Reflow Soldering and Wave Soldering in the SMT Line
An assembly line is a sequence, and the sequence is not arbitrary. Each station performs an operation that assumes the previous one was done correctly and prepares the board for the next, so a mistake at the printing stage cannot be corrected by adjusting the oven.
This article explains the order of operations on a surface mount line, why reflow soldering and wave soldering are used for different things, and how a board that contains both surface mount and through-hole components is handled.
The Order of Operations on an SMT Line
The line runs from printing to placement to reflow to inspection. Solder paste is printed onto the pads, components are placed into the paste, the assembly passes through a reflow oven where the paste melts and forms the joints, and the result is inspected. Cleaning and rework are inserted where the process requires them rather than being fixed stations.
The reason the order cannot be rearranged is that the paste is the only thing holding the components in position between placement and reflow. Everything upstream of the oven exists to get the right amount of paste in the right place with the right components on top of it, and everything downstream exists to check that it happened.
Paste Printing: the Step That Decides the Rest
Most assembly defects originate at the printer. The stencil aperture determines the volume of paste deposited, and the volume determines whether the joint forms properly. An aperture that is too small leaves insufficient solder; one that is too large produces bridging and, on fine pitch, solder balls thrown clear of the joint during reflow.
The stencil itself has to be made for the board. Aperture size, the area ratio of the aperture to its walls, and the stencil thickness all interact, and the relationship is what limits how fine a pitch can be printed reliably. Cleaning the stencil between prints and checking that the paste has not dried in the apertures are operational matters, but they determine the yield as much as any design decision.
<img src="https://www.gopcba.com/wp-content/uploads/2026/09/Exploring-Semi-Additive-Processes-SAP-in-Ultra-HDI-PCB-Fabrication-95263.jpg" alt="SMT placement machine on a surface mount assembly line” />
The SMT Placement Machine
The placement machine follows the printer. Its job is to take components from feeders and place them accurately on the printed pads, and its two important characteristics are placement accuracy and throughput. Accuracy determines the finest pitch the machine can handle; throughput determines how the line is balanced, since a single slow machine sets the pace for everything behind it.
Placement is where adhesive may be used rather than paste. Adhesive is dispensed at a fixed position on the board before placement, and it exists to hold the component in place for processes that would otherwise disturb it, such as a subsequent wave soldering step on a double-sided assembly. The choice between paste and adhesive, or a combination of the two, follows from what happens to the board after placement rather than from the component itself.
Reflow Soldering and the Profile
Reflow soldering heats the assembly to a temperature above the melting point of the paste, holds it there long enough for the alloy to wet the surfaces, and then cools it. The oven may use air or nitrogen, and the atmosphere choice matters on fine pitch and on finishes that oxidise readily, because an inert atmosphere widens the process window.
The profile is the controllable part. It has four regions: preheat, which brings the board up without thermal shock; soak, which activates the flux and equalises temperature across components of different mass; reflow, where the alloy melts; and cooling, which determines the grain structure of the joint. A profile that heats too quickly will produce solder spatter and tombstoning, and one that cools too slowly produces a coarse joint with poor fatigue behaviour.
Wave Soldering and Through-Hole Components
Wave soldering works on a different principle. The board passes over a standing wave of molten solder, and the leads of through-hole components are immersed in it as they cross the wave. The solder fills the hole and forms a fillet on both sides, which is a connection that surface mount cannot make.
The wave is used for through-hole and mixed-technology assemblies, and the process has its own constraints. Components must be able to survive immersion, which excludes most surface mount parts unless they are protected, and the board must be oriented so that the wave reaches every joint without shadowing behind a tall component. Solder mask, pad shape and thermal relief on the pads all affect whether the joint fills properly.
Mixed-Technology Boards and Double-Sided Assembly
A board with components on both sides is assembled in two passes. The first side is printed, populated and reflowed, and the second side is then printed and populated. The difficulty is holding the first side in place during the second reflow, which is done either with adhesive or by a reflow process that keeps the underside temperature below the melting point of the already-formed joints.
Where a board combines surface mount with through-hole parts, the usual sequence is to reflow the surface mount side first and then wave solder the through-hole joints, using a pallet or selective tooling to protect the reflowed side. The alternative is selective soldering, which applies solder to individual joints by a small nozzle and avoids immersing the whole board. The choice depends on the number of through-hole joints and on how much of the board can tolerate the wave.
Inspection and Rework
Inspection follows the process rather than replacing it. Optical inspection verifies that the components are present, correctly oriented and within their placement tolerance, and it catches bridges and missing parts. It cannot see beneath an area array package, which is where X-ray inspection is used, and functional test confirms that the assembly behaves as intended.
Rework requires the same temperature control as the original process. A joint that is reworked with a hand iron at the wrong temperature, or reheated without a proper profile, may pass a continuity test and still be weaker than the joints around it. The causes of component shift during reflow are worth reviewing before any rework, because the defect being repaired is often a symptom of a process setting rather than an isolated event.
Design Decisions That Help the Line
The assembly process is easier when the design anticipates it. Consistent orientation of similar components reduces the risk of a placement error, fiducials on the panel allow the machine to align accurately, and enough space between components lets the nozzle reach its target without disturbing a neighbour. Panelisation and the position of break-away rails are part of the same decision because the board has to be handled through the line.
Thermal relief on through-hole pads, adequate solder mask clearance and a sensible pad-to-pad spacing all reduce defects at the wave. The placement order and pad positioning rules used for assembly and the broader design rules that keep a board manufacturable describe the constraints from the fabricator and assembler side, and applying them at layout time is far cheaper than correcting them in production.
FAQ
Why is the placement machine before the reflow oven? Because the paste that holds the components is printed first and the joints are formed last. The placement machine only positions parts, so it must run before the oven, and it can only work if the printer has already deposited the paste.
Can surface mount parts be wave soldered? Some can, with adhesive holding them in place, but the thermal and shadowing constraints limit the technique. Where a board has both component types, reflow first and wave afterwards is the usual sequence.
What determines whether an inert atmosphere is worth using? The pitch and the surface finish. Nitrogen widens the process window and reduces oxidation on fine-pitch assemblies and on finishes that oxidise readily, and the benefit falls away as the geometry becomes coarser.
