SMT Assembly vs Through-Hole: Process and Cost

Almost every assembly question eventually becomes a process question: does this board go through reflow, through wave solder, or through both? The answer determines the cost, the defect risks, and the sequence the board follows down the line. Surface-mount and through-hole technologies are not competitors so much as different tools, and most products need a considered mix of the two.

How the Two Processes Differ

In SMT assembly the components sit on the surface of the board and are joined by solder paste printed through a stencil. Component density is high, the board is lighter and smaller, and the process is fully automated and suited to volume. The joints are made by reflowing the paste in a controlled thermal profile.

Through-hole components have leads that pass through the board and are soldered on the opposite side. Their pitch is coarser, they consume far more board area, and assembly is slower. What they offer is mechanical strength: a through-hole joint resists pull and shear far better than a surface joint, which is why connectors, transformers, and heavy parts are still through-hole even on otherwise surface-mount boards.

Solder Paste Printing

Solder paste printing is the step that decides most of the yield. A squeegee pushes paste through apertures in a stencil, leaving a controlled deposit on each pad. Stencil cost is modest, but it is not free, and routine maintenance of the stencil itself, including cleaning and guarding against scratches and dried paste on the web, is part of the operating cost. Aperture design determines paste volume, and paste volume determines whether a joint is starved, adequate, or bridged.

Paste handling matters as much as the printing. Paste must be brought to room temperature naturally before use; warming it quickly, with a heater or a hot plate, causes the flux system to separate and produces a paste that behaves unpredictably. Paste that has been open too long, or stirred too little, prints inconsistently, and inconsistent printing is the root cause of more assembly defects than any other single factor.

Reflow Soldering

Reflow soldering passes the board through a heated atmosphere that melts the printed paste, wets the pads and terminations, and then cools to form the joint. The profile has four regions: preheat, soak, reflow, and cooling, and each has a purpose. Preheat brings the assembly up without thermal shock. Soak activates the flux and equalises temperature across parts of different mass. Reflow melts the alloy and lets it wet. Cooling, if too fast, produces a brittle and stressed joint.

Reflow is a one-sided process. A double-sided board goes through the oven twice, and the second pass does not disturb the first side, because the alloy that has already solidified does not remelt at the profile temperature used for the second side. That is why the sequence of sides, and the thermal mass on each, has to be planned rather than discovered.

Wave Soldering

Wave soldering brings the board into contact with a standing wave of molten solder, which wets the exposed leads and pads. It is fast and well suited to through-hole work, and it is usually the cheaper option when the joint count is high and the components are simple. The limitation is that the solder has to reach the joint, so components with fine pitch or with hidden terminations cannot use it.

Mixed-Technology Boards

When a board carries both types, both processes are used, and the order is not optional. Wave soldering must come after reflow. Two reasons. First, a board that has already been through the wave will not survive another pass through the reflow oven, because the through-hole parts stand off the board and will not sit flat on the conveyor. Second, the reflow profile is hot enough to disturb joints, and running the sequence backwards risks reworking connections that were already good.

The practical consequence is that mixed technology forces a decision: the through-hole parts must tolerate the reflow profile, or be excluded from it. Where they cannot, the alternative is selective soldering, which applies molten solder to individual joints and avoids exposing the whole assembly to a second thermal excursion.

What Drives the Cost

Three factors dominate. Component mix is the first: a board that needs both processes pays for two setups and two thermal passes. Package complexity is the second, since fine-pitch parts need accurate paste deposits and often X-ray inspection, while coarse-pitch parts do not. Volume is the third, because the stencil, the programming, and the line setup are fixed costs that spread across the order.

Where cost has to come down, the geometry is usually the place to look. Fewer unique components, footprints that match what the line already runs, and a layout that groups surface-mount parts on one side reduce both setup time and defect risk. The layout decisions that affect production are usually cheaper to change than the process.

Choosing the Right Mix

Connectors, transformers, high-current parts, and anything that will see mechanical load belong through-hole if the design allows it. Everything else should be surface-mount unless there is a reason not to be. Solder paste quality and printing discipline determine whether that choice pays off, and alloy selection follows from the product compliance requirements rather than from process preference, as the lead-free versus leaded comparison sets out.

One more risk is worth planning for: parts that move during reflow because the paste does not hold them. The causes are catalogued under component shift during reflow, and most of them are addressed at the stencil and footprint stage.

Inspection After Assembly

Inspection follows the process, and each method covers a different defect family. Optical inspection catches placement errors, missing parts, and visible bridging, and it is the only method that verifies the presence of a component. X-ray is needed where joints are hidden, which means ball-grid arrays, bottom-terminated packages, and any through-hole joint on a dense board; it reveals voids, opens, and insufficient solder. In-circuit or flying-probe test verifies electrical continuity and catches the faults that vision cannot, including a joint that looks perfect but has not wetted. Functional test then exercises the assembly as a working circuit. The sequence matters, because each stage is progressively more expensive to fail: a placement defect caught at optical inspection costs a rework pass, while the same defect caught at functional test costs a full diagnostic cycle. Where the board carries fine-pitch parts, all four stages are normally required rather than optional.

FAQ

Can a board be all through-hole? It can, and for low volumes it sometimes still is. The penalty is board area, assembly time, and joint count, all of which push unit cost up as volume rises.

Why must wave soldering come after reflow? Because the reflow profile is hot enough to disturb joints, and because through-hole parts standing off the board do not sit flat on a reflow conveyor.

How many times can a board go through reflow? Twice is normal for a double-sided assembly. Each additional pass stresses the laminate and the plating, so the number of passes should be minimised by design.

What is the single biggest cause of assembly defects? Paste volume, and therefore the stencil and the printing process. Joint problems are usually printing problems that only become visible at reflow.