Through-Hole PCB Assembly Process
See how a leaded component becomes a soldered, inspected connection.
Through-hole assembly places component leads into PCB holes, then solders them using wave, selective or controlled hand soldering. The complete process also includes build review, component preparation, inspection and the agreed tests.
Follow the illustrated sequence, compare soldering methods, and check the dimensions that matter before releasing your assembly.
Follow the joint through production
Select wave, selective or hand soldering, then move through each stage. Use Play for a guided sequence or choose an individual step.
The interactive illustration loads with JavaScript. The complete process is also described below.
Educational animation, not a machine simulation. Shape, solder fill, motion and timing are illustrative; they do not set process limits or certify a joint.
The through-hole assembly process, step by step
Review files and choose the process
Match the BOM, board revision and assembly drawing. Identify which components need through-hole insertion, where they sit, and whether the underside contains SMT parts. Establish the solder alloy, component restrictions, inspection requirements and electrical test scope.
Output: a reviewed assembly route and a list of resolved engineering questions.
Prepare components and insert leads
Verify part identity and polarity before insertion. Form leads only where the component manufacturer permits, using support that avoids stress at the body or seal. Seat components to the drawing and provide support for heavy or mechanically loaded parts. Axial, radial, DIP and connector packages need different handling instructions.
Check: orientation, seating height, pin position and approved lead projection. Trimming may be before or after soldering; it is not a universal fixed step.
Apply flux to the intended joints
Wave and selective routes need controlled flux delivery to the soldering locations. Use a flux compatible with the solder alloy and the required residue or cleaning strategy. Hand soldering may instead use flux-cored wire with additional compatible flux where specified.
Check: the supplier鈥檚 flux recommendations and coverage of the intended joint locations.
Preheat within the qualified process
Preheat prepares the assembly for solder contact. Board thickness, copper distribution, fixtures and component limits influence the thermal behavior. Measure representative board locations when establishing the process; do not copy a generic temperature from an article.
Check: a suitable thermal profile and the component limits. Separate preheat is not required for every hand-soldering operation.
Solder with wave, selective or a controlled hand method
A wave contacts a broad underside region; a selective nozzle addresses defined joints; a soldering iron delivers local heat with solder wire. The chosen route must let solder wet the relevant lead and pad surfaces while respecting nearby components. Pin-in-paste is a separate, package-qualified option, not a default replacement.
Check: access, wetting and a process window appropriate to the complete assembly.
Cool, trim and clean as specified
Avoid movement while joints solidify. Where post-solder trimming is specified, avoid transferring excessive cutting loads into the joint. Cleaning depends on the flux system and the end-use requirements; 鈥渘o-clean鈥� does not automatically mean residues are acceptable for every application.
Check: component security, damage, residues and the released cleaning instructions.
Inspect the assembly and run agreed tests
Inspect component orientation, physical condition, solder connections and unwanted solder. Assess workmanship using the standard, revision, class and any customer conditions agreed for the order. Continuity, in-circuit or functional testing answer different questions from visual inspection.
Check: acceptance records, defect disposition and actual test results鈥攏ot appearance alone.
Release the verified revision and package it
Close open issues and identify the released assembly revision. Apply the agreed traceability records, labels and protective packaging. Support tall or heavy parts where shipment could otherwise apply damaging loads.
Output: the agreed inspection/test records and an identifiable assembly ready for dispatch.
For the preceding paste printing, placement and reflow stages on mixed boards, see the complete PCB assembly process.
Wave, selective or hand soldering?
Choose around board access, component heat limits and repeatability needs. Quantity affects economics, but it does not decide the route by itself.
| Method | Useful starting point | What to review |
|---|---|---|
| Wave soldering | Many accessible through-hole joints across the intended underside. | Underside SMT compatibility, pallet openings, masking and connector orientation; review measured wave-soldering temperature and contact. |
| Selective soldering | Defined groups of through-hole joints on mixed-technology boards. | Review selective-nozzle access and local thermal demand, lead projection, travel path and fixture access. |
| Controlled hand soldering | Restricted joints, unusual components or operations suited to a defined manual method. | Tip access; operator instructions; heating consistency; repeatability and inspection. |
| Pin-in-paste / intrusive reflow | A possible route when the connector or package is explicitly suitable for reflow. | Package heat rating; paste volume; hole geometry; stencil strategy and qualified profile. |
Ask for engineering review if access is uncertain. Equipment ownership alone does not establish which process is suitable for your board. View the assembly equipment page.
Watch the process, then check the details
YouTube process video
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What to look for
- How are components seated and held?
- Which surfaces contact solder, and which remain protected?
- How does soldering access differ from component placement access?
- Which checks still need an inspection or test record?
A demonstration can explain motion and equipment. It cannot establish a process window or acceptance result for a different assembly.
Where through-hole fits on a mixed SMT board
A common route is SMT placement and reflow 鈫� SMT inspection 鈫� through-hole insertion and soldering 鈫� final inspection and test. A second SMT side, heat-sensitive parts, adhesives or a special fixture can change the sequence.
For through-hole versus SMT, the practical distinction is how a component connects to the board. SMT supports dense surface placement; through-hole places leads through holes and needs suitable access. Package choice alone does not guarantee higher power handling or better reliability.
For a component explicitly compatible with reflow, review pin-in-paste soldering and the paste-volume worksheet before choosing a separate soldering operation. Geometry, retained deposition and the exact component limits determine whether that route is practical.
Review double-sided assembly when underside parts restrict soldering. For hidden ball-grid joints, use the separate BGA assembly guide rather than applying through-hole inspection assumptions.
Check the finished hole against the lead
Use the minimum finished plated-hole diameter, after the fabrication tolerance, and the maximum lead envelope. Drill-tool diameter is not the finished hole.
Check finished-hole / lead clearance
A simple worked example
A 1.00 mm finished hole with a 鈭�0.05 mm tolerance has a 0.95 mm minimum diameter. With a 0.70 mm maximum round lead, the diametral clearance is 0.95 鈭� 0.70 = 0.25 mm. This is a geometry result, not a recommended clearance or an insertion approval.
For a rectangular lead in a round hole, use the diagonal envelope: 鈭�(width虏 + thickness虏). Also review hole position, lead pitch and alignment across multi-pin parts. Positive clearance does not predict solder fill.
The planner calculates from your inputs. It does not approve manufacturability. Download the result to attach to your enquiry; Contact does not automatically receive the parameters.
Inspection: identify the symptom before changing the process
| Observation | Possible areas to investigate | Useful evidence |
|---|---|---|
| Bridge between adjacent leads | Lead geometry, wetting, flux condition, solder separation and tooling. | Joint photographs, pin spacing and process records. |
| Insufficient barrel fill | Solderability, thermal demand, flux delivery, hole/lead geometry and access. | Finished-hole/lead data, copper connections and a measured profile. |
| Non-wetting or dewetting | Surface condition, storage, contamination and material/process compatibility. | Lot identity, storage history and solderability findings. |
| Mis-seated or reversed component | Insertion instructions, polarity markings, fixtures and verification. | Released drawing and first-piece inspection. |
| Cracked or disturbed connection | Movement during solidification, handling, trimming stress or mechanical loading. | Handling sequence, support method and defect examination. |
These are investigation paths, not diagnoses from a photograph. Increasing heat alone may not correct a poor joint and can damage the assembly.
Define acceptance before the build
IPC J-STD-001 addresses soldered-assembly requirements and process/material controls; IPC-A-610 provides assembly acceptability criteria. Specify the agreed revision, class and applicable customer conditions. Do not apply one generic barrel-fill percentage to every order.
Quality and review evidence explains the supplier-review discussion; the test-method guide helps separate inspection, electrical test and functional test.
Send a build package that can be quoted
Include these files and decisions
- PCB fabrication data and revision.
- BOM with manufacturer part numbers and sourcing responsibility.
- Assembly drawing with polarity, seating and lead-forming notes.
- Quantity, fitted variants and any staged delivery requirements.
- Alloy, cleaning/coating, acceptance and test requirements.
- Component data sheets for unusual heat or mechanical limits.
Unknown items can be marked 鈥渆ngineering advice needed.鈥� You do not need to select a soldering machine before contacting us.
What changes the price?
Insertion effort, joint count and access, fixture work, component sourcing, test scope and build quantity affect the quotation. A count of through-hole pins alone cannot produce an exact build price.
Use the assembly estimate page for preliminary budget planning. Exact price and lead time require the actual files and scope.
Send files for an exact quoteOpens Contact. No files or project data are sent from this page.
Through-hole assembly questions
Is through-hole assembly the same as DIP assembly?
DIP is one through-hole package style. Through-hole assembly also covers axial and radial parts, connectors and other leaded packages.
Do all through-hole parts require wave soldering?
No. Wave, selective and controlled hand soldering are different routes. Access and component limits matter. Some suitable packages can use a qualified pin-in-paste route.
Is lead cutting always done after soldering?
No. The component, insertion method and released work instruction determine the sequence. Avoid damaging the body, seal or finished joint.
What temperature and contact time should I specify?
Provide the component restrictions and assembly requirements. The production process needs a qualified profile for the actual materials, board and equipment; there is no single universal setting.
Does every no-clean assembly need no cleaning?
No-clean identifies a flux process category, not unconditional residue acceptability. Electrical environment, coating and customer cleanliness requirements can change the decision.
Can I combine SMT and through-hole components?
Yes. Review underside access, thermal exposure, support and inspection between operations. Mixed assembly is a process-planning task.
Can a shiny joint prove quality?
No. Appearance varies with materials and does not establish electrical function or all acceptance criteria. Use the agreed inspection requirements and tests.
How can I request a quote if my soldering route is undecided?
Send the BOM, PCB data, assembly drawing, quantity and known restrictions through Contact. Mark the route as needing review rather than guessing.
For sourcing, files, test fixtures and delivery questions, see the PCB assembly FAQ.
Technical references
Use component-specific instructions and the agreed order requirements when setting a production process.
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