{"id":265,"date":"2026-08-07T09:00:00","date_gmt":"2026-08-07T09:00:00","guid":{"rendered":"https:\/\/rlxsolutions.com\/blog\/through-hole-pcb-assembly-soldering\/"},"modified":"2026-08-07T09:00:00","modified_gmt":"2026-08-07T09:00:00","slug":"through-hole-pcb-assembly-soldering","status":"publish","type":"post","link":"https:\/\/rlxsolutions.com\/blog\/through-hole-pcb-assembly-soldering\/","title":{"rendered":"Through-Hole PCB Assembly & Soldering Explained"},"content":{"rendered":"

Through-hole PCB assembly<\/a> mounts component leads through drilled holes in a printed circuit board, then solders those leads to create electrical and mechanical connections. RLX Solutions<\/a>, a contract electronics manufacturer in Concord, Ontario, uses through-hole assembly for boards where component format, mechanical loading or mixed-technology construction makes leaded mounting the practical choice.<\/p>\n

Through-hole assembly starts with the bare board<\/h2>\n

Through-hole assembly is often described as a soldering operation, but many of its constraints are established during bare PCB fabrication. The hole has to be drilled, plated where required, registered to the copper features and sized for the component lead before assembly begins. For the full picture, see our PCB Assembly Guide<\/a>.<\/p>\n

A plated through-hole (PTH) is a drilled hole whose wall is metallized so the hole can provide electrical connection between copper layers. A non-plated through-hole has no conductive barrel and is commonly used where the hole serves a mechanical rather than electrical purpose.<\/p>\n

That distinction matters because a component lead inserted into a PTH is interacting with more than the visible pad. The solder joint involves the lead, pad and plated barrel. Hole geometry therefore affects insertion, solder flow and the finished joint.<\/p>\n

Consider a 1.6 mm thick PCB with a 0.80 mm finished plated hole and a component lead measuring 0.65 mm across its widest section. The 0.15 mm nominal difference provides insertion clearance before lead shape, plating variation and positional tolerance are considered. Reducing the finished hole to 0.68 mm may appear to save routing space, but only 0.03 mm remains around the nominal lead diameter. A slightly bent lead or non-circular lead section can turn assembly into a force-fit operation.<\/p>\n

The engineering consequence appears later. Operators may struggle to seat components consistently, solder may have less room to move through the annular space, and rework becomes harder.<\/p>\n

Hole size should therefore be selected from the actual component lead geometry and the intended assembly process, rather than copied from a generic footprint library.<\/p>\n

Component insertion determines what soldering has to solve<\/h2>\n

Once the bare circuit board is fabricated, through-hole components must be inserted with the correct orientation, seating height and lead condition.<\/p>\n

Some parts can sit close to the board. Others need controlled stand-off for mechanical, electrical or thermal reasons. Connectors may also need their body aligned with a panel, enclosure or mating assembly rather than simply pushed as far as possible into the PCB.<\/p>\n

Insertion creates several practical checks:<\/p>\n

    \n
  1. Lead-to-hole fit.<\/strong> The lead needs enough clearance to enter without damaging the plating or component.<\/li>\n
  2. Polarity and orientation.<\/strong> Polarized capacitors, diodes and keyed connectors have to match the assembly documentation.<\/li>\n
  3. Component seating.<\/strong> The drawing should make intentional stand-offs or body heights clear.<\/li>\n
  4. Mechanical retention.<\/strong> Heavy or awkward components may need to remain stable before soldering.<\/li>\n
  5. Lead protrusion.<\/strong> Excessive or inconsistent protrusion can interfere with soldering, trimming or nearby hardware.<\/li>\n<\/ol>\n

    A footprint can therefore be electrically correct and still be difficult to manufacture.<\/p>\n

    For example, imagine a connector with two rows of ten leads. If the electrical pin pattern is correct but the component outline is misplaced by 1.0 mm relative to the PCB edge, every solder joint can pass inspection while the finished circuit board still fails enclosure integration. Through-hole design needs to treat mechanical datum relationships as assembly requirements, not cosmetic drawing information.<\/p>\n

    Soldering must heat the lead, barrel and pad as a system<\/h2>\n

    A through-hole solder joint forms when solder wets the surfaces intended to participate in the connection. The challenge is that those surfaces do not all heat at the same rate.<\/p>\n

    A small signal lead connected to a narrow trace presents a different thermal load from a connector pin attached to a large copper region. Copper conducts heat away from the joint. Component mass, copper distribution, board construction and adjacent hardware can therefore change how readily the joint reaches the required soldering condition.<\/p>\n

    This is why identical-looking holes on a layout may behave differently during production.<\/p>\n

    Take two connector pins using the same hole and pad geometry. Pin 1 connects to a narrow signal trace. Pin 2 connects into a large copper area on an internal layer. During soldering, Pin 2 can draw heat away from the joint faster. Treating both pins as thermally identical can produce good wetting on the signal pin while the higher-thermal-mass connection remains more difficult to solder.<\/p>\n

    The design lesson is not simply to add more heat. More thermal energy can affect nearby components, laminate and solder mask. The better approach is to identify thermally demanding joints during design for manufacturability (DFM) and plan the pad, copper connection and soldering process around them.<\/p>\n

    Selective soldering changes the mixed-technology design problem<\/h2>\n

    Many modern circuit boards combine surface-mount technology (SMT<\/a>) with a smaller number of through-hole connectors, relays, switches or other leaded parts. In that situation, through-hole assembly is one stage in a larger process rather than the entire build.<\/p>\n

    Selective soldering applies solder to defined through-hole locations instead of exposing the full underside of the board to the same soldering operation. It is useful for mixed-technology assemblies, but component placement still has to provide workable access around the intended solder locations.<\/p>\n

    That creates an important layout question: what is physically close to the joint on the solder side?<\/p>\n

    A through-hole connector may fit comfortably in the CAD model while nearby surface-mount components occupy the space needed by the soldering process. Moving one small component early in layout can be much less disruptive than discovering after fabrication that the intended process has poor access to a row of connector pins.<\/p>\n

    The assembly drawing also matters. It should communicate component side, orientation, special seating requirements and any assembly conditions that cannot be inferred safely from the bill of materials (BOM) and centroid data alone.<\/p>\n

    Same layer count does not mean the same through-hole assembly cost<\/h2>\n

    Layer count is a poor shortcut for judging assembly difficulty. Two boards with identical dimensions and layer counts can place very different demands on fabrication, insertion, soldering, inspection and test.<\/p>\n

    Consider two hypothetical eight-layer industrial control boards:<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n
    Decision factor<\/th>\nBoard A<\/th>\nBoard B<\/th>\n<\/tr>\n<\/thead>\n
    Layer count<\/td>\n8<\/td>\n8<\/td>\n<\/tr>\n
    Board thickness<\/td>\n1.6 mm<\/td>\n1.6 mm<\/td>\n<\/tr>\n
    Through-hole content<\/td>\nTwo small headers<\/td>\nFour large connectors plus leaded power parts<\/td>\n<\/tr>\n
    Typical PTH geometry<\/td>\n0.8 mm holes<\/td>\nMix of 0.8 mm and 1.2 mm holes<\/td>\n<\/tr>\n
    Copper around PTH joints<\/td>\nPrimarily signal connections<\/td>\nSeveral connections into larger copper regions<\/td>\n<\/tr>\n
    Assembly sequence<\/td>\nMostly SMT, limited PTH insertion<\/td>\nSMT plus substantial secondary insertion<\/td>\n<\/tr>\n
    Soldering demand<\/td>\nRelatively uniform<\/td>\nWider thermal variation between joints<\/td>\n<\/tr>\n
    Inspection burden<\/td>\nSmall PTH population<\/td>\nLarger joint population with mixed geometry<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    Board B is not more involved because it has eight layers. Both boards do. Its cost pressure comes from the assembly content: more manual or automated insertion activity, more solder joints, different thermal loads and a greater inspection burden.<\/p>\n

    This distinction matters during sourcing. A quotation based mainly on PCB dimensions, layer count and BOM line count misses the process information that determines how the board will actually move through production.<\/p>\n

    The manufacturer needs enough assembly data to identify that difference before the build reaches the line.<\/p>\n

    Hole geometry can turn a fabrication decision into an assembly problem<\/h2>\n

    Through-hole designers need to distinguish drill size from finished hole size. Plating changes the geometry of a plated hole, so the number that matters to component insertion is the finished condition.<\/p>\n

    Aspect ratio also matters in fabrication. For a through-hole, aspect ratio relates board thickness to hole diameter. As the board becomes thicker or the hole becomes smaller, creating a consistent plated barrel becomes more demanding.<\/p>\n

    Consider a hypothetical 2.4 mm thick PCB with a 0.30 mm plated through-hole. Its thickness-to-hole relationship is 8:1. If the same design moves to a 3.0 mm board while retaining the 0.30 mm hole, that relationship becomes 10:1.<\/p>\n

    Nothing changed in the schematic. The component may not have changed either. But the fabrication condition did.<\/p>\n

    The assembly engineer cares because the solder joint depends on the fabricated hole that arrives at assembly. A design should therefore be reviewed as a chain:<\/p>\n

    Component lead geometry → finished hole → drilling and plating → pad and annular ring → insertion → soldering → inspection<\/strong><\/p>\n

    Breaking that chain into separate purchasing decisions can hide problems until the physical board reaches assembly.<\/p>\n

    Through-hole and SMT should be designed as one manufacturing sequence<\/h2>\n

    Mixed-technology assembly is not simply SMT plus through-hole. The order of operations affects component access, inspection and test.<\/p>\n

    Surface-mount components normally depend on solder paste deposition, placement and reflow. Through-hole components introduce insertion and a subsequent soldering operation. Components, fixtures and board features therefore have to survive and remain accessible through the complete manufacturing sequence selected for the board.<\/p>\n

    The practical design review should look beyond individual footprints. Engineers should examine where large through-hole bodies sit relative to surface-mount devices, whether connector geometry blocks inspection, and whether the final configuration still permits test access.<\/p>\n

    A hypothetical six-layer board illustrates the issue. Suppose a 2.00 mm pitch through-hole connector sits on the top side, while small surface-mount devices occupy the underside directly around its pins. Moving those underside components 3 mm during layout may create much cleaner access for the later through-hole operation. Waiting until assembly means the PCB layout, stencil, procurement and fabrication data may already be released.<\/p>\n

    That is the value of design for assembly (DFA): it examines whether the physical design supports the intended production sequence rather than merely whether every component can be placed in CAD.<\/p>\n

    Inspection has to match the joint and the build class<\/h2>\n

    Through-hole solder joints are inspected for workmanship after soldering, but inspection should not be treated as a cosmetic final gate.<\/p>\n

    IPC-A-610 provides workmanship criteria for electronic assemblies, with Class 2 and Class 3 representing different performance expectations. Class 3 applies tighter workmanship requirements than Class 2. The build class should therefore be established as a programme requirement rather than selected informally after boards have been assembled.<\/p>\n

    J-STD-001 addresses soldered electrical and electronic assemblies and is directly relevant to the soldering process. These standards interact with design documentation, process controls and acceptance requirements.<\/p>\n

    Visual inspection under magnification can evaluate accessible through-hole joints. The broader inspection plan for a mixed-technology PCB may also include solder paste inspection (SPI) before SMT reflow, automated optical inspection (AOI) after reflow and X-ray inspection for packages such as ball grid arrays (BGAs), land grid arrays (LGAs) and quad flat no-lead (QFN) devices.<\/p>\n

    The adjacent processes matter because a mixed board does not become a through-hole-only inspection problem merely because its final assembly stage contains leaded parts.<\/p>\n

    Electrical test catches problems that appearance cannot<\/h2>\n

    A solder joint can look acceptable while the circuit still fails electrically. Conversely, a visually unusual feature may require engineering interpretation rather than an assumption that the board is electrically defective.<\/p>\n

    Inspection and test answer different questions.<\/p>\n

    Inspection asks whether the physical assembly meets the applicable workmanship requirements. Electrical test asks whether the intended connections and functions are present.<\/p>\n

    Flying-probe in-circuit test (ICT) can provide net coverage without a dedicated fixture for prototype and lower-volume work. At production volume, bed-of-nails ICT uses a fixture to access designated test points. Functional test (FCT) goes further by exercising the board against customer-defined behaviour and recording pass or fail results.<\/p>\n

    Through-hole components should therefore be considered during test-access planning.<\/p>\n

    A large connector placed over test-point access may create a fixture problem even though the connector itself is easy to solder. Likewise, a board that will eventually sit inside an enclosure may need programming or functional access before mechanical integration blocks those interfaces.<\/p>\n

    Test strategy belongs upstream in PCB layout and assembly planning.<\/p>\n

    Rework gets harder as thermal mass and access increase<\/h2>\n

    Through-hole rework requires heating the existing joint, removing solder sufficiently to release the lead, extracting the component without damaging the barrel or pad, then preparing the location for replacement.<\/p>\n

    That sequence becomes harder when several leads must release together.<\/p>\n

    A two-lead component and a 40-pin connector are fundamentally different rework problems. With the connector, individual joints may be acceptable, yet all required leads must be freed without imposing excessive mechanical load on plated holes. Pulling against solder that has not fully released can damage the board.<\/p>\n

    Thermal mass compounds the issue. A connector tied into large copper features can require a different rework approach from a small signal component.<\/p>\n

    This creates a design trade-off. Through-hole mounting may be selected partly for mechanical reasons, but the same mechanical retention and large lead population can make replacement more difficult. If a component is expected to be serviced, programmed, calibrated or replaced during the product life, access around it deserves explicit review before layout release.<\/p>\n

    Give the manufacturer enough data to control the process<\/h2>\n

    A complete through-hole assembly package should communicate the electrical design and the physical assembly intent. Gerber RS-274X, ODB++ or IPC-2581 data can define the PCB fabrication information, with NC drill data establishing the drilled features. Assembly requires additional information.<\/p>\n

    For a mixed-technology PCB, the manufacturing package commonly needs:<\/p>\n