder - the parabolic bubble of moving solder - at the top of the nozzle. Different suppliers use different methods of achieving this, from controlling pump speed to various nozzle design factors. What we find effective is a machined radial groove near the top of the inside of the nozzle that provides universal back pressure at the periphery, so that the solder in the center of the nozzle will rise higher than it would normally if the flow of the solder column were uniform.
The concept is simple, but the effect is to provide the greater freeboard of the parabolic dome of molten solder emerging from the top of the bullet nozzle. It also permits a high rate of solder exchange; this in turn provides a high rate of heat exchange, thermal transfer to the...