A PCB can pass electrical design review and still fail in production.Mixed PCB assembly sounds simple: place SMT components, insert through-hole parts, solder everything, and ship.

In real production, mixed SMT and DIP designs can become difficult or even impossible to manufacture if the through-hole layout is not designed for the assembly process. Hole size, component position, wave solder tooling, and clearance to nearby SMT parts all matter.

Locating Holes: Too Large Can Make Connectors Sink or Tilt

Connectors such as USB Type-C often include positioning posts or locating pins. These pins help the part sit in the correct location before soldering.

A common design rule is that the PCB locating hole should be slightly larger than the component post, usually by about 0.10 mm to 0.15 mm. If the hole is too small, insertion becomes difficult. If the hole is too large, the part can move, sink, or tilt during placement and reflow.

In one case, a Type-C connector had oversized PCB locating holes. During SMT placement, the front side of the connector sank downward. This created alignment risk and reduced soldering consistency.

Design takeaway:
Mechanical locating holes should control position, not simply “make room” for the part.

DIP Hole Diameter: Bigger Is Not Always Safer

For DIP components, the plated through hole must fit the pin while still allowing solder to fill the barrel. A practical rule is to make the finished hole around 0.3 mm larger than the component lead diameter, depending on the component and soldering method.

In one case, a pin header needed to align with another board after assembly. The component lead diameter was about 0.5 mm, and the recommended PCB hole size was about 0.8 mm. However, the actual PCB hole was around 1.05 mm. After manual soldering, the header tilted and could not align accurately with the mating board.

The issue was not soldering skill. The hole was too large to hold the component position during assembly.

Design takeaway:
Through-hole clearance must support both solder filling and mechanical alignment.

SMT-to-DIP Clearance: Leave Space for Wave Solder Fixtures

If a board uses both SMT and DIP parts, the soldering process must be considered from the layout stage.

For double-sided boards using wave soldering, a fixture may be required to protect SMT components on the soldering side. These fixtures need physical space around DIP pads. If SMT pads are placed too close to DIP soldering areas, the fixture cannot cover the SMT components properly.

In one production review, several DIP pads were too close to nearby SMT pads. Manual soldering became difficult, and wave soldering could not be implemented reliably. The product was not suitable for mass production without layout changes.

A practical minimum clearance between SMT pads and DIP soldering areas is often around 2 mm, because the fixture wall and protection area need enough material thickness.

Design takeaway:
When designing SMT+DIP boards, leave room for the soldering process, not just the component body.

DIP Parts on Both Sides: Possible in CAD, Difficult in Production

A double-sided PCB can technically place DIP components on both sides. But in manufacturing, this can create major process challenges.

If DIP parts are distributed on both top and bottom sides, it becomes harder to choose a stable soldering method. Wave soldering may not be practical, fixtures become complicated, and manual soldering may increase labor cost and variation.

In one case, a double-sided PCB placed through-hole parts on both sides. During DFM review, the layout was found unsuitable for efficient mass production. The recommended improvement was to place DIP components on one side as much as possible.

Design takeaway:
A layout that is electrically correct can still be operationally poor. Keep DIP assembly flow simple when production volume matters.

The Manufacturing Review Question Designers Should Ask

When placing through-hole parts, ask this question early:

How will this part actually be soldered in production?

If the answer is manual soldering, check access, alignment, and operator repeatability.

If the answer is wave soldering, check direction, fixture clearance, shadowing, and bottom-side SMT protection.

If the answer is selective soldering, check nozzle access and thermal spacing.

Practical Checklist for Through-Hole and Mixed Assembly Layouts

Before releasing a mixed SMT+DIP PCB, review:

• Are locating holes matched to the connector’s positioning posts?
• Is the DIP hole size matched to the actual lead diameter?
• Can the component stay vertical before soldering?
• Is there enough clearance between DIP solder pads and nearby SMT pads?
• Can a wave solder or selective solder fixture be designed?
• Are through-hole parts concentrated on one side where possible?
• Can operators inspect and rework the solder joints?

Final Thoughts

Through-hole design is not old-fashioned. It is still critical for connectors, headers, power parts, mechanical interfaces, and high-reliability assemblies.

The cost of a poorly designed through-hole layout is not only a bad solder joint. It can create alignment failure, slow manual work, unstable yield, or a board that cannot be scaled into production.

At NexPCB, DFM review connects PCB layout decisions with real assembly methods, helping Clients avoid production problems before the first build reaches the line.