SMD and Through-Hole Components in SMT Batch Assembly

A surface mount line can place thousands of parts per hour, yet most production boards still carry a handful of through-hole components. The mix is not a legacy accident. It reflects a set of requirements that surface mount technology serves poorly, and understanding those requirements is what makes a component selection decision defensible rather than habitual. This article examines what each family offers and where the trade-offs land in a batch assembly environment.

Why the Mix Persists

Through-hole parts are inserted into drilled holes and soldered on the opposite side, which gives a mechanical anchor that a surface joint does not have. The lead passes through the board and the solder fillet forms on both the barrel and the pad, so the connection resists peeling and shearing far better than a flat joint.

That single property explains most of the remaining demand. Connectors that will be plugged and unplugged, large inductors and transformers that carry real mass, and parts that will be stressed during final product assembly all benefit from a joint whose strength comes from geometry rather than from adhesion.

What Surface Mount Brings

Surface mount parts sit on pads rather than through holes. They are smaller, lighter and cheaper in volume, and they allow the placer to populate both sides of the board in a single pass. Removing the drilled hole frees routing space underneath the component, which is often what makes a dense layout possible at all.

The electrical behaviour improves as well. Short leads mean low inductance and low resistance, which matters for high frequency and high current paths. A surface mount decoupling capacitor placed a millimetre from the pin it serves does its job far better than an equivalent part with a long lead. For more on how these decisions interact with the rest of the build, see our notes on PCBA soldering requirements.

Where Through-Hole Components Still Win

Thermal performance is the clearest case. A through-hole package can conduct heat into the board and into a heatsink through its leads, and it can be mounted against a metal chassis with a screw. Surface mount parts rely on the copper area around them, which is limited on a crowded board.

Mechanical strength under vibration follows the same logic. The lead acts as a compliant spring that absorbs movement instead of transferring it into the solder joint, which is why through-hole parts survive shock and vibration testing that cracks surface joints.

Process Consequences on the Line

The presence of through-hole components forces a second operation. After reflow, the board passes through wave soldering, selective soldering or hand soldering, and each of those steps adds tooling, time and a new set of defects to control.

Wave soldering is efficient when the through-hole count is high and the parts are on one side. Selective soldering suits mixed assemblies where wave exposure would damage parts already placed on the underside. Hand soldering covers the remainder and depends entirely on operator skill. Our SMT prototype BOM checklist covers how to flag these parts before the build starts.

Design Choices That Follow

Decide early. A part that moves from surface mount to through-hole after the board is released forces a re-spin for the hole pattern, the keep-out and often the routing. Adding a through-hole part also consumes inner layer area on every layer it passes through.

When a through-hole part is unavoidable, place it on one side where possible so that a single soldering operation covers all of them. Group them near the board edge for easier tooling, and keep the thermal relief on the pad barrel consistent so that heat does not sink away from the joint during soldering.

Reliability and Tolerance

Component tolerance and reliability interact with package choice more than most designers expect. A through-hole resistor can absorb board flex through its lead, while a large surface mount part can crack at the termination when the board bends during depanelisation.

Where a product will see thermal cycling, the mismatch between the expansion of the package and the expansion of the board drives the stress. A compliant lead decouples the two, and that is often worth the extra process step and the extra board area.

Placement Accuracy in Mixed Technology

Mixing package families changes what placement accuracy has to deliver. A board carrying fine pitch surface mount parts and heavy through-hole connectors has features at both extremes of the tolerance scale, so the placer cannot be optimised for one and still satisfy the other. Reference points must be chosen where both the fine features and the connector positions can be reached within the field of view.

The through-hole parts themselves do not need micron accuracy, but the pads they share a board with do. Placing the fine pitch parts first, before the heavy inserted components disturb the panel, keeps the fiducial recognition stable and preserves the accuracy that the fine parts require.

Inspection After Mixed Assembly

Inspection has to cover two joint types with different failure modes. Surface joints fail as voids, insufficient solder or tombstoning, which are visible optically. Through-hole joints fail as insufficient barrel fill or voids inside the barrel, which are not visible from the surface at all.

That difference drives the equipment list. Automated optical inspection handles the surface joints efficiently, while barrel fill needs X-ray sampling or destructive cross-sectioning on a periodic basis. Planning both from the start prevents the common situation where the through-hole joints are simply assumed to be good.

Cost Comparison in Practice

Comparing the two families by component price alone is misleading. The real comparison includes the cost of the extra soldering operation, the tooling, the additional inspection and the board area consumed by holes and keep-outs.

Those costs are mostly fixed per build rather than per unit, which is why the economics shift with volume. A small batch can absorb a handful of hand soldered connectors easily, while a large batch benefits from designing them out entirely and paying the redesign cost once.

Documentation and Traceability for Mixed Builds

A mixed technology board needs two sets of process documentation. The reflow profile covers the surface mount stage, and a separate profile covers the wave or selective soldering stage that follows. Both should be recorded for the build rather than left as defaults from a previous product.

Traceability follows the same split. Components from one reel may pass through two different processes, so the record has to link the reel to the board at each stage. When a fault appears later, that link is what allows the affected units to be identified instead of the whole batch being quarantined.

When to Convert a Part

Conversion is worth considering when a part is available in both families and the only reason for the current choice is history. Moving a small resistor from through-hole to surface mount removes two holes, frees routing space and deletes a soldering step from the process.

It is not worth considering when the part carries real current, dissipates real heat or experiences mechanical load. Those are precisely the conditions where the lead does useful work, and replacing it with a flat joint trades a process cost for a reliability risk.

FAQ

Can all surface mount boards be assembled without through-hole parts? Yes for many products, but not for those that need heavy connectors, large magnetic components or joints that must survive repeated mechanical load. In those cases the through-hole part is the cheaper answer overall.

Does adding through-hole components slow production? It adds a second soldering operation, which affects tooling and cycle time. The impact is small when the parts are few and grouped together, and significant when they are scattered across the board.

Which approach costs less in a small batch? Surface mount assembly usually costs less because it needs no additional tooling, but a design that forces hand soldering of a large number of leads loses that advantage quickly.