{"id":277,"date":"2026-08-11T09:00:00","date_gmt":"2026-08-11T09:00:00","guid":{"rendered":"https:\/\/rlxsolutions.com\/blog\/smt-vs-through-hole-assembly\/"},"modified":"2026-08-11T09:00:00","modified_gmt":"2026-08-11T09:00:00","slug":"smt-vs-through-hole-assembly","status":"publish","type":"post","link":"https:\/\/rlxsolutions.com\/blog\/smt-vs-through-hole-assembly\/","title":{"rendered":"SMT vs Through-Hole: Surface Mount vs THT Assembly"},"content":{"rendered":"

Surface mount technology (SMT) places components directly onto PCB pads, while through-hole technology (THT) inserts leads through drilled holes before soldering. SMT usually suits dense, automated assemblies; THT remains useful where component geometry or mechanical loading calls for leaded mounting. RLX Solutions<\/a>, a contract electronics manufacturer in Concord, Ontario, supports boards that use either method or combine both.<\/p>\n

SMT vs Through-Hole at a Glance<\/h2>\n

The practical difference is not simply component size. Choosing SMT<\/a> or THT changes the printed circuit board layout, fabrication requirements, assembly sequence, inspection strategy and often the amount of manual processing required.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Decision Factor<\/th>\nSMT<\/th>\nThrough-Hole<\/th>\n<\/tr>\n<\/thead>\n
Component mounting<\/td>\nComponent terminations land on surface pads<\/td>\nLeads pass through drilled PCB holes<\/td>\n<\/tr>\n
Board density<\/td>\nSupports high component density and fine-pitch packages<\/td>\nConsumes board area with holes and annular rings<\/td>\n<\/tr>\n
Assembly method<\/td>\nWell suited to automated placement and reflow<\/td>\nMay involve insertion plus wave, selective or manual soldering<\/td>\n<\/tr>\n
Use of both PCB sides<\/td>\nComponents can be mounted on both sides<\/td>\nLead and solder-side geometry can restrict routing and placement<\/td>\n<\/tr>\n
Mechanical attachment<\/td>\nSolder joints connect terminations to pads<\/td>\nLeads extend through the circuit board<\/td>\n<\/tr>\n
PCB fabrication impact<\/td>\nPads do not inherently require component holes<\/td>\nPlated through-holes must be included in fabrication data<\/td>\n<\/tr>\n
Common package types<\/td>\nChip passives, QFN, BGA and other surface packages<\/td>\nLeaded connectors, switches and other insertion components<\/td>\n<\/tr>\n
Inspection challenge<\/td>\nHidden or fine-pitch joints can require X-ray<\/td>\nMany solder joints remain visually accessible<\/td>\n<\/tr>\n
Mixed assembly<\/td>\nOften processed first<\/td>\nOften inserted and soldered after SMT operations<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The distinction matters before layout is complete. Replacing a surface-mount connector with a through-hole version late in design can require more than a footprint change. Drill data, routing, clearances, solder access and assembly tooling may all change.<\/p>\n

Surface Mount Technology Packs More Function Into Board Area<\/h2>\n

SMT avoids drilling a component hole for every lead. A resistor, capacitor or integrated circuit can terminate on copper pads without occupying corresponding plated holes through the stackup. That gives the layout engineer more freedom to route internal and opposite-side features.<\/p>\n

Consider a hypothetical 6-layer, 1.6 mm board containing a 0.5 mm-pitch surface-mount integrated circuit and small chip passives around it. Those components can sit close to the device while signals transition through nearby vias. Changing several surrounding components to leaded through-hole packages introduces holes through the stack and clearance requirements around those holes.<\/p>\n

That difference propagates through the layout. A through-hole lead is not confined to the component side. Its plated barrel passes through the PCB, so it can remove routing space from multiple layers even when the electrical connection only needs one or two of them.<\/p>\n

SMT density also creates its own manufacturing constraints. Fine-pitch pads put more emphasis on solder paste deposition, placement accuracy and reflow control. Solder paste inspection (SPI) can catch paste volume, area and offset problems before reflow, while automated optical inspection (AOI) checks the assembled board<\/a> after reflow. Packages with joints hidden beneath the body, such as ball grid arrays (BGAs), land grid arrays (LGAs) and QFNs, can require X-ray inspection.<\/p>\n

The engineering trade-off is therefore routing freedom versus process sensitivity. SMT can free substantial PCB area, but finer geometry shifts more assembly risk into printing, placement and reflow.<\/p>\n

Through-Hole Assembly Changes the PCB, Not Just the Assembly Process<\/h2>\n

A through-hole component requires a relationship between the finished hole, component lead, plating and surrounding pad. The hole is therefore part of both the mechanical fit and the electrical interconnect.<\/p>\n

For a plated through-hole (PTH), fabrication starts with drilling and includes plating the hole wall to establish an electrical connection through the required copper layers. The assembly operation later inserts the component lead into that fabricated feature and solders it.<\/p>\n

Hole geometry deserves attention because board thickness affects the drilled feature. As an illustrative comparison, take a 1.5 mm-thick PCB with a 0.30 mm finished plated hole. Dividing thickness by hole diameter gives a 5:1 aspect ratio. On a 3.0 mm board using the same 0.30 mm hole, the ratio becomes 10:1.<\/p>\n

Nothing changed in the component lead diameter, but the second circuit board presents a much deeper hole relative to its width. That affects fabrication and makes hole specification part of manufacturability rather than a detail that can be left until assembly.<\/p>\n

Lead fit matters at assembly as well. An unnecessarily large hole may provide poor component location, while a hole that does not accommodate the actual lead and applicable tolerances creates an insertion problem. The footprint should therefore come from the physical component and manufacturing requirements, not from a generic library symbol alone.<\/p>\n

Mechanical Loading Is a Strong Reason to Consider THT<\/h2>\n

Through-hole mounting can be useful where the component is expected to experience significant insertion, extraction or handling forces. The lead passes through the board rather than terminating only on a surface pad, changing how mechanical loads enter the assembly.<\/p>\n

Connectors are a common design case. A connector repeatedly subjected to cable insertion forces can impose bending and peeling loads on its soldered attachment. Through-hole leads, mounting posts or other mechanical retention features can help transfer those forces through the PCB structure.<\/p>\n

That does not mean every connector should be THT. A low-force internal connector may be entirely suitable as a surface-mount component. Nor does a through-hole electrical lead automatically solve every mechanical problem. Connector body support, board flex, enclosure attachment and the direction of applied force still matter.<\/p>\n

The useful design question is where the load goes. If normal product use repeatedly pushes or pulls on the component, analyse that load path before choosing the footprint.<\/p>\n

Board A vs Board B: Same Layer Count, Different Assembly Cost Drivers<\/h2>\n

Layer count alone tells you little about assembly effort. Two 8-layer boards can have very different manufacturing flows even if their bare-board dimensions and electrical function are similar.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Factor<\/th>\nBoard A<\/th>\nBoard B<\/th>\n<\/tr>\n<\/thead>\n
PCB stack<\/td>\n8 layers<\/td>\n8 layers<\/td>\n<\/tr>\n
Thickness<\/td>\n1.6 mm<\/td>\n1.6 mm<\/td>\n<\/tr>\n
Primary component technology<\/td>\nSurface mount<\/td>\nSurface mount plus THT<\/td>\n<\/tr>\n
Fine-pitch device<\/td>\n0.5 mm-pitch BGA<\/td>\n0.5 mm-pitch BGA<\/td>\n<\/tr>\n
Connector<\/td>\nSurface-mount<\/td>\n20-lead through-hole<\/td>\n<\/tr>\n
Main SMT sequence<\/td>\nPaste, place, reflow<\/td>\nPaste, place, reflow<\/td>\n<\/tr>\n
Additional assembly operation<\/td>\nNone from the connector<\/td>\nConnector insertion and through-hole soldering<\/td>\n<\/tr>\n
Hidden-joint inspection<\/td>\nBGA X-ray<\/td>\nBGA X-ray<\/td>\n<\/tr>\n
Through-hole considerations<\/td>\nNone from the connector<\/td>\nHole fit, solder access and THT inspection<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Board A can keep the connector inside the SMT placement and reflow sequence. Board B adds another assembly technology after the surface-mount process.<\/p>\n

That additional operation is the reason two assemblies with the same layer count and similar component counts can price differently. Component count is not a complete proxy for manufacturing effort. The number of process transitions, manual touches, inspection requirements and soldering operations can matter more.<\/p>\n

Board B may still be the better engineering choice if the connector experiences repeated mechanical loading. Paying for another process step can be rational when that step supports a product requirement. The mistake is selecting THT by habit when the application does not need what it adds.<\/p>\n

Mixed Technology Often Solves the False SMT-or-THT Choice<\/h2>\n

Many boards do not need to choose one technology exclusively. Mixed-technology assembly uses surface-mount components where density and automated placement matter, then through-hole parts where package availability or mechanical requirements point in that direction.<\/p>\n

A hypothetical industrial control board illustrates the logic. Suppose a 4-layer, 1.6 mm PCB uses surface-mount ICs and passives across most of the design but has two large leaded connectors along one edge. The SMT components can go through solder paste printing, placement and reflow first. The connectors can then be inserted and processed through an appropriate through-hole soldering operation.<\/p>\n

Selective soldering is particularly relevant when through-hole joints need to be processed without exposing the entire populated assembly to the same soldering operation. The physical layout still has to provide access for the intended process.<\/p>\n

This is where design for assembly (DFA) becomes more useful than a blanket rule such as \u201cuse SMT whenever possible.\u201d A mixed board may add a manufacturing stage, but it can preserve dense routing around semiconductors while using leaded components only where their geometry or attachment is justified.<\/p>\n

Assembly Sequence Can Decide Whether a Mixed Design Is Easy or Expensive<\/h2>\n

A mixed-technology circuit board should be laid out around the intended manufacturing sequence. Components that are individually easy to solder can become difficult when nearby parts obstruct access later in the process.<\/p>\n

    \n
  1. Apply solder paste to SMT pads and inspect the deposit.<\/li>\n
  2. Place surface-mount components.<\/li>\n
  3. Reflow the SMT side and perform post-reflow inspection.<\/li>\n
  4. Insert through-hole components.<\/li>\n
  5. Complete the required through-hole soldering operation.<\/li>\n
  6. Inspect the THT joints and perform the specified electrical or functional testing.<\/li>\n<\/ol>\n

    The exact process depends on the board, but the sequence exposes an important layout issue. A tall component installed too early can obstruct later soldering or inspection. A through-hole lead protruding into a restricted area can also interfere with fixtures or access.<\/p>\n

    The manufacturer therefore needs more than a bill of materials (BOM). Gerber RS-274X, ODB++ or IPC-2581 manufacturing data, NC drill information, pick-and-place data and assembly drawings help connect the electrical design to the intended manufacturing flow.<\/p>\n

    Inspection Requirements Differ Between SMT and THT<\/h2>\n

    SMT and THT fail in different ways, so relying on one inspection method for both can leave gaps.<\/p>\n

    For SMT, the process starts before a solder joint exists. SPI checks the printed solder paste for conditions such as volume, area and offset. After reflow, AOI can detect visible placement and soldering defects. X-ray inspection addresses joints that optical systems cannot see directly beneath packages such as BGAs, LGAs and QFNs.<\/p>\n

    Through-hole joints introduce a different geometry. Lead insertion, solder distribution and visible workmanship become part of the inspection problem. Manual visual inspection under magnification remains relevant, particularly when assessing workmanship against IPC-A-610 Class 2 or Class 3 requirements.<\/p>\n

    Inspection should then connect to electrical verification. Flying-probe in-circuit test (ICT) can provide net coverage without a dedicated fixture for prototype and lower-volume builds, while bed-of-nails ICT can suit production where a fixture is justified. Functional test (FCT) verifies behaviour against the customer’s specified test procedure.<\/p>\n

    Choosing SMT or THT therefore changes more than soldering. It changes where defects can hide and which inspection step has the best chance of detecting them before the product ships.<\/p>\n

    Use the Product Requirement to Choose SMT, THT or Both<\/h2>\n

    Start with package availability and electrical requirements, then examine density, mechanical loading and assembly access. This produces a more defensible decision than choosing a technology from a general cost rule. Learn more about SMT, through-hole and mixed-technology PCB assembly<\/a>.<\/p>\n

    Choose SMT when board area and routing density are major constraints, components are available in appropriate surface-mount packages, and the design suits automated paste, placement and reflow operations.<\/p>\n

    Consider THT where a required component is leaded or where the physical attachment provides a useful load path for a mechanically stressed component. Check hole geometry, lead fit and solder access as part of the footprint decision rather than after layout release.<\/p>\n

    Use mixed technology when neither method fits the whole board. Surface-mount semiconductors and passives can coexist with selected through-hole connectors or other leaded parts without forcing the entire design into one assembly method.<\/p>\n

    Most importantly, make the decision while the board is still editable. Once routing, enclosure geometry and component placement are locked, changing mounting technology can affect both bare PCB fabrication and assembly.<\/p>\n

    Get the Assembly Process Reviewed Before Release<\/h2>\n

    If your design combines fine-pitch SMT, BGAs, through-hole connectors or mixed assembly, RLX reviews design for manufacturability<\/a> and design for assembly as part of every quote. Send the fabrication data, BOM, pick-and-place file and assembly drawing so the proposed build sequence, inspection requirements and manufacturing constraints can be reviewed before production release.<\/p>\n

    FAQ<\/h2>\n

    Is SMT cheaper than through-hole assembly?<\/h3>\n

    SMT can reduce manual insertion and fits automated placement and reflow processes well, but technology alone does not determine the final assembly cost. Component package, placement density, board sides, inspection requirements, testing and mixed-process operations all affect manufacturing effort. A board that adds a few through-hole parts after SMT may require an additional soldering and inspection stage.<\/p>\n

    Can SMT and through-hole components be used on the same PCB?<\/h3>\n

    Yes. Mixed-technology PCB assembly combines surface-mount and through-hole components on the same circuit board. The manufacturing sequence needs to account for both processes. Layout decisions such as component spacing, lead location, solder access and fixture clearance can determine whether the through-hole stage remains straightforward after the SMT components have been populated.<\/p>\n

    Does a through-hole component require a plated hole?<\/h3>\n

    For an electrical through-hole lead that must connect to PCB copper, the design typically uses a plated through-hole. Mechanical features may use different hole constructions depending on their function. The fabrication data must clearly distinguish the required hole types so drilling, plating and assembly match the component footprint and electrical design.<\/p>\n

    How are BGA solder joints inspected?<\/h3>\n

    Ball grid array solder joints sit underneath the package and cannot be fully evaluated by ordinary optical inspection. X-ray inspection can examine BGA connections and is also applicable to packages such as LGAs and QFNs where important solder features are hidden beneath the component. Electrical and functional testing can then verify aspects of assembled-board performance.<\/p>\n

    What files are needed to quote PCB assembly?<\/h3>\n

    A complete assembly package should include the PCB manufacturing data, BOM, pick-and-place or centroid file and assembly drawing. Relevant fabrication information can include Gerber RS-274X, ODB++, IPC-2581 and NC drill data. Stackup information, impedance targets and fabrication notes should also be supplied where they apply to the circuit board.<\/p>\n","protected":false},"excerpt":{"rendered":"

    Surface mount technology (SMT) places components directly onto PCB pads, while through-hole technology (THT) inserts leads through drilled holes before […]<\/p>\n","protected":false},"author":2,"featured_media":276,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"nf_dc_page":"","site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[17],"tags":[],"class_list":["post-277","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-pcb-assembly"],"_links":{"self":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/posts\/277","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/comments?post=277"}],"version-history":[{"count":0,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/posts\/277\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/media\/276"}],"wp:attachment":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/media?parent=277"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/categories?post=277"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/tags?post=277"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}