Surface Mount Assembly versus Through Hole: How to Choose

The choice between surface mount and through hole technology is no longer a question of which process is modern. Both are mature, both are used in high reliability products, and most boards today carry a mixture of the two. The decision belongs to the individual component and to the mechanical and thermal requirements of the joint, not to the board as a whole.

What Each Process Does

Surface mount assembly places components on pads on the board surface, holds them with solder paste printed through a stencil, and reflows the whole assembly in an oven. The joints are formed between a pad and a termination, with no hole through the board.

Through hole technology inserts leads into drilled and plated holes filling them with solder, usually by wave soldering or by selective soldering where the assembly is mixed. The joint is a fillet inside the barrel as well as on the pad, which makes it mechanically much stronger.

Why Surface Mount Dominates

Surface mount is the default because it allows higher density. Components can be placed on both sides of the board, the pad is only slightly larger than the part, and the lack of a drill pattern frees the layers beneath for routing. Microvias and fine pitch packages would be impossible if every connection needed a hole.

The process is also faster and cheaper at volume. Paste printing, placement and reflow are continuous, there is no per hole cost, and the assembly can be inspected automatically. This is the reason passive components have moved almost entirely to surface mount packages, and the reason through hole parts now look old fashioned even when they are the better engineering choice.

Where Through Hole Still Wins

Mechanical strength is the first reason. A connector that will be plugged and unplugged repeatedly, a terminal block that takes a screwdriver, or a part that carries the weight of a cable all benefit from a joint that passes through the board rather than sitting on its surface.

Thermal and electrical reasons follow. Power devices with a large lead frame can dissipate heat through the leads into the barrel, and a through hole connection to an internal plane has a lower thermal and electrical resistance than a surface pad with vias. High voltage parts also favour the through hole arrangement, since the creepage path through the board is longer than across the surface.

Joint Strength and Reliability

A through hole joint tolerates bending and thermal cycling better than a surface mount joint, because the solder fills a barrel and the stress is distributed along the lead rather than concentrated at the termination. Where a board will be flexed during assembly or service, that difference decides the outcome.

A surface mount joint, by contrast, is loaded in shear and depends on the termination geometry. Component shift, tombstoning and cracked fillets are the characteristic defects, and their causes are covered in SMT component shift causes.

Process Steps and Their Control Points

The surface mount sequence begins with stencil printing, which sets the paste volume and therefore the joint. Aperture design, stencil thickness, squeegee pressure and separation speed all affect the deposit, and a deposit that is short of paste produces a weak joint even though the placement is perfect.

Reflow soldering follows, with a profile that has to soak the flux, activate it, bring every joint above liquidus and cool at a controlled rate. Through hole wave soldering has its own variables: flux application, preheat, wave height, conveyor speed and the thermal shadow cast by tall components. Both processes depend on control rather than on equipment alone.

Mixed Technology Assemblies

Most modern boards mix the two. The usual sequence prints paste, places surface mount parts, reflows, and then inserts the through hole parts, which are soldered by wave, selective soldering or by hand depending on the volume and the thermal sensitivity of the assembly.

Selective soldering has largely replaced wave soldering for mixed boards, because it applies heat only where it is needed and leaves the reflowed joints and the surface mount parts untouched. Manual soldering is still appropriate for a handful of parts in a prototype or for repairs, and its quality depends on operator skill and on a defined workmanship standard.

Rework and Repair

Rework is easier on through hole joints, which can be desoldered with a pump or wick and refilled without special equipment. A fine pitch surface mount package needs hot air or infrared with a controlled profile, a preheat stage to avoid thermal shock, and a fixture to protect the surrounding parts.

Design for rework matters. Leaving space around a large package for a nozzle, avoiding placing a thermally sensitive part next to a device that runs hot during rework, and providing a defined footprint that can be re-used if a joint has to be redone all reduce the cost of a repair.

Cost Considerations

Surface mount assembly is cheaper per connection at volume, but the comparison changes with quantity. A stencil and a placement programme carry a fixed cost, and for a handful of prototypes hand assembly of surface mount parts is often cheaper than setting up the line.

Through hole adds a per hole cost through drilling and plating and an additional assembly operation, but it saves the cost of a more complex board where the alternative would be a plated through via plus a surface pad. The comparison should be made on the whole assembly rather than on the component alone, and the pad geometry in both cases should follow the design rules in PCB pad design standards.

Choosing Per Component

A practical rule is to choose the process by the mechanical and thermal duty of the joint. Signal and low power parts go to surface mount. Connectors, terminals, parts subject to mechanical load and devices that must conduct heat into the board stay in through hole packages, even when a surface mount equivalent exists.

The alloy question is common to both. A lead free process changes the melting point and the wetting behaviour of every joint, and the consequences are set out in lead-free versus leaded solder. The decision between technologies should not be driven by the alloy, but the alloy has to be compatible with the components that were chosen.

Quality and Inspection

Surface mount inspection relies on automated optical inspection for the visible joints and on X-ray for the area array packages that cannot be seen. Through hole joints are inspected visually from the solder side, with a requirement on the fillet and on barrel fill for high reliability classes.

The acceptance standard should be agreed before production. Class 2 and Class 3 differ mainly in what is rejected, and for a product with a long service life the stricter class is usually worth the yield cost. The specification should also state how rework is to be recorded, because a reworked joint that is not documented is an unknown quantity in the field.

FAQ

Is through hole technology obsolete? No. It is the correct choice for connectors, power devices and anything mechanically loaded, and it remains common in industrial and automotive assemblies.

Can surface mount parts be used on a single sided board? Yes, and they usually are. The limitation is routing rather than the assembly process.

Which is more reliable? Neither in general. A well designed surface mount joint and a well designed through hole joint both last the life of the product, but the through hole joint tolerates more bending and carries more current.