Through-Hole vs Surface Mount Assembly: Choosing the Process
The choice between through-hole and surface mount assembly is usually settled by the component list rather than by the board, but the consequences reach back into the layout. Where both technologies appear on one board, the order in which they are assembled decides which side is populated first and which features the layout has to protect.
Why Through-Hole Parts Survive
Through-hole components remain in use for reasons that have nothing to do with nostalgia. A connector that must withstand repeated mechanical insertion, a component that dissipates enough heat to need a large lead frame, and a part that must survive a high voltage transient are all better served by a lead that passes through the board and is soldered on the far side.
The joint itself is also more inspectable. A through-hole joint can be examined from the solder side without an X-ray, and a joint that is visibly filled is a joint that can be trusted. That property keeps through-hole construction in aerospace, defence and industrial products where the inspection record matters.
Where Surface Mount Wins
Surface mount packages are smaller, cheaper and available in a far wider range, and they are the only option for the fine-pitch devices that carry modern processors. The assembly process is also cheaper at volume, because the placement is automatic and the whole side is reflowed in one pass rather than soldered joint by joint.
The trade is that a surface mount joint is small and hidden. Inspection depends on optical or X-ray methods, and the joint’s strength depends on the pad geometry and the amount of paste printed, which are process variables rather than design variables once the layout is fixed. Our automated optical inspection notes describe how the verification is carried out.
Mixed Technology and the Assembly Order
A board that carries both technologies is assembled in a defined order. The surface mount parts are placed and reflowed first, on one side and then the other if both sides carry them, and the through-hole parts are inserted afterwards and soldered by wave, by selective soldering or by hand.
That order imposes constraints on the layout. Any through-hole part placed on the side that is reflowed second must be able to survive the reflow oven, and the adhesive that holds the second-side surface mount parts in place must not be disturbed by the insertion of the through-hole leads. Planning the order before the placement is fixed avoids a redesign.
Through-Hole Joints in a Wave Solder Machine
Wave soldering remains the fastest way to solder many through-hole joints at once, and it depends on the board being able to pass over a standing wave of molten solder without dislodging anything already fitted. That is why the parts on the wave side of a mixed board are normally held by adhesive, and why the layout keeps heavy parts away from the leading edge.
Thermal relief matters more in wave soldering than anywhere else, because a pad connected to a large plane will not reach soldering temperature while the wave is in contact. The relief spokes control how quickly the pad heats, and a pad without relief produces a joint that looks soldered and is not. Our solderability notes describe how the wetting behaviour is verified.
Selective Soldering for Mixed Boards
Selective soldering applies a small nozzle of solder to individual joints, which suits a mixed board with a handful of through-hole parts and a large number of surface mount ones. It avoids the thermal shock that a wave imposes on the whole assembly and reduces the masking that would otherwise be needed.
The technique is slower per joint but faster overall on a board with few through-hole joints, because the masking and de-masking steps disappear. The layout requirement is that the joints remain accessible to the nozzle, which means keeping tall components away from the through-hole row.
Hand Soldering and Its Limits
Hand soldering is the fallback for prototypes, rework and the small number of joints that neither wave nor selective soldering can reach. It is also the least repeatable of the methods, because the result depends on the operator’s choice of tip, temperature and dwell time.
Where hand soldering is expected, the layout should make it easy: pads large enough to be reached with a tip, thermal relief on plane connections, and a mask opening that leaves a visible fillet. A joint that is hard to reach will be made badly regardless of the skill of the operator.
Design Rules That Serve Assembly
The placement rules that make assembly reliable are the same ones that make hand assembly possible: pin one marked on the silkscreen, polarity marks that survive the mask process, adequate courtyard space between parts, and fiducials that the placement machine can find. None of these cost anything at the layout stage and all of them cost time if they are missing.
Where a board will be assembled in more than one factory, the rules should be set to the tighter of the two process capabilities rather than to the average. Our cost reduction notes describe how much of the assembly cost is decided by these layout choices.
Cost Comparison at Different Volumes
At low volume the assembly method is chosen for convenience, because the setup cost dominates and the number of joints barely matters. As volume rises the balance shifts toward the method with the lowest cost per joint, which is wave or reflow, and the design should be arranged so that it can use them.
The crossover also depends on the number of through-hole joints. A board with a single connector may be cheaper to build with selective soldering at any volume, because the alternative would require a wave machine setup that the job cannot justify. Our fabrication notes guidance explains how these choices are recorded.
Process Control and Verification
Reviewing the design before the data is released is far cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
FAQ
Can a surface mount board be reworked by hand? Yes, and the practical limit is the pitch rather than the technology. A package with visible leads can be removed and replaced with a hot air tool or an iron, while an area array package needs either a rework station with a localised preheater or a stencil-printed reball. Planning for rework means leaving space around the part on the layout.
Which side should the through-hole parts go on? The side that is soldered last, which in a mixed board is normally the bottom. That keeps them out of the reflow oven and away from the adhesive cure, but it also means the bottom-side surface mount parts must be able to survive the soldering step that follows.
How does gopcb decide between wave and selective soldering? We look at the number of through-hole joints, the thermal mass of the assembly and the accessibility of each joint. Where the count is low and the surface mount content is high, selective soldering usually wins on total cost even though it is slower per joint.
