Selective Soldering: Preparation, Placement and Process Control

Selective soldering solders the through hole parts on a board that has already been reflowed, one joint or one group of joints at a time. The nozzle is the interface between the machine and the joint, and it decides how much heat arrives, how much flux is activated and how much turbulence disturbs the neighbouring parts.

What a Nozzle Has to Do

A nozzle has to deliver a stable wave of molten alloy to a defined area, transfer enough heat to wet the barrel and the lead, and then break away cleanly without leaving a bridge. It also has to reach into the space left between tall components.

The three requirements compete. A wider nozzle transfers heat faster but spreads flux further and risks touching a neighbouring pad, while a narrow nozzle is precise but slow and prone to cooling at the tip.

Nozzle Geometry and Flow

Nozzle bore sets the footprint of the wave, and the shape of the tip controls how the alloy behaves as it lifts away. A nozzle with a generous internal volume delivers a steadier wave than a thin one, because the flow is less sensitive to small pressure changes in the pump.

Where a joint sits beside a tall part, an angled or extended tip can be used to keep the body of the nozzle clear. The trade is heat loss along the extension, which shows up as a longer dwell time and more flux consumption.

Heat Transfer and Thermal Mass

The nozzle supplies heat, but the board and the component absorb it. A heavy connector ground plane pulls heat away faster than a signal pin, so the same nozzle setting produces different results across a single board.

The usual answer is a longer dwell or a preheat step rather than a hotter pot. Raising the pot temperature shortens nozzle life and oxidises the alloy, so the profile should be built from dwell and preheat. Our article on mixed thermal mass reflow describes the same problem in the reflow oven.

Flux Delivery and Residue

Flux is applied ahead of the nozzle, either as a spray, a foam or a droplet, and the nozzle then passes through the wetted area. Too little flux leaves an unwetted barrel, and too much leaves residue that has to be removed from a no clean board.

Flux chemistry also decides how the nozzle behaves over a shift. A flux that carbonises leaves deposits on the tip that change the flow, so the cleaning interval is a process parameter rather than a maintenance preference.

Choosing Nozzle Size for the Joint

The working rule is that the nozzle bore should be a little larger than the joint so that the wave wraps the lead without flooding the pad. A bore matched to the hole diameter, rather than to the pad, is usually the better starting point.

Where a connector has a row of pins, a multi hole nozzle or a slot nozzle can solder several joints at once, which shortens the cycle and reduces the number of thermal cycles the board sees. The limits are the pitch of the pins and the flatness of the row.

Nozzle Wear and Replacement

Alloy erodes the nozzle, and the bore grows over time. A growing bore widens the wave, increases bridging and consumes more alloy, so the wear shows up as a quality trend before it shows up as a visible defect.

Measuring the bore at a defined interval and tracking it against the deposit is the practical way to set the replacement point. The pot itself needs the same attention, as described in our article on solder pot maintenance.

Defects Tied to the Nozzle

Bridging between adjacent pins is the classic nozzle related defect, and it usually follows from an oversized bore, a dwell that is too long or a tip that has lost its shape. Icicles appear when the wave breaks away late, leaving a tail that freezes before it retracts.

Incomplete fill points the other way, to insufficient heat or flux rather than to excess. The diagnostic sequence for both is set out in our article on wave solder defect troubleshooting, because the two processes share a mechanism.

Machine Settings Around the Nozzle

Pump speed, travel speed, dwell time, preheat and flux volume are the parameters that surround the nozzle choice, and they should be recorded as a set. Changing one without the others is the usual reason a proven setup stops working.

The result should be verified on the product rather than on a test piece, with the fill and the fillet inspected on the first article and a sample through the run.

Verification and Records

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

Points to Confirm at First Article

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected. Where the supplier and the user both measure the same property, they should agree on the method before the first delivery.

FAQ

How often should a nozzle be replaced? When the measured bore has grown beyond the tolerance that still produces an acceptable wave, which is typically a fraction of a millimetre. The trend in bridging rate usually gives the warning first.

Can one nozzle cover a whole product? Often yes, if the joints are similar in size. Where a board mixes heavy connectors with small pins, two nozzles and two programs are usually faster than one compromise setting.

Does a wider nozzle solder faster? It transfers heat faster, but it also wets a larger area and increases bridging risk, so the gain is limited by the space between the joints rather than by the nozzle alone.