The wider the diameter, the faster solder joints will form.

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 solder site, permitting proper hole filling in the barrel, a feat often difficult with multilayer, high heat sink boards. A higher freeboard actually lets the machine “pump” solder into the holes that need complete filling; but at the same time, we want a constantly controlled solder wave or fountain shape.

A rule of thumb in nozzle selection is to always use the largest diameter nozzle available that will handle every selective soldering task on the board; don’t skimp by using a small nozzle. If a bigger one can be used, all the better; the bigger the nozzle, the more thermal energy will be imparted to the board, and also the quicker the solder joints will form, thus reducing dwell time. Try to use the same nozzle for every site on the same board, if possible.

Practically all selective soldering equipment manufacturers provide wetted nozzles, and these deliver excellent performance. With these nozzles, the outside diameter of the nozzle is actually wetted by the solder. Everyone has their own particular approach; in our case, a proprietary alloy lets us achieve this wetted effect. Wetted nozzles provide more uniformity and control of the flow of the molten solder as it travels back toward the pot.

In many cases, for expediency, uniquely shaped nozzles can be designed to solder a number of sites and multiple solder joints simultaneously, in a stamping or a branding type of operation. For example, there might be multiple nozzles of various shapes in the same nozzle manifold that come up and stamp a certain pattern time and again. They might be a mix of square, rectangular, round and half-round, etc.

When Pb-free selective soldering, remember that even stainless steel nozzles are scavenged by the tin in the solder. SnPb alloys seem to have little effect on the longevity of stainless steel nozzles, but Pb-free alloys, and particularly SAC types, will erode the nozzle material, even stainless. One remedy is to coat the inside of the nozzles with a resistant coating to prevent or at least slow that erosion process. Erosion of stainless will add nickel content to the solder pot, and any type of dissolved metals contamination can impact solder joint reliability.

Have uniform control of the “freeboard” of solder above the nozzles. However, selective soldering nozzles, particularly smaller-diameter nozzles, are notorious for gradually losing the height of the solder freeboard over time due to the buildup of contamination inside the nozzle. Real-time measurement of the solder height that will automatically adjust pump speed in order to maintain a pre-set desired height is valuable here. That’s a critical part of the process. If solder height diminishes and you aren’t aware of it, soon you’re not soldering at all, or at least not getting the desired quality.

Selection, shape, design and function of selective soldering nozzles are a critical focal point of successful soldering. Know your nozzles! Understand their features, how they affect and control the solder wave, and what their vulnerabilities may be to the specific alloy used.

Alan Cable is president of A.C.E. Production Technologies (ace-protech.com); acable@ace-protech.com.