Reflow vs Wave vs Selective Soldering: PCB Assembly Methods

Most modern printed circuit boards are assembled using SMD components. Solder paste, component placement, a convection reflow oven — and almost the entire PCB can be soldered in a single pass. But what if the PCB also has THT through-hole components? Connectors, transformer, display, relays, or other components.

Solder them by hand? Sure. And for low-volume production, this is often exactly what manufacturers do. But if we are producing thousands or tens of thousands of PCBs, manual soldering becomes a separate manufacturing operation that requires people, time, and money.

And this brings us to an interesting question: how can we quickly solder through-hole components onto PCBs where the SMD components have already been soldered in a convection reflow oven during the first production cycle?

There are several main soldering methods used in high-volume PCB soldering. The most common are reflow soldering, wave soldering, THT soldering in a pot of molten solder, and selective soldering. Each of these processes is intended for different applications.

Can through-hole components be soldered in convection reflow oven? Yes.

This process called Pin-in-Paste, also known as Intrusive reflow. Solder paste is applied to the pad and around the plated through-hole, the component lead is inserted into the hole, and then the entire assembly goes through the standard reflow process in a convection oven.

But this process cannot always be used. We need enough solder paste volume to fill the hole, and the component itself must be able to withstand the reflow temperature profile. This is not always convenient and not always practical from a manufacturing standpoint. And this is where the first problem appears.

If pin-in-paste method allows us to solder through-hole components in a convection reflow oven, why do we need another process at all?

Because some THT components may have temperature-profile limitations and can be damaged during heating in the oven. And most importantly, the PCB design itself does not always allow all components to be conveniently assembled in a single reflow process. This is why a completely different process is used for high-volume soldering of through-hole components — Wave soldering.

Wave soldering works on a completely different principle. Through-hole components are simply inserted with machine or manually into the PCB with their leads trimmed. Their leads and the exposed solder pads are located on the bottom side of the PCB. Flux is also applied to the bottom side of the PCB, the board is then preheated, and after that the bottom side of the PCB passes over a wave of molten solder.

Over the wave — the entire PCB is not dipped into the solder pot. The molten solder in a wide, but narrow wave contacts the exposed metal surfaces, wets the component leads and pads, and flows into the plated through-holes, filling them.

The main advantage of wave soldering is the soldering speed. If several dozen THT components are installed on the PCB, we don’t need to solder every lead. But wave soldering has one limitation — the solder wave contacts the entire accessible portion of the bottom side of the PCB. And sometimes this becomes a problem.

Let's imagine a modern PCB. The SMD components have already been soldered on the both sides, and there are also unsoldered connectors, different THT through-hole components, display, and some components that should not come into contact with the solder wave at all. The solder wave can simply wash SMD components, soldered to the bottom side of the PCB, if they have not been glued in place beforehand — and gluing SMD components is a separate manufacturing process.

Certain areas on the bottom side can be masked to prevent contact with the solder wave, but all of these measures make PCB assembly more complicated and make wave soldering less convenient.

So we get the next problem — wave soldering is very fast. A huge number of THT component leads can be soldered in a single pass, but its main advantage is also its limitation: The wave contacts the entire bottom side of the PCB, unless its width is restricted.

But what if we don't need to solder the entire bottom side of the PCB and only need to solder one large through-hole connector with several dozen leads? Sending the entire PCB through a solder wave just for that connector does not always make sense.

And this is where another method appears — Selective soldering.

The basic principle of Selective soldering is similar to Wave soldering, because molten solder is also used. But instead of a wide solder wave that passes across almost the entire width of the PCB, a small nozzle is used to deliver solder individually to each required THT component lead and PCB pad. The small nozzle moves according to a programmed path and solders THT leads on the PCB, only where soldering is required.

Selective soldering method makes it possible to work with complex PCBs where SMD components can be located on the bottom side of the PCB close to through-hole components. These SMD components cannot be exposed to the solder wave, and they don’t need to be masked. But the price for this flexibility is lower soldering speed.

Selective soldering only solders specific joints and nozzle physically moves between them. Therefore, when there are a large number of through-hole components, this process can be relatively slow. And this raises the opposite question - if selective soldering is so precise and allows us to apply solder only where it is needed, why do manufacturers still use wave soldering?

The answer - is slow soldering speed of selective soldering. If the PCB design allows wave soldering and we need to solder a large number of THT components, there is no reason to move a selective soldering nozzle from one joint to another.

To make this possible, the PCB should be designed from the beginning so that there are no SMD components on the bottom side, that interfere with wave soldering and would need to be glued and additionally masked.

Or you can design your PCBs with SMD and THT components distributed across both sides, but the assembly cost will be significantly higher, because selective soldering will have to be used. Selective soldering is always more expensive.

The choice between soldering processes actually begins not on the production line, but during PCB design. The placement of through-hole and SMD components, the spacing between them, components on the bottom side of the PCB, connectors, and displays can determine which soldering process can be used.

A PCB designer can create a design that is perfectly suited for automated assembly. Or they can create a PCB where just a few poorly positioned THT components force the manufacturer to use selective soldering or even perform some operations manually. So good design is about how that PCB will later be soldered and assembled.

The job of a SMT process engineer is not to choose the most modern soldering method, but to develop a manufacturing process that can consistently produce a specific PCB with maximum quality and minimum manual labor.

PCB Production volume also matters, so nobody is likely to buy several different soldering systems to solder ten boards per month. This kind of equipment starts to make sense, when we are talking about tens of thousands of PCBs per month. That completely changes the production economics.

No single “best” PCB soldering method. Oven Reflow is excellent for high-volume soldering of SMD components.

Wave soldering provides high speed, when soldering large numbers of through-hole components.

DIP soldering in the pot is also used. In this process, the entire bottom side of the printed circuit board comes into contact with molten solder in a solder pot, soldering all the connections on the bottom side at once.

Selective soldering makes it possible to automate the soldering of individual through-hole components on complex PCBs, where using a solder wave becomes a problem.

And pin-in-paste can eliminate a separate through-hole soldering process entirely in some PCB designs.

Thank you for reading!