The through-hole vs surface mount decision is usually framed as old against new. That framing is useless at the bench, because most boards need both. The question worth asking per part is narrower: what does this mounting style commit the assembly process to, and what does it cost in board area, inspection, and supply? Below are the numbers, the standards, and the cases where each still wins.
Key takeaways
- The same die in a through-hole package occupies roughly four times the board area of its surface-mount version. Worked below for the ATmega328P.
- The two technologies have separate land-pattern standards: IPC-7251 for through-hole, IPC-7351 for surface mount. They are not interchangeable references.
- IPC-A-610 sets a 75 % minimum vertical fill for plated through-hole joints in Classes 2 and 3. Class 2 permits a documented exception; Class 3 does not.
- Moisture sensitivity binds surface mount, not through-hole. An MSL 3 part has 168 hours of floor life once the bag is opened.
- Mount style is often a family option rather than a different part, so check the supplier’s profile list before treating it as a redesign.
[IMAGE 1: same PCB region shown twice, once populated with through-hole parts and once with the surface-mount equivalents, with occupied area shaded | alt: “Through-hole vs surface mount board area for the same circuit”]
What separates through-hole from surface mount
Through-hole parts have leads that pass through drilled, plated holes and are soldered on the far side. Surface-mount parts have terminations that sit on pads on the same side as the body, held only by the solder joint. That single difference sets board area, mechanical strength, soldering process, inspection method, and handling rules.
Everything else in the comparison follows from it. The hole gives mechanical anchorage and a barrel the solder must fill. The pad gives density and lets both sides of the board carry parts.
One die, three packages: the area and supply comparison
Microchip ships the ATmega328P in a 28-pin PDIP, a 32-pin TQFP at 7 × 7 mm, and a 32-pin VQFN at 5 × 5 mm. Same silicon, three mounting outcomes.
Take footprint area. A 28-lead 0.300 in DIP with 1.5 mm pads spans about 9.1 mm across the pad rows and about 34.5 mm along them:
34.5 mm × 9.1 mm ≈ 314 mm²
The TQFP body is 7 × 7 mm; allowing a courtyard of roughly 9 × 9 mm gives 81 mm². The through-hole version costs about 3.9× the board area for identical function, and it forecloses placing anything on the opposite side beneath it.
| Package | Mounting | Body / footprint | Reported supply position, June 2026 |
| ATMEGA328P-PU, 28-PDIP | Through-hole | 0.300 in rows, ≈ 314 mm² footprint | ≈ 43,000 at DigiKey; $2.89 at qty 1; 6-week DigiKey manufacturer standard lead time |
| ATMEGA328P-AU, 32-TQFP | Surface mount | 7 × 7 mm body, ≈ 81 mm² courtyard | Reel position ≈ 70,000 at DigiKey |
| ATMEGA328P-MU, 32-VQFN | Surface mount | 5 × 5 mm body | ≈ 312,000 direct from Microchip |
Supply data from a Findchips survey published 29 June 2026; PDIP price and lead time from the DigiKey product listing.
Read the right-hand column carefully. The surface-mount versions of the same part are stocked an order of magnitude deeper. Choosing through-hole for “availability” reasons is, for this part, backwards.
The parametric comparison
| Factor | Through-hole | Surface mount |
| Land-pattern standard | IPC-7251 | IPC-7351 |
| Typical soldering process | Wave or selective solder | Reflow |
| Board sides usable | One side blocked by lead exit | Both sides |
| Mechanical anchorage | Lead in a plated barrel | Solder joint on a pad only |
| Inspection | Visual both sides; X-ray for hidden barrels | AOI, X-ray for BGA and QFN |
| Moisture handling | Generally outside MSL floor-life control | MSL rated, floor life per J-STD-033 |
| Rework | Iron and desoldering tool | Hot air, stencil, or rework station |
| Smallest practical passive | Axial or radial body, several mm | 01005 metric chip |
| Prototyping | Breadboard and hand assembly | Requires paste, stencil, or hot air |
Two technologies, two land-pattern standards
Most comparisons treat footprint design as one topic. It is two. IPC-7251 covers through-hole land patterns; IPC-7351 covers surface-mount land patterns. A library built to one says nothing about the other.
The through-hole side starts from the lead, not the pad. Take a rectangular lead measuring 0.36 mm by 0.56 mm at maximum material condition. The diagonal is:
√(0.36² + 0.56²) = 0.67 mm
IPC-7251 adds a density-level allowance to that figure: 0.25 mm at Level A, 0.20 mm at Level B, 0.15 mm at Level C. At Level B the finished hole lands at 0.87 mm, so a 0.9 mm drill with a 1.5 mm pad. Nothing in the surface-mount standard resembles this calculation, because there is no barrel to fill.
[INTERNAL LINK: IPC-7251 through-hole land patterns -> through-hole footprint design]
[IMAGE 2: side-by-side land pattern construction, a plated barrel with annular ring against a surface-mount toe-heel-side pad | alt: “Through-hole vs surface mount land pattern construction under IPC-7251 and IPC-7351”]
What the acceptance standard requires of each
IPC-A-610 sets the accept-reject line, and the through-hole criteria are stricter than most designers expect. Classes 2 and 3 both require a minimum 75 % vertical fill of the plated barrel.
The difference is the exception. Sierra Circuits’ summary of IPC-A-610H describes a Class 2 allowance dropping to 50 % vertical fill, or 1.2 mm, whichever is less, provided specific conditions hold: the barrel connects to internal planes, the lead is visible on the source side, and the destination side has wetted 360°. L3Harris supplier specification IS-003 states plainly that Class 3 permits no exception to the 75 % minimum, citing IPC-A-610 section 7.3.5 and Table 7-4.
Circumferential wetting requirements also tighten from Class 2 to Class 3, but published summaries disagree on the exact figures. Read the revision you are building to rather than a secondary source, including this one.
The practical consequence: a through-hole joint hides most of its evidence inside the board. Achieving 75 % fill on a thick multilayer with internal planes is a thermal problem, and verifying it usually means X-ray. A surface-mount fillet is visible to AOI in one pass.
Moisture: a clock that runs on one side only
Surface-mount plastic packages absorb moisture and can crack during reflow. IPC/JEDEC J-STD-020 classifies the sensitivity; J-STD-033 governs handling. Floor life is the time a part may sit exposed at 30 °C and 60 % RH after the moisture barrier bag is opened.
| MSL | Floor life at ≤30 °C / 60 % RH |
| 1 | Unlimited |
| 2 | 1 year |
| 2a | 4 weeks |
| 3 | 168 hours |
| 4 | 72 hours |
| 5 | 48 hours |
| 5a | 24 hours |
| 6 | Per label, bake before reflow |
An MSL 3 part left out over a long weekend has spent a third of its budget. Only baking resets the clock; resealing halts it but does not restore it. Dry storage below 5 % RH is treated as equivalent to a sealed bag.
Through-hole parts sit largely outside this regime, because wave and selective soldering do not drive the whole body through a reflow profile. That is a genuine operational advantage of through-hole and one that no ranking page mentions.
Where through-hole still wins on merit
Mechanical load paths. Connectors, terminal blocks, transformers, and anything a user pushes, pulls, or plugs into belongs in a hole. A surface-mount joint resists peel with solder alone.
High current and high voltage. A plated barrel plus a lead carries more current than a pad of the same footprint, and the extra spacing between through-hole terminals helps creepage.
Field service and socketing. Sockets, replaceable firmware, and parts a technician will change in the field all need leads.
Prototype iteration. Hand assembly, breadboarding, and probing are faster with leads. That is a development-cost argument, not a production one.
Shock and vibration. The barrel shares load across the board thickness rather than across a surface joint. Where a design must survive drops, the anchorage is worth the area.
Thermal performance runs the other way. A through-hole body sits above the board on its standoff and couples to copper only through its leads. Surface-mount packages with an exposed pad conduct straight into a via field, which is why power devices moved to surface mount even though their mechanical case for leads was strong.
Cost, supply, and the availability argument
The cost picture is not uniform. Surface-mount placement is fast and automated, but the process carries stencil, paste, and reflow overhead that a small run may not absorb. Through-hole insertion is slower per part and increasingly manual, which is why it dominates unit cost at volume.
Supply is where received wisdom fails most often. Several ranking pages assert that through-hole parts are getting hard to find. The ATmega328P data above shows the opposite for that part: the through-hole version is stocked, priced, and shipping, while carrying the longest factory lead time of the three.
Check per part, not per technology. And check whether the mount style is even a different part number, since Bourns lists surface-mount SIP profiles (4300H, 4300M, 4300R, 4600H, 4600M, 4600X) alongside its through-hole resistor networks in the same families. Switching mount style there is a suffix change, not a redesign.
[INTERNAL LINK: component obsolescence and second-source qualification -> lifecycle management]
Mixed technology is the real answer
Almost every current board is mixed: surface-mount actives and passives, through-hole connectors and power entry. The cost of mixing is process steps. A board with parts on both surfaces plus through-hole parts may need two reflow passes and a selective solder pass.
Group through-hole parts geographically so selective soldering can reach them in one path. Scattering six through-hole parts across a dense board costs more than clustering twelve at one edge.
Watch the interaction rules. Through-hole parts on the wave-solder side impose spacing and shadowing constraints on nearby surface-mount parts, and any surface-mount part on the wave side has to survive the pot.
[IMAGE 3: mixed-technology board with through-hole parts clustered at one edge and a selective solder path overlaid | alt: “Mixed through-hole vs surface mount assembly with clustered selective soldering path”]
Through-hole vs surface mount FAQ
What is the difference between through-hole and surface mount?
Through-hole parts have leads that pass through drilled, plated holes and solder on the opposite side. Surface-mount parts have terminations soldered to pads on the same side as the body. The difference sets board area, mechanical strength, soldering process, inspection method, and moisture handling rules.
Is through-hole stronger than surface mount?
For mechanical load, yes. A lead anchored in a plated barrel shares stress through the board thickness, while a surface-mount joint resists peel with solder alone. That is why connectors, terminal blocks, and user-accessed hardware stay through-hole even on otherwise all-SMT boards.
Why is surface mount cheaper?
Placement is automated and fast, parts are smaller and cheaper to package, and both board sides are usable. The savings appear at volume. For a handful of boards, stencil, paste, and reflow setup can make surface mount the more expensive route.
Can you mix through-hole and surface mount on one board?
Yes, and most current boards do. The cost is process steps: mixed boards often need two reflow passes plus a selective solder pass. Cluster the through-hole parts so the selective solder path is short, rather than scattering them across the board.
Do through-hole components need an MSL rating?
Generally not, because wave and selective soldering do not put the whole body through a reflow profile. Surface-mount plastic packages are MSL rated under J-STD-020, with floor life controlled by J-STD-033 — 168 hours for MSL 3 at 30 °C and 60 % RH.
Is through-hole obsolete?
No. It remains standard for connectors, power entry, high-current paths, sockets, and anything mechanically loaded. What has changed is that it is no longer the default for actives and passives, and package availability for new logic and microcontrollers now favors surface mount.
What to do next
Assign mount style per part, not per board. Anything carrying mechanical load, high current, or a user connection goes through-hole; everything else defaults to surface mount unless a specific reason overrides it.
If board area is the binding constraint, run the comparison for your actual part rather than assuming. The ATmega328P case gives 314 mm² against 81 mm² for the same die, a figure large enough to decide an enclosure.
If you are specifying the assembly, put the class on the drawing. “Build to IPC-A-610” with no class named leaves the 75 % vertical fill exception open to interpretation, and that exception is the difference between a Class 2 and a Class 3 product on the same board.
Primary sources
- IPC-A-610 Class 2 and Class 3 barrel fill criteria, summarized — https://www.protoexpress.com/blog/ipc-class-2-vs-class-3-different-design-rules/
- L3Harris supplier specification IS-003, Class 3 vertical fill requirement citing IPC-A-610 §7.3.5 and Table 7-4 — https://suppliers.bcs.l3harris.com/docs/quality/internal-specifications/is-003
- IPC/JEDEC J-STD-033 floor life reference conditions and MSL scale — https://en.wikipedia.org/wiki/Moisture_sensitivity_level
- ATmega328P package variants and supply survey, 29 June 2026 — https://blog.findchips.com/atmega328p-2026-pinout-packages-availability/
- Bourns 4600X Series datasheet, REV. 10/1/20, listing surface-mount SIP profiles — https://www.bourns.com/docs/Product-Datasheets/4600x.pdf
- IPC-7251 through-hole density-level hole sizing summary — https://www.nwengineeringllc.com/article/plated-through-hole-size-for-pcb-component-leads.php
