Selective soldering needs more than a list of through-hole components. Before layout release, review whether the proposed machine, nozzle, fluxing, heating and support arrangement can reach each joint while respecting nearby parts and the assembly's thermal constraints.
The mixed SMT/THT planning guide compares overall assembly routes. This article focuses on the geometry and trial evidence needed when selective soldering is the proposed through-hole operation.
1. State why selective soldering is being proposed
Identify the components and joints assigned to the operation, the sequence relative to reflow, the expected quantities and any alternative route still under consideration. Distinguish repeatable automated processing from occasional authorized hand repair. Neither route should be treated as acceptable solely because the board contains through-hole parts.
Ask the assembler to identify the equipment configuration and process assumptions behind its DFM response. Ersa's VERSAFLOW ONE equipment data identifies distinct fluxing, preheating and soldering modules. This illustrates why the proposed configuration matters; it does not supply universal clearances or settings for another machine.
2. Review a three-dimensional access envelope
Supply the board outline, actual assembly-side orientation, bottom-side component bodies and heights, lead protrusion, connectors, fasteners and proposed fixture interfaces. Identify the nozzle approach, travel path and any parts that restrict access before or after the target joint.
On small screens, scroll the table sideways to compare columns.
| Feature | Question for the assembler | Evidence before release |
|---|---|---|
| Joint near an SMT body | Can the qualified nozzle and motion path reach the joint without interference? | Access drawing that includes the actual tool envelope and placement tolerances |
| Connector body or locating peg | Does the full component geometry obstruct fluxing, solder access or support? | Assembly model and an agreed insertion/protrusion condition |
| Large copper connection | Can the planned route form an acceptable joint within the component and board constraints? | Representative thermal and soldering trial evidence |
| Support or pallet feature | Does it maintain access while supporting the actual board? | Controlled tooling drawing and loaded-board trial |
| Unusual lead length | Is the variation compatible with the proposed path and joint requirements? | Approved preparation range and verification method |
A machine's maximum component clearance does not establish the local spacing needed around a particular solder joint. For example, an equipment brochure may state board-handling capacities while the nozzle, component geometry and programmed movement still determine whether a local feature is accessible.
3. Include tolerances rather than checking nominal CAD alone
Review component placement variation, lead preparation, board position, tooling fit and the relevant equipment capability. Agree how these contributors are represented in the access assessment. Do not assume that the nominal footprint courtyard is the complete manufacturing keepout.
Hypothetical teaching example: a preliminary layout shows a 3.0 mm nominal gap to an adjacent feature. The agreed review allowance for that specific configuration consumes 1.0 mm, leaving 2.0 mm in the review model. That arithmetic does not approve the layout. The remaining gap must still be compared with the actual qualified access requirement, which this example intentionally does not supply.
If no validated requirement is available, record the question as open and arrange the appropriate tooling or trial review. Avoid changing the allowance until the drawing appears to pass. Preserve the assumptions and identify which revision was actually evaluated.
4. Evaluate soldering and heating as one route
Discuss board construction, copper distribution, connector materials, component restrictions and previous thermal processing. Review the proposed flux, alloy, preheating, contact conditions and process sequence as a set. The approved trial should demonstrate suitability at representative difficult features, rather than only at easily soldered pins.
Set the joint acceptance criteria and inspection method before the trial. Where IPC requirements are invoked, state the applicable documents, revision, class and customer-specific requirements. The official IPC revision table identifies documents and revisions; it does not replace the actual requirements or authorize a generic hole-fill percentage.
Keep cleaning and downstream coating in view. Flux residues and any later cleaning process must be assessed through the relevant cleaning control plan. Do not assume that a process-material label alone resolves every finished-assembly requirement.
5. Define a trial matrix that can support the decision
- Identify representative joints: thermally demanding connections, dense groups, edge features and restricted-access locations.
- Record the board and component revisions, machine configuration, nozzle, fixture and controlled program identity.
- Document the relevant process-material identities and actual trial conditions.
- Apply the agreed inspection and any additional evidence needed to answer the specific risk questions.
- Record defects and repairs without allowing a repaired result to conceal an unsuccessful original process.
- Define the action owner and evidence required before an open issue can be closed.
Choose the trial quantity and verification depth from the product's requirements and identified risks. One attractive joint image does not demonstrate process capability across every pin, board configuration or future material lot. State what the trial did and did not establish.
6. Preserve the process for repeat orders
Release the relevant layout keepouts, fixture revision, program identity, material specification, acceptance requirements and restart conditions together. Identify changes that require review, such as a connector supplier change, lead-forming change, revised copper geometry, replacement nozzle or modified support tooling.
Use the release-package control guide to prevent a later order from combining an old fixture with a changed board. Where hand touch-up is permitted, define its authorization, inspection and record requirements; it should not become an undocumented workaround for an unresolved access problem.
Selective-soldering questions
Is there one keepout distance for every selective-soldering job?
No. The actual tool, nozzle, movement, component geometry, tolerances and support arrangement determine the access review. Obtain a requirement tied to the proposed configuration.
Can selective soldering use the same settings as an earlier board?
An earlier process may be useful starting information, but suitability must be reviewed against the new board construction, features, materials and thermal constraints before release.
Does acceptable hand touch-up prove the automated route is ready?
It demonstrates only the authorized repair result and its verification. Preserve the original process outcome and resolve the reason touch-up was needed before claiming the automated route is established.
Source scope: equipment guidance and IPC document information support the review questions. Clearances, settings and acceptance limits must come from the applicable controlled requirements. The numeric gap example is hypothetical and is not a design rule.